From c8a05beac77b438f49a3887aaccdb421cf6c1c32 Mon Sep 17 00:00:00 2001
From: Luca Toniolo <10792599+grandixximo@users.noreply.github.com>
Date: Thu, 3 Sep 2026 18:21:23 +1000
Subject: [PATCH 1/6] xyzbc-trt-kins: apply the direction sign to the offsets
 in the inverse

The inverse turns the x and z offsets through the B tilt in the
conventional sense, while the forward turns them in whichever sense the
conventional-directions pin selects.  With the pin false, which is the
default, and any of x-offset, z-offset or tool-offset set, a pose does
not survive the round trip: the position comes back out by twice sin(b)
times the offset.  xyzac has no term of this kind.

Found by tests/kins-jacobian, which multiplies the derivative of the
inverse by differences of the forward and expects the identity.
---
 src/emc/kinematics/trtfuncs.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/emc/kinematics/trtfuncs.c b/src/emc/kinematics/trtfuncs.c
index 31c9ff1a6de..0cb4b5eb7aa 100644
--- a/src/emc/kinematics/trtfuncs.c
+++ b/src/emc/kinematics/trtfuncs.c
@@ -362,11 +362,14 @@ int xyzbcKinematicsInverse(const EmcPose * pos,
     const double          dz = hal_get_real(haldata->z_offset) + dt;
     const double       b_rad = pos->b*TO_RAD;
     const double       c_rad = pos->c*TO_RAD;
-    const double         dpx = -cos(b_rad)*dx + sin(b_rad)*dz + dx;
-    const double         dpz = -sin(b_rad)*dx - cos(b_rad)*dz + dz;
 
     rtapi_real con = hal_get_bool(haldata->conventional_directions) ? 1.0 : -1.0;
 
+    // the offsets seen from the tilted table: the same rotation the
+    // forward applies to them, in the same sense
+    const double         dpx = -cos(b_rad)*dx + con * sin(b_rad)*dz + dx;
+    const double         dpz = -con * sin(b_rad)*dx - cos(b_rad)*dz + dz;
+
     EmcPose P; // computed position
 
     P.tran.x   = +       cos(c_rad) * cos(b_rad) * (pos->tran.x - x_rot_point)

From 336054b23b855ec017181147ba7e10a3deb5e90d Mon Sep 17 00:00:00 2001
From: Luca Toniolo <10792599+grandixximo@users.noreply.github.com>
Date: Thu, 3 Sep 2026 18:21:23 +1000
Subject: [PATCH 2/6] scarakins: set the elbow flag for a negative elbow angle

The forward sets the flag that makes the inverse negate its arc cosine
when joint 1 is below 90 degrees, so for an elbow between 0 and 90 the
inverse returns the other arm and the pose does not survive the round
trip.  The sign of the arc cosine is the sign of the elbow angle, so the
test is against zero.
---
 src/emc/kinematics/scarakins.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/emc/kinematics/scarakins.c b/src/emc/kinematics/scarakins.c
index 8db57eb7b0c..3b0e69ee7e4 100644
--- a/src/emc/kinematics/scarakins.c
+++ b/src/emc/kinematics/scarakins.c
@@ -96,8 +96,9 @@ int scaraKinematicsForward(const double * joint,
     z = D1 + D3 - joint[2] - D5;
     c = a3;
 
+    // the elbow flag: which sign the inverse gives the acos of joint 1
     *iflags = 0;
-    if (joint[1] < 90)
+    if (joint[1] < 0)
         *iflags = 1;
 
     world->tran.x = x;

From cd07b56e21a75fc23069e497705c0ebad16e10c2 Mon Sep 17 00:00:00 2001
From: Luca Toniolo <10792599+grandixximo@users.noreply.github.com>
Date: Thu, 3 Sep 2026 18:21:23 +1000
Subject: [PATCH 3/6] pumakins, three21kins: compare the branch flags modulo a
 whole turn

The forward decides the shoulder, elbow and wrist branches by comparing a
joint angle with the value the inverse's formula gives, within a fuzz,
and does not wrap the difference.  A joint standing a whole turn from
that value, which the differences of two atan2 results produce freely,
fails the comparison and the inverse is sent down the other branch.  The
difference is brought into (-pi, pi] first.
---
 src/emc/kinematics/pumakins.c    | 20 +++++++++++++++-----
 src/emc/kinematics/three21kins.c | 20 +++++++++++++++-----
 2 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/src/emc/kinematics/pumakins.c b/src/emc/kinematics/pumakins.c
index 98dc83aa4c5..049d384b424 100644
--- a/src/emc/kinematics/pumakins.c
+++ b/src/emc/kinematics/pumakins.c
@@ -29,6 +29,16 @@ struct haldata {
     hal_real_t a2, a3, d3, d4, d6;
 } *haldata = NULL;
 
+/* the difference of two angles, brought into (-pi, pi] so that a joint a
+   whole turn from the formula still matches it */
+static double angleDiff(double a, double b)
+{
+   double d = a - b;
+   while (d > PM_PI) { d -= 2*PM_PI; }
+   while (d <= -PM_PI) { d += 2*PM_PI; }
+   return d;
+}
+
 /* The flange orientation for a joint set: the ISO 9787 mechanical interface
    frame, whose z points out of the interface towards the work.  Shared by the
    forward kinematics and the tool frame so the two cannot drift apart. */
@@ -152,16 +162,16 @@ static int pumaKinematicsForward(const double * joint,
    *iflags = 0;
 
    /* Set shoulder-up flag if necessary */
-   if (fabs(joint[0]*PM_PI/180 - atan2(hom.tran.y, hom.tran.x) +
-       atan2(PUMA_D3, -sqrt(sumSq))) < FLAG_FUZZ)
+   if (fabs(angleDiff(joint[0]*PM_PI/180, atan2(hom.tran.y, hom.tran.x) -
+       atan2(PUMA_D3, -sqrt(sumSq)))) < FLAG_FUZZ)
    {
      *iflags |= PUMA_SHOULDER_RIGHT;
    }
 
    /* Set elbow down flag if necessary */
-   if (fabs(joint[2]*PM_PI/180 - atan2(PUMA_A3, PUMA_D4) +
+   if (fabs(angleDiff(joint[2]*PM_PI/180, atan2(PUMA_A3, PUMA_D4) -
        atan2(k, -sqrt(PUMA_A3 * PUMA_A3 +
-       PUMA_D4 * PUMA_D4 - k * k))) < FLAG_FUZZ)
+       PUMA_D4 * PUMA_D4 - k * k)))) < FLAG_FUZZ)
    {
       *iflags |= PUMA_ELBOW_DOWN;
    }
@@ -177,7 +187,7 @@ static int pumaKinematicsForward(const double * joint,
 
    /* if not singular set wrist flip flag if necessary */
    else{
-     if (! (fabs(joint[3]*PM_PI/180 - atan2(t1, t2)) < FLAG_FUZZ))
+     if (! (fabs(angleDiff(joint[3]*PM_PI/180, atan2(t1, t2))) < FLAG_FUZZ))
      {
        *iflags |= PUMA_WRIST_FLIP;
      }
diff --git a/src/emc/kinematics/three21kins.c b/src/emc/kinematics/three21kins.c
index a5a7b5dfa8f..2a3dfe08ee5 100644
--- a/src/emc/kinematics/three21kins.c
+++ b/src/emc/kinematics/three21kins.c
@@ -32,6 +32,16 @@ struct haldata {
     hal_real_t a1, a2, a3, d1, d2, d3, d4, d6;
 } *haldata = NULL;
 
+/* the difference of two angles, brought into (-pi, pi] so that a joint a
+   whole turn from the formula still matches it */
+static double angleDiff(double a, double b)
+{
+   double d = a - b;
+   while (d > PM_PI) { d -= 2*PM_PI; }
+   while (d <= -PM_PI) { d += 2*PM_PI; }
+   return d;
+}
+
 static int three21KinematicsForward(const double * joint,
                                     EmcPose * world,
                                     const KINEMATICS_FORWARD_FLAGS * fflags,
@@ -132,8 +142,8 @@ static int three21KinematicsForward(const double * joint,
    *iflags = 0;
 
    /* set shoulder flag */
-   if (fabs(joint[0]*PM_PI/180 - atan2(hom.tran.y, hom.tran.x) +
-       atan2(d23, -sqrt(sumSq))) < FLAG_FUZZ)
+   if (fabs(angleDiff(joint[0]*PM_PI/180, atan2(hom.tran.y, hom.tran.x) -
+       atan2(d23, -sqrt(sumSq)))) < FLAG_FUZZ)
    {
      *iflags |= THREE21_SHOULDER_RIGHT;
    }
@@ -143,8 +153,8 @@ static int three21KinematicsForward(const double * joint,
    if (discr < 0.0) {
        discr = 0.0;
    }
-   if (fabs(joint[2]*PM_PI/180 - atan2(a3, d4) +
-       atan2(k, -sqrt(discr))) < FLAG_FUZZ)
+   if (fabs(angleDiff(joint[2]*PM_PI/180, atan2(a3, d4) -
+       atan2(k, -sqrt(discr)))) < FLAG_FUZZ)
    {
       *iflags |= THREE21_ELBOW_DOWN;
    }
@@ -158,7 +168,7 @@ static int three21KinematicsForward(const double * joint,
    }
    else
    {
-     if (! (fabs(joint[3]*PM_PI/180 - atan2(t1, t2)) < FLAG_FUZZ))
+     if (! (fabs(angleDiff(joint[3]*PM_PI/180, atan2(t1, t2))) < FLAG_FUZZ))
      {
        *iflags |= THREE21_WRIST_FLIP;
      }

From 4a3f7a2831e2232aac7371768ce1ed072c21b06d Mon Sep 17 00:00:00 2001
From: Luca Toniolo <10792599+grandixximo@users.noreply.github.com>
Date: Thu, 3 Sep 2026 18:21:24 +1000
Subject: [PATCH 4/6] userkins, millturn, matrixkins, xyzab_tdr_kins: set the
 kinematics pins up once, at load

These four build their pins on the first kinematicsType() call behind an is_setup flag nothing sets, so every later call runs the setup again, reassigns haldata and fails to create pins already taken; motion asks twice when num_extrajoints is set.

Move the setup to EXTRA_SETUP(), which halcompile runs once before the component is ready, as the two trsrn modules already do. The pins exist from load, kinematicsType() only answers, and the hal_set_unready()/hal_ready() dance goes with it. userkins is the template for out of tree modules, so its description says where the pins go.
---
 src/hal/components/matrixkins.comp     | 14 +++++---------
 src/hal/components/millturn.comp       | 13 ++++++-------
 src/hal/components/userkins.comp       | 18 +++++++++---------
 src/hal/components/xyzab_tdr_kins.comp | 11 +++++------
 4 files changed, 25 insertions(+), 31 deletions(-)

diff --git a/src/hal/components/matrixkins.comp b/src/hal/components/matrixkins.comp
index b12dedc2fcf..aac6c04d913 100644
--- a/src/hal/components/matrixkins.comp
+++ b/src/hal/components/matrixkins.comp
@@ -176,6 +176,7 @@ the adjustment values should be added to the old values instead of replacing the
 """;
 see_also "kins(9)";
 pin out bool dummy=1; // halcompile requires at least one pin
+option extra_setup;
 license "GPL";
 ;;
 
@@ -191,15 +192,15 @@ static struct haldata {
     hal_real_t C_zz;
 } *haldata;
 
-static int matrixkins_setup(void) {
+EXTRA_SETUP() {
+    (void)__comp_inst;
+    (void)prefix;
+    (void)extra_arg;
     int res=0;
 
     // inherit comp_id from rtapi_main()
     if (comp_id < 0) goto error;
 
-    res = hal_set_unready(comp_id);
-    if (res) goto error;
-
     haldata = hal_malloc(sizeof(struct haldata));
     if (!haldata) goto error;
 
@@ -215,9 +216,6 @@ static int matrixkins_setup(void) {
 
     if (res) goto error;
 
-    res = hal_ready(comp_id);
-    if (res) goto error;
-
     rtapi_print("*** %s setup ok\n",__FILE__);
     return 0;
 error:
@@ -235,8 +233,6 @@ EXPORT_SYMBOL(kinematicsForward);
 
 KINEMATICS_TYPE kinematicsType()
 {
-    static bool is_setup=0;
-    if (!is_setup) matrixkins_setup();
     return KINEMATICS_BOTH;
 }
 
diff --git a/src/hal/components/millturn.comp b/src/hal/components/millturn.comp
index d1271de882e..45ec650ad96 100644
--- a/src/hal/components/millturn.comp
+++ b/src/hal/components/millturn.comp
@@ -27,9 +27,10 @@ chapter (docs/src/motion/switchkins.txt)
 
 """;
 // The fpin pin is not accessible in kinematics functions.
-// Use the *_setup() function for pins and params used by kinematics.
+// Use EXTRA_SETUP() for pins and params used by kinematics.
 pin out si32 fpin=0"pin to demonstrate use of a conventional (non-kinematics) function fdemo";
 option period no;
+option extra_setup;
 function fdemo;
 license "GPL";
 author "David Mueller";
@@ -59,14 +60,15 @@ FUNCTION(fdemo) {
    fpin_set(fpin + 1);
 }
 
-static int millturn_setup(void) {
+EXTRA_SETUP() {
+    (void)__comp_inst;
+    (void)prefix;
+    (void)extra_arg;
 #define HAL_PREFIX "millturn"
     int res=0;
 
     // inherit comp_id from rtapi_main()
     if (comp_id < 0) goto error;
-    // set unready to allow creation of pins
-    if (hal_set_unready(comp_id)) goto error;
 
     haldata = hal_malloc(sizeof(*haldata));
     if (!haldata) goto error;
@@ -85,7 +87,6 @@ static int millturn_setup(void) {
     res += hal_pin_new_bool(comp_id, HAL_OUT, &haldata->kinstype_is_1, 0, "kinstype.is-1");
 
     if (res) goto error;
-    hal_ready(comp_id);
     rtapi_print("*** %s setup ok\n",__FILE__);
     return 0;
 error:
@@ -142,8 +143,6 @@ int kinematicsSwitch(int new_switchkins_type)
 
 KINEMATICS_TYPE kinematicsType()
 {
-static bool is_setup=0;
-    if (!is_setup) millturn_setup();
     return KINEMATICS_BOTH; // set as required
            // Note: If kinematics are identity, using KINEMATICS_BOTH
            //       may be used in order to allow a gui to display
diff --git a/src/hal/components/userkins.comp b/src/hal/components/userkins.comp
index a2a25d88c29..a7af5d29a75 100644
--- a/src/hal/components/userkins.comp
+++ b/src/hal/components/userkins.comp
@@ -53,14 +53,16 @@ change all instances of `userkins` to `mykins`.
 * The *fpin* pin is included to satisfy the requirements of the halcompile
   utility but it is not accessible to kinematics functions.
 * HAL pins and parameters needed in kinematics functions (kinematicsForward(),
-  kinematicsInverse()) must be setup in a function (*userkins_setup()*) invoked
-  by the initial motion module call to kinematicsType().
+  kinematicsInverse()) must be setup in the *EXTRA_SETUP()* function, which
+  halcompile runs once when the module is loaded, before the component is
+  made ready.
 
 """;
 // The fpin pin is not accessible in kinematics functions.
-// Use the *_setup() function for pins and params used by kinematics.
+// Use EXTRA_SETUP() for pins and params used by kinematics.
 pin out si32 fpin=0"pin to demonstrate use of a conventional (non-kinematics) function fdemo";
 option period no;
+option extra_setup;
 function fdemo;
 license "GPL";
 author "Dewey Garrett";
@@ -91,14 +93,15 @@ FUNCTION(fdemo) {
    fpin_set(fpin + 1);
 }
 
-static int userkins_setup(void) {
+EXTRA_SETUP() {
+    (void)__comp_inst;
+    (void)prefix;
+    (void)extra_arg;
 #define HAL_PREFIX "userkins"
     int res=0;
 
     // inherit comp_id from rtapi_main()
     if (comp_id < 0) goto error;
-    // set unready to allow creation of pins
-    if (hal_set_unready(comp_id)) goto error;
 
     haldata = hal_malloc(sizeof(struct haldata));
     if (!haldata) goto error;
@@ -112,7 +115,6 @@ static int userkins_setup(void) {
     res += hal_param_new_real(comp_id, HAL_RO, &haldata->param_ro, 0.0, "%s.param-ro", HAL_PREFIX);
 
     if (res) goto error;
-    hal_ready(comp_id);
     rtapi_print("*** %s setup ok\n",__FILE__);
     return 0;
 error:
@@ -130,8 +132,6 @@ EXPORT_SYMBOL(kinematicsForward);
 
 KINEMATICS_TYPE kinematicsType()
 {
-static bool is_setup=0;
-    if (!is_setup) userkins_setup();
     return KINEMATICS_IDENTITY; // set as required
            // Note: If kinematics are identity, using KINEMATICS_BOTH
            //       may be used in order to allow a gui to display
diff --git a/src/hal/components/xyzab_tdr_kins.comp b/src/hal/components/xyzab_tdr_kins.comp
index b059f32c103..dd0350e44f7 100644
--- a/src/hal/components/xyzab_tdr_kins.comp
+++ b/src/hal/components/xyzab_tdr_kins.comp
@@ -31,6 +31,7 @@ chapter (docs/src/motion/switchkins.txt)
 """;
 
 pin out si32 dummy=0"one pin needed to satisfy halcompile requirement";
+option extra_setup;
 
 license "GPL";
 author "David Mueller";
@@ -54,13 +55,14 @@ static struct haldata {
   hal_bool_t kinstype_is_1;
 } *haldata;
 
-static int xyzab_tdr_setup(void) {
+EXTRA_SETUP() {
+    (void)__comp_inst;
+    (void)prefix;
+    (void)extra_arg;
 #define HAL_PREFIX "xyzab_tdr_kins"
     int res=0;
     // inherit comp_id from rtapi_main()
     if (comp_id < 0) goto error;
-    // set unready to allow creation of pins
-    if (hal_set_unready(comp_id)) goto error;
 
     haldata = hal_malloc(sizeof(*haldata));
     if (!haldata) goto error;
@@ -80,7 +82,6 @@ static int xyzab_tdr_setup(void) {
     res += hal_pin_new_bool(comp_id, HAL_OUT, &haldata->kinstype_is_1, 0, "kinstype.is-1");
 
     if (res) goto error;
-    hal_ready(comp_id);
     rtapi_print("*** %s setup ok\n",__FILE__);
     return 0;
 error:
@@ -137,8 +138,6 @@ int kinematicsSwitch(int new_switchkins_type)
 
 KINEMATICS_TYPE kinematicsType()
 {
-static bool is_setup=0;
-    if (!is_setup)  xyzab_tdr_setup();
     return KINEMATICS_BOTH; // set as required
            // Note: If kinematics are identity, using KINEMATICS_BOTH
            //       may be used in order to allow a gui to display

From 77f4173c3684ea287abeb321a7bfa10e4b27c795 Mon Sep 17 00:00:00 2001
From: Luca Toniolo <10792599+grandixximo@users.noreply.github.com>
Date: Tue, 15 Sep 2026 18:12:53 +1000
Subject: [PATCH 5/6] maxkins: make the forward and the inverse describe the
 same machine

Two faults left kinematicsForward and kinematicsInverse disagreeing whenever the head is tilted, and nothing in tree could show it: max5 is the only config that loads the module and it ships B_PIVOT_LENGTH = 0 with six joints, so the pivot to tip distance is zero and U, V and W never leave zero.

The forward took the U axis apart into xv and zv and subtracted both from the position it reports, while the inverse added xv back and subtracted zv again: X inverted and Z did not, in the same function, the shape of a typo rather than of a convention. The inverse now adds zv back.

The forward also rotated the table and saddle into the workpiece frame and then subtracted the B, U and V corrections from the result, so those landed in the rotated frame; the inverse un-rotates first and adds them back in the machine frame. The vismach model says which is right: max5gui builds the head as tool, spindle, head, brotary, zslide, column, and the workpiece separately as crotary, table, saddle, so the head hangs off the Z slide and does not turn with the C table. The corrections are machine frame, they apply before the rotation, and the inverse had it right. The forward now subtracts them from the joint values and rotates the sum.

A sweep of 40000 random poses, pivot lengths to 300 and tool lengths to 37.5, round trips to 2.7e-13, against 666 mm before. Reported world coordinates move for anyone running maxkins with the head tilted and the table turned; they were wrong by that amount, and the DRO and the machine now agree. Nothing moves at C0, and nothing moves at all on the max5 sim.

tests/maxkins drives the machine through poses with the pivot length, the tool length and each of B, C, U, V and W away from zero and compares the position that comes back with the one commanded: motion runs the inverse to turn the pose into joints and the forward to turn the joint feedback into a position, so a pose that survives the round trip is one the two directions agree on. Both settings of conventional-directions are covered. Against the old code the pose with B 40 and U 15 came back 19.284 mm out in Z, twice u sin(b), and the pose with B 40 and C 30 landed 10.765 mm out in X and 40.174 mm out in Y.
---
 src/emc/kinematics/maxkins.c |  20 ++--
 tests/maxkins/README         |   7 ++
 tests/maxkins/checkresult    |   2 +
 tests/maxkins/core_sim.hal   |  28 +++++
 tests/maxkins/test-ui.py     | 132 ++++++++++++++++++++++
 tests/maxkins/test.ini       | 210 +++++++++++++++++++++++++++++++++++
 tests/maxkins/test.sh        |   4 +
 tests/maxkins/tool.tbl       |   0
 8 files changed, 396 insertions(+), 7 deletions(-)
 create mode 100644 tests/maxkins/README
 create mode 100755 tests/maxkins/checkresult
 create mode 100644 tests/maxkins/core_sim.hal
 create mode 100755 tests/maxkins/test-ui.py
 create mode 100644 tests/maxkins/test.ini
 create mode 100755 tests/maxkins/test.sh
 create mode 100644 tests/maxkins/tool.tbl

diff --git a/src/emc/kinematics/maxkins.c b/src/emc/kinematics/maxkins.c
index 773525b3566..d2623e0ad62 100644
--- a/src/emc/kinematics/maxkins.c
+++ b/src/emc/kinematics/maxkins.c
@@ -50,10 +50,6 @@ int kinematicsForward(const double *joints,
     // B correction
     const double zb = (pivot_length + joints[8] + tool_length) * cos(d2r(joints[4]));
     const double xb = (pivot_length + joints[8] + tool_length) * sin(d2r(joints[4]));
-        
-    // C correction
-    const double xyr = hypot(joints[0], joints[1]);
-    const double xytheta = atan2(joints[1], joints[0]) + d2r(joints[5]);
 
     // U correction
     const double zv = joints[6] * sin(d2r(joints[4]));
@@ -61,8 +57,18 @@ int kinematicsForward(const double *joints,
 
     // V correction is always in joint 1 only
 
-    pos->tran.x = xyr * cos(xytheta) - (con * xb) - xv;
-    pos->tran.y = xyr * sin(xytheta) - joints[7];
+    // B, U and V are all machine frame: the head hangs off the Z slide and
+    // does not turn with the C table, so they apply before the rotation into
+    // the workpiece frame rather than after it.
+    const double mx = joints[0] - (con * xb) - xv;
+    const double my = joints[1] - joints[7];
+
+    // C correction
+    const double xyr = hypot(mx, my);
+    const double xytheta = atan2(my, mx) + d2r(joints[5]);
+
+    pos->tran.x = xyr * cos(xytheta);
+    pos->tran.y = xyr * sin(xytheta);
     pos->tran.z = joints[2] - zb - (con * zv) + pivot_length + tool_length;
 
     pos->a = joints[3];
@@ -103,7 +109,7 @@ int kinematicsInverse(const EmcPose * pos,
 
     joints[0] = xyr * cos(xytheta) + (con * xb) + xv;
     joints[1] = xyr * sin(xytheta) + pos->v;
-    joints[2] = pos->tran.z + zb - (con * zv) - pivot_length - tool_length;
+    joints[2] = pos->tran.z + zb + (con * zv) - pivot_length - tool_length;
 
     joints[3] = pos->a;
     joints[4] = pos->b;
diff --git a/tests/maxkins/README b/tests/maxkins/README
new file mode 100644
index 00000000000..cdf964410db
--- /dev/null
+++ b/tests/maxkins/README
@@ -0,0 +1,7 @@
+Tests that maxkins forward and inverse kinematics agree with each other.
+
+The machine is driven to a series of poses with the pivot length, the tool
+length and every one of B, C, U, V and W away from zero, and the position
+that comes back through the forward kinematics is compared with the one that
+was commanded.  A pose that survives the round trip is one where the two
+directions of the transform describe the same machine.
diff --git a/tests/maxkins/checkresult b/tests/maxkins/checkresult
new file mode 100755
index 00000000000..24dc9aa53e3
--- /dev/null
+++ b/tests/maxkins/checkresult
@@ -0,0 +1,2 @@
+#!/bin/sh
+exit 0 # test failure is indicated by test.sh exit value
diff --git a/tests/maxkins/core_sim.hal b/tests/maxkins/core_sim.hal
new file mode 100644
index 00000000000..daf9b8f4c25
--- /dev/null
+++ b/tests/maxkins/core_sim.hal
@@ -0,0 +1,28 @@
+# core HAL config file for simulation
+
+loadrt [KINS]KINEMATICS
+loadrt [EMCMOT]EMCMOT servo_period_nsec=[EMCMOT]SERVO_PERIOD num_joints=[KINS]JOINTS
+
+# add motion controller functions to servo thread
+addf motion-command-handler servo-thread
+addf motion-controller servo-thread
+
+# create HAL signals for position commands from motion module
+# loop position commands back to motion module feedback
+net Xpos joint.0.motor-pos-cmd => joint.0.motor-pos-fb
+net Ypos joint.1.motor-pos-cmd => joint.1.motor-pos-fb
+net Zpos joint.2.motor-pos-cmd => joint.2.motor-pos-fb
+net Apos joint.3.motor-pos-cmd => joint.3.motor-pos-fb
+net Bpos joint.4.motor-pos-cmd => joint.4.motor-pos-fb
+net Cpos joint.5.motor-pos-cmd => joint.5.motor-pos-fb
+net Upos joint.6.motor-pos-cmd => joint.6.motor-pos-fb
+net Vpos joint.7.motor-pos-cmd => joint.7.motor-pos-fb
+net Wpos joint.8.motor-pos-cmd => joint.8.motor-pos-fb
+
+# the geometry the kinematics works from.  Both are away from zero so that
+# the pivot to tip distance is not degenerate.
+setp maxkins.pivot-length [MAX]B_PIVOT_LENGTH
+setp maxkins.tool-length  [MAX]TOOL_LENGTH
+
+# estop loopback
+net estop-loop iocontrol.0.user-enable-out iocontrol.0.emc-enable-in
diff --git a/tests/maxkins/test-ui.py b/tests/maxkins/test-ui.py
new file mode 100755
index 00000000000..d65e561e516
--- /dev/null
+++ b/tests/maxkins/test-ui.py
@@ -0,0 +1,132 @@
+#!/usr/bin/env python3
+
+import linuxcnc
+import hal
+
+import time
+import sys
+
+
+def wait_for_linuxcnc_startup(status, timeout=10.0):
+
+    """Poll the Status buffer waiting for it to look initialized,
+    rather than just allocated (all-zero).  Returns on success, throws
+    RuntimeError on failure."""
+
+    start_time = time.time()
+    while time.time() - start_time < timeout:
+        status.poll()
+        if (status.angular_units == 0.0) \
+            or (status.axis_mask == 0) \
+            or (status.cycle_time == 0.0) \
+            or (status.exec_state != linuxcnc.EXEC_DONE) \
+            or (status.interp_state != linuxcnc.INTERP_IDLE) \
+            or (status.inpos == False) \
+            or (status.linear_units == 0.0) \
+            or (status.max_acceleration == 0.0) \
+            or (status.max_velocity == 0.0) \
+            or (status.program_units == 0.0) \
+            or (status.rapidrate == 0.0) \
+            or (status.state != linuxcnc.RCS_DONE) \
+            or (status.task_state != linuxcnc.STATE_ESTOP):
+            time.sleep(0.1)
+        else:
+            # looks good
+            return
+
+    # timeout, throw an exception
+    raise RuntimeError("Timeout")
+
+
+c = linuxcnc.command()
+s = linuxcnc.stat()
+e = linuxcnc.error_channel()
+
+# Wait for LinuxCNC to initialize itself so the Status buffer stabilizes.
+wait_for_linuxcnc_startup(s)
+
+# Because the kinematics is non-trivial, a homing is needed.
+# HOME_ABSOLUTE_ENCODER = 1 is used in the ini file.
+c.state(linuxcnc.STATE_ESTOP_RESET)
+c.state(linuxcnc.STATE_ON)
+for joint in range(9):
+    c.home(joint)
+c.wait_complete()
+
+LETTERS = 'XYZABCUVW'
+
+
+def absdelta(a, b):
+    '''Maximum absolute difference between components of two coordinate points'''
+    return max(abs(a[i] - b[i]) for i in range(len(a)))
+
+
+def test_pose(pose):
+    '''Command LinuxCNC to go to 'pose', then check that the position that
+    comes back matches it.  Motion runs the inverse kinematics to turn the
+    pose into joint values and the forward kinematics to turn the joint
+    feedback back into a position, so a pose that comes back unchanged is one
+    the two directions agree on.
+    '''
+    words = ' '.join('%s%0.9f' % (letter, value)
+                     for letter, value in zip(LETTERS, pose))
+
+    c.mode(linuxcnc.MODE_MDI)
+    c.mdi('G0 ' + words)
+    c.wait_complete()
+
+    # The delay seems to be needed for trajectory to fully settle, otherwise
+    # there is about 1e-5 error between target and actual position.
+    time.sleep(0.05)
+    s.poll()
+    joints = s.joint_actual_position[:9]
+    position = s.actual_position[:9]
+
+    print("Commanded %s, joints %s, position %s" % (pose, joints, position))
+
+    # Accuracy limit is set to 1e-9 here.
+    # For practical purposes, a numerical accuracy of 1e-6 would be perfectly
+    # acceptable.  Current implementation using doubles achieves about 1e-14
+    # precision.
+    if absdelta(position, pose) > 1e-9:
+        raise RuntimeError(
+            "Forward and inverse kinematics disagree: commanded %s, joints %s, got %s"
+            % (pose, joints, position))
+
+
+#                X    Y    Z    A    B    C    U    V    W
+POSES = [
+    # The pose everything starts from.
+    (       0,   0,   0,   0,   0,   0,   0,   0,   0),
+    # Linear axes only, with the head upright.
+    (      10,  20,  30,   0,   0,   0,   0,   0,   0),
+    # B alone tilts the head, which moves the tip in X and Z.
+    (      10,  20,  30,   0,  40,   0,   0,   0,   0),
+    # C alone turns the table under the tip.
+    (      10,  20,  30,   0,   0,  30,   0,   0,   0),
+    # B and C together: the head correction is in the machine frame and the
+    # table rotation is not, so the order the two are applied in matters.
+    (      10,  20,  30,   0,  40,  30,   0,   0,   0),
+    # U alone shifts the head along its own axis, which with B tilted has a
+    # component in both X and Z.
+    (      10,  20,  30,   0,  40,   0,  15,   0,   0),
+    # V is a plain shift of the saddle.
+    (      10,  20,  30,   0,  40,   0,   0,   8,   0),
+    # W extends the pivot to tip distance.
+    (      10,  20,  30,   0,  40,   0,   0,   0,  12),
+    # Everything at once.
+    (      50,  50,  50,   5,  40,  30,  15,   8,  12),
+    # And with the signs the other way round.
+    (     -50, -50, -50,  -5, -40, -30, -15,  -8, -12),
+    # Back to the origin, where the pose is the same whichever direction the
+    # axes are taken to run in.
+    (       0,   0,   0,   0,   0,   0,   0,   0,   0),
+]
+
+for conventional in (0, 1):
+    hal.set_p('maxkins.conventional-directions', str(conventional))
+    print("conventional-directions %d" % conventional)
+    for pose in POSES:
+        test_pose(pose)
+
+sys.exit(0)
diff --git a/tests/maxkins/test.ini b/tests/maxkins/test.ini
new file mode 100644
index 00000000000..7cf23c05738
--- /dev/null
+++ b/tests/maxkins/test.ini
@@ -0,0 +1,210 @@
+[EMC]
+# The version string for this INI file.
+VERSION = 1.1
+
+DEBUG = 0
+
+[MAX]
+# distance from the end of the reference tool to the centre of the head
+# tilt axis, and the length of the tool on top of it
+B_PIVOT_LENGTH = 100
+TOOL_LENGTH = 25
+
+[DISPLAY]
+DISPLAY = ./test-ui.py
+
+[FILTER]
+#No Content
+
+[RS274NGC]
+PARAMETER_FILE = sim.var
+
+[EMCMOT]
+EMCMOT = motmod
+COMM_TIMEOUT = 4.0
+BASE_PERIOD = 0
+SERVO_PERIOD = 1000000
+
+[TASK]
+TASK = milltask
+CYCLE_TIME = 0.001
+
+[HAL]
+HALUI = halui
+HALFILE = core_sim.hal
+
+[HALUI]
+#No Content
+
+[TRAJ]
+COORDINATES =           X Y Z A B C U V W
+HOME =                  0 0 0 0 0 0 0 0 0
+LINEAR_UNITS =          mm
+ANGULAR_UNITS =         degree
+DEFAULT_LINEAR_VELOCITY =       10
+MAX_LINEAR_VELOCITY =          100
+MAX_LINEAR_ACCELERATION =     1000
+DEFAULT_ANGULAR_VELOCITY =      10
+MAX_ANGULAR_VELOCITY =         100
+MAX_ANGULAR_ACCELERATION =    1000
+
+[EMCIO]
+TOOL_TABLE = tool.tbl
+RANDOM_TOOLCHANGER = 0
+
+[KINS]
+KINEMATICS = maxkins
+JOINTS = 9
+
+[AXIS_X]
+MIN_LIMIT = -1000
+MAX_LIMIT = 1000
+MAX_VELOCITY = 100
+MAX_ACCELERATION = 1000
+
+[JOINT_0]
+TYPE =             LINEAR
+HOME =             0.000
+HOME_ABSOLUTE_ENCODER = 1
+MAX_VELOCITY =     100
+MAX_ACCELERATION = 1000
+BACKLASH =         0.000
+MIN_LIMIT =        -1000
+MAX_LIMIT =        1000
+FERROR =           0.001
+
+[AXIS_Y]
+MIN_LIMIT = -1000
+MAX_LIMIT = 1000
+MAX_VELOCITY = 100
+MAX_ACCELERATION = 1000
+
+[JOINT_1]
+TYPE =             LINEAR
+HOME =             0.000
+HOME_ABSOLUTE_ENCODER = 1
+MAX_VELOCITY =     100
+MAX_ACCELERATION = 1000
+BACKLASH =         0.000
+MIN_LIMIT =        -1000
+MAX_LIMIT =        1000
+FERROR =           0.001
+
+[AXIS_Z]
+MIN_LIMIT = -1000
+MAX_LIMIT = 1000
+MAX_VELOCITY = 100
+MAX_ACCELERATION = 1000
+
+[JOINT_2]
+TYPE =             LINEAR
+HOME =             0.000
+HOME_ABSOLUTE_ENCODER = 1
+MAX_VELOCITY =     100
+MAX_ACCELERATION = 1000
+BACKLASH =         0.000
+MIN_LIMIT =        -1000
+MAX_LIMIT =        1000
+FERROR =           0.001
+
+[AXIS_A]
+MIN_LIMIT = -360
+MAX_LIMIT = 360
+MAX_VELOCITY = 100
+MAX_ACCELERATION = 1000
+
+[JOINT_3]
+TYPE =             ANGULAR
+HOME =             0.000
+HOME_ABSOLUTE_ENCODER = 1
+MAX_VELOCITY =     100
+MAX_ACCELERATION = 1000
+BACKLASH =         0.000
+MIN_LIMIT =        -360
+MAX_LIMIT =        360
+FERROR =           0.001
+
+[AXIS_B]
+MIN_LIMIT = -360
+MAX_LIMIT = 360
+MAX_VELOCITY = 100
+MAX_ACCELERATION = 1000
+
+[JOINT_4]
+TYPE =             ANGULAR
+HOME =             0.000
+HOME_ABSOLUTE_ENCODER = 1
+MAX_VELOCITY =     100
+MAX_ACCELERATION = 1000
+BACKLASH =         0.000
+MIN_LIMIT =        -360
+MAX_LIMIT =        360
+FERROR =           0.001
+
+[AXIS_C]
+MIN_LIMIT = -360
+MAX_LIMIT = 360
+MAX_VELOCITY = 100
+MAX_ACCELERATION = 1000
+
+[JOINT_5]
+TYPE =             ANGULAR
+HOME =             0.000
+HOME_ABSOLUTE_ENCODER = 1
+MAX_VELOCITY =     100
+MAX_ACCELERATION = 1000
+BACKLASH =         0.000
+MIN_LIMIT =        -360
+MAX_LIMIT =        360
+FERROR =           0.001
+
+[AXIS_U]
+MIN_LIMIT = -1000
+MAX_LIMIT = 1000
+MAX_VELOCITY = 100
+MAX_ACCELERATION = 1000
+
+[JOINT_6]
+TYPE =             LINEAR
+HOME =             0.000
+HOME_ABSOLUTE_ENCODER = 1
+MAX_VELOCITY =     100
+MAX_ACCELERATION = 1000
+BACKLASH =         0.000
+MIN_LIMIT =        -1000
+MAX_LIMIT =        1000
+FERROR =           0.001
+
+[AXIS_V]
+MIN_LIMIT = -1000
+MAX_LIMIT = 1000
+MAX_VELOCITY = 100
+MAX_ACCELERATION = 1000
+
+[JOINT_7]
+TYPE =             LINEAR
+HOME =             0.000
+HOME_ABSOLUTE_ENCODER = 1
+MAX_VELOCITY =     100
+MAX_ACCELERATION = 1000
+BACKLASH =         0.000
+MIN_LIMIT =        -1000
+MAX_LIMIT =        1000
+FERROR =           0.001
+
+[AXIS_W]
+MIN_LIMIT = -1000
+MAX_LIMIT = 1000
+MAX_VELOCITY = 100
+MAX_ACCELERATION = 1000
+
+[JOINT_8]
+TYPE =             LINEAR
+HOME =             0.000
+HOME_ABSOLUTE_ENCODER = 1
+MAX_VELOCITY =     100
+MAX_ACCELERATION = 1000
+BACKLASH =         0.000
+MIN_LIMIT =        -1000
+MAX_LIMIT =        1000
+FERROR =           0.001
diff --git a/tests/maxkins/test.sh b/tests/maxkins/test.sh
new file mode 100755
index 00000000000..6edf34d77ca
--- /dev/null
+++ b/tests/maxkins/test.sh
@@ -0,0 +1,4 @@
+#!/bin/bash
+
+rm -f sim.var
+linuxcnc -r test.ini
diff --git a/tests/maxkins/tool.tbl b/tests/maxkins/tool.tbl
new file mode 100644
index 00000000000..e69de29bb2d

From 91dfff8f5ea8ef86c6603b6fc328add04d024bd9 Mon Sep 17 00:00:00 2001
From: Luca Toniolo <10792599+grandixximo@users.noreply.github.com>
Date: Tue, 15 Sep 2026 18:31:31 +1000
Subject: [PATCH 6/6] motion: a kinematics switch whose forward cannot solve
 the joints is refused

handle_kinematicsSwitch() switched the module, then ran the new forward kinematics on the joints and, when that failed, reported it and kept the position it knew. The module stayed in the new type, so from the next cycle the inverse ran the joints to wherever that kept position lands in the new kinematics: on the hexapod that is a jump of twenty units on every strut, with no move commanded.

The joints are where they are, and a kinematics whose forward cannot solve them is not one the machine can run in from there. Switch the module back before anything else sees the new type, report which type is still in force, and abort as before. Nothing moves and a later switch is taken on its own merits.

tests/kins-switch-unsolved loads the hexapod with identity first, homes at zero, asks for the hexapod and checks that the type, the joints and the position are all as they were, that the failure was reported, that a move afterwards follows the identity inverse and that a switch to the type in force is accepted.
---
 src/emc/motion/control.c               |  20 ++--
 tests/kins-switch-unsolved/README      |   9 ++
 tests/kins-switch-unsolved/checkresult |   3 +
 tests/kins-switch-unsolved/sim.hal     |  16 +++
 tests/kins-switch-unsolved/test-ui.py  | 112 +++++++++++++++++++++
 tests/kins-switch-unsolved/test.ini    | 130 +++++++++++++++++++++++++
 tests/kins-switch-unsolved/test.sh     |   4 +
 tests/kins-switch-unsolved/tool.tbl    |   1 +
 8 files changed, 288 insertions(+), 7 deletions(-)
 create mode 100644 tests/kins-switch-unsolved/README
 create mode 100755 tests/kins-switch-unsolved/checkresult
 create mode 100644 tests/kins-switch-unsolved/sim.hal
 create mode 100755 tests/kins-switch-unsolved/test-ui.py
 create mode 100644 tests/kins-switch-unsolved/test.ini
 create mode 100755 tests/kins-switch-unsolved/test.sh
 create mode 100644 tests/kins-switch-unsolved/tool.tbl

diff --git a/src/emc/motion/control.c b/src/emc/motion/control.c
index 50ddadc6c07..b509a133d3e 100644
--- a/src/emc/motion/control.c
+++ b/src/emc/motion/control.c
@@ -363,10 +363,6 @@ static void handle_kinematicsSwitch(void) {
         return; // the kinematics in force is unchanged
     }
 
-    switchkins_type = requested_type;
-    hal_set_real(emcmot_hal_data->kins_type, (double)switchkins_type);
-    emcmotStatus->switchkins_type = switchkins_type;
-
     KINEMATICS_FORWARD_FLAGS tmpFFlags = fflags;
     KINEMATICS_INVERSE_FLAGS tmpIFlags = iflags;
 #ifdef SWITCHKINS_DEBUG
@@ -376,15 +372,25 @@ static void handle_kinematicsSwitch(void) {
         beforePose[anum] = *pcmd_p[anum];
     }
 #endif
+    /* the joints stay where they are, so a kinematics whose forward cannot
+       solve them is one the machine cannot run in from here: put the old
+       one back, or the inverse would run the joints to wherever the pose
+       we know lands in the new one */
     EmcPose poseKinsSwitch = emcmotStatus->carte_pos_cmd;
     if (kinematicsForward(joint_posKinsSwitch, &poseKinsSwitch,
                           &tmpFFlags, &tmpIFlags)) {
-        reportError(_("kinematicsForward failed for kinematics type %d"),
-                    switchkins_type);
+        kinematicsSwitch(switchkins_type);
+        reportError(_("kinematicsForward failed for kinematics type %d,"
+                      " type %d is still in force"),
+                    requested_type, switchkins_type);
         SET_MOTION_ERROR_FLAG(1);  // abort
-        return; // keep the position we know rather than an unsolved one
+        return; // the kinematics in force and the position are unchanged
     }
     emcmotStatus->carte_pos_cmd = poseKinsSwitch;
+
+    switchkins_type = requested_type;
+    hal_set_real(emcmot_hal_data->kins_type, (double)switchkins_type);
+    emcmotStatus->switchkins_type = switchkins_type;
 #ifdef SWITCHKINS_DEBUG
     fprintf(stderr,"kswitch type=%d (%s:%d)\n",switchkins_type,__FUNCTION__,__LINE__);
     for (anum = 0; anum < EMCMOT_MAX_AXIS; anum++) {
diff --git a/tests/kins-switch-unsolved/README b/tests/kins-switch-unsolved/README
new file mode 100644
index 00000000000..c3f263e4f7f
--- /dev/null
+++ b/tests/kins-switch-unsolved/README
@@ -0,0 +1,9 @@
+A kinematics switch whose forward kinematics cannot solve the current
+joint position must leave the type in force, the joints and the position
+alone, report the failure and abort; motion goes on working in the
+kinematics it kept.
+
+The hexapod module starts in identity kinematics here, so the joints are
+put at a few units each as coordinates.  Six struts that short are not a
+platform position the hexapod forward can converge to, so G12.1 P1 asks
+for exactly that switch.
diff --git a/tests/kins-switch-unsolved/checkresult b/tests/kins-switch-unsolved/checkresult
new file mode 100755
index 00000000000..9d48d3f180e
--- /dev/null
+++ b/tests/kins-switch-unsolved/checkresult
@@ -0,0 +1,3 @@
+#!/bin/sh
+# the test script counts its own failures
+grep -q "^Exiting with 0 errors" "$1"
diff --git a/tests/kins-switch-unsolved/sim.hal b/tests/kins-switch-unsolved/sim.hal
new file mode 100644
index 00000000000..e92c60eb526
--- /dev/null
+++ b/tests/kins-switch-unsolved/sim.hal
@@ -0,0 +1,16 @@
+loadrt [KINS]KINEMATICS
+loadrt [EMCMOT]EMCMOT servo_period_nsec=[EMCMOT]SERVO_PERIOD num_joints=[KINS]JOINTS
+
+addf motion-command-handler servo-thread
+addf motion-controller servo-thread
+
+net J0 joint.0.motor-pos-cmd => joint.0.motor-pos-fb
+net J1 joint.1.motor-pos-cmd => joint.1.motor-pos-fb
+net J2 joint.2.motor-pos-cmd => joint.2.motor-pos-fb
+net J3 joint.3.motor-pos-cmd => joint.3.motor-pos-fb
+net J4 joint.4.motor-pos-cmd => joint.4.motor-pos-fb
+net J5 joint.5.motor-pos-cmd => joint.5.motor-pos-fb
+
+net estop-loop iocontrol.0.user-enable-out iocontrol.0.emc-enable-in
+net tool-prep-loop iocontrol.0.tool-prepare iocontrol.0.tool-prepared
+net tool-change-loop iocontrol.0.tool-change iocontrol.0.tool-changed
diff --git a/tests/kins-switch-unsolved/test-ui.py b/tests/kins-switch-unsolved/test-ui.py
new file mode 100755
index 00000000000..1995bcd1e23
--- /dev/null
+++ b/tests/kins-switch-unsolved/test-ui.py
@@ -0,0 +1,112 @@
+#!/usr/bin/env python3
+# A kinematics switch the new forward cannot solve.  Motion must keep the
+# position it knows, say so, abort, and take a switch back afterwards.
+import hal
+import linuxcnc
+import sys
+import time
+
+c = linuxcnc.command()
+s = linuxcnc.stat()
+e = linuxcnc.error_channel()
+
+errors = 0
+
+def error(what):
+    global errors
+    errors += 1
+    print("*** ERROR %s" % what)
+
+def kins_type():
+    return int(hal.get_value("motion.kins-type"))
+
+def position():
+    s.poll()
+    return tuple(round(v, 6) for v in s.position[:6])
+
+def drain():
+    said = []
+    while True:
+        m = e.poll()
+        if not m:
+            return said
+        said.append(m[1])
+
+def wait_idle():
+    deadline = time.time() + 30
+    while time.time() < deadline:
+        s.poll()
+        if s.interp_state == linuxcnc.INTERP_IDLE and not s.queue:
+            return
+        time.sleep(0.05)
+    error("timed out waiting for the interpreter")
+
+def mdi(cmd):
+    c.mdi(cmd)
+    c.wait_complete(30)
+    wait_idle()
+    time.sleep(0.2)   # let the error channel and the status catch up
+
+c.state(linuxcnc.STATE_ESTOP_RESET)
+c.state(linuxcnc.STATE_ON)
+c.wait_complete(30)
+c.home(-1)
+c.wait_complete(60)
+c.mode(linuxcnc.MODE_MDI)
+c.wait_complete(30)
+drain()
+
+if kins_type() != 0:
+    error("starts in kinematics type %d, expected identity (0)" % kins_type())
+
+def joints():
+    s.poll()
+    return tuple(round(v, 6) for v in s.joint_position[:6])
+
+# a known position, in identity: joints and coordinates are the same thing
+mdi("G0 X1 Y2 Z3 A4 B5 C6")
+before = position()
+if before != (1, 2, 3, 4, 5, 6):
+    error("position before the switch is %s, expected (1, 2, 3, 4, 5, 6)" % (before,))
+drain()
+
+# the switch the hexapod forward cannot solve: six struts of a few units
+# are no platform position.  Nothing may change but the report.
+mdi("G12.1 P1")
+said = drain()
+if not any("kinematicsForward failed" in m for m in said):
+    error("no report of the failed forward kinematics, got %s" % said)
+if kins_type() != 0:
+    error("kinematics type %d after the failed switch, expected 0 still" % kins_type())
+if position() != before:
+    error("position after the failed switch is %s, expected %s unchanged" % (position(), before))
+if joints() != before:
+    error("joints after the failed switch are %s, expected %s unchanged" % (joints(), before))
+s.poll()
+if s.task_state != linuxcnc.STATE_ON:
+    error("task state %d after the failed switch, expected still ON" % s.task_state)
+
+# motion goes on working in the kinematics it kept, and the module is
+# back in it too: the joints follow the identity inverse, not the hexapod's
+c.mode(linuxcnc.MODE_MDI)
+c.wait_complete(30)
+mdi("G0 X30 Y30 Z30 A30 B30 C30")
+if joints() != (30, 30, 30, 30, 30, 30):
+    error("joints after the move are %s, expected all 30" % (joints(),))
+said = drain()
+if said:
+    error("unexpected messages after the failed switch: %s" % said)
+
+# a switch to the kinematics in force is nothing to do
+mdi("G13.1")
+if kins_type() != 0:
+    error("kinematics type %d after G13.1, expected identity (0)" % kins_type())
+if position() != (30, 30, 30, 30, 30, 30):
+    error("position after G13.1 is %s, expected all 30" % (position(),))
+said = drain()
+if said:
+    error("unexpected messages after the switch back: %s" % said)
+
+print("Exiting with %d errors" % errors)
+c.state(linuxcnc.STATE_ESTOP)
+sys.exit(1 if errors else 0)
diff --git a/tests/kins-switch-unsolved/test.ini b/tests/kins-switch-unsolved/test.ini
new file mode 100644
index 00000000000..c93bc09563f
--- /dev/null
+++ b/tests/kins-switch-unsolved/test.ini
@@ -0,0 +1,130 @@
+[EMC]
+VERSION = 1.1
+DEBUG = 0
+
+[DISPLAY]
+DISPLAY = ./test-ui.py
+
+[RS274NGC]
+RS274NGC_STARTUP_CODE = G17 G21 G40 G49 G54 G64 P0.001 G80 G90 G92.1 G94 G97 G98
+PARAMETER_FILE = sim.var
+
+[KINS]
+# switchkins-type 0 is identity, 1 is the hexapod, 2 is the userk template
+KINEMATICS = genhexkins sparm=identityfirst
+JOINTS = 6
+
+[HAL]
+HALFILE = sim.hal
+
+[TRAJ]
+COORDINATES = XYZABC
+LINEAR_UNITS = mm
+ANGULAR_UNITS = deg
+DEFAULT_LINEAR_VELOCITY = 200
+MAX_LINEAR_VELOCITY = 346
+MAX_LINEAR_ACCELERATION = 800
+DEFAULT_LINEAR_ACCELERATION = 800
+MAX_ANGULAR_VELOCITY = 360
+
+[EMCMOT]
+EMCMOT = motmod
+SERVO_PERIOD = 1000000
+COMM_TIMEOUT = 4
+
+[TASK]
+TASK = milltask
+CYCLE_TIME = 0.010
+
+[EMCIO]
+TOOL_TABLE = tool.tbl
+
+[AXIS_X]
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 200
+MAX_ACCELERATION = 800
+
+[AXIS_Y]
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 200
+MAX_ACCELERATION = 800
+
+[AXIS_Z]
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 200
+MAX_ACCELERATION = 800
+
+[AXIS_A]
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 60
+MAX_ACCELERATION = 200
+
+[AXIS_B]
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 60
+MAX_ACCELERATION = 200
+
+[AXIS_C]
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 60
+MAX_ACCELERATION = 200
+
+[JOINT_0]
+TYPE = LINEAR
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 200
+MAX_ACCELERATION = 800
+HOME_SEARCH_VEL = 0
+HOME_SEQUENCE = 0
+
+[JOINT_1]
+TYPE = LINEAR
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 200
+MAX_ACCELERATION = 800
+HOME_SEARCH_VEL = 0
+HOME_SEQUENCE = 0
+
+[JOINT_2]
+TYPE = LINEAR
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 200
+MAX_ACCELERATION = 800
+HOME_SEARCH_VEL = 0
+HOME_SEQUENCE = 0
+
+[JOINT_3]
+TYPE = ANGULAR
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 60
+MAX_ACCELERATION = 200
+HOME_SEARCH_VEL = 0
+HOME_SEQUENCE = 0
+
+[JOINT_4]
+TYPE = ANGULAR
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 60
+MAX_ACCELERATION = 200
+HOME_SEARCH_VEL = 0
+HOME_SEQUENCE = 0
+
+[JOINT_5]
+TYPE = ANGULAR
+MIN_LIMIT = -500
+MAX_LIMIT = 500
+MAX_VELOCITY = 60
+MAX_ACCELERATION = 200
+HOME_SEARCH_VEL = 0
+HOME_SEQUENCE = 0
diff --git a/tests/kins-switch-unsolved/test.sh b/tests/kins-switch-unsolved/test.sh
new file mode 100755
index 00000000000..765cf14fed6
--- /dev/null
+++ b/tests/kins-switch-unsolved/test.sh
@@ -0,0 +1,4 @@
+#!/bin/bash -e
+# a failed run leaves the var file behind, and it carries offsets
+rm -f sim.var sim.var.bak
+linuxcnc -r test.ini
diff --git a/tests/kins-switch-unsolved/tool.tbl b/tests/kins-switch-unsolved/tool.tbl
new file mode 100644
index 00000000000..d793e2d60ed
--- /dev/null
+++ b/tests/kins-switch-unsolved/tool.tbl
@@ -0,0 +1 @@
+T1 P1 D0.0 Z12.5 ;