Fix for #2410

See the issue on github for more details, but the cause seems to be a naive
attempt to handle zero-length moves at zero feed rate without considering
very short moves at a suitably small feed rate.


Signed-off-by: andypugh <andy@bodgesoc.org>

src/emc/rs274ngc/interp_convert.cc

@@ -4750,11 +4750,10 @@ The approach to operating in incremental distance mode (g91) is to
 put the the absolute position values into the block before using the
 block to generate a move.
 
-In inverse time feed mode, a lower bound of 0.1 is placed on the feed
-rate so that the feed rate is never set to zero. If the destination
-point is the same as the current point, the feed rate would be
-calculated as zero otherwise.
-
+If the destination point is the same as the current point, the feed rate
+will be calculated as zero, so a default of 0.1 is applied in this case
+(It doesn't matter how wrong the feed rate is on a zero-length move)
+ 
 If cutter compensation is in use, the path's length may increase or
 decrease.  Also an arc may be added, to go around a corner, before the
 straight move.  For the purpose of calculating the feed rate when in

src/emc/rs274ngc/interp_inverse.cc

@@ -62,10 +62,13 @@ int Interp::inverse_time_rate_arc(double x1,     //!< x coord of start point of
   if (settings->feed_mode != INVERSE_TIME) return -1;
 
   length = find_arc_length(x1, y1, z1, cx, cy, turn, x2, y2, z2);
-  rate = std::max(0.1, (length * block->f_number));
+  if (length == 0){
+    rate = 0.1; // See https://github.com/LinuxCNC/linuxcnc/issues/2410
+  } else {
+    rate = length * block->f_number;
+  }
   enqueue_SET_FEED_RATE(rate);
   settings->feed_rate = rate;
-
   return INTERP_OK;
 }
 
@@ -120,10 +123,14 @@ int Interp::inverse_time_rate_straight(double end_x,     //!< x coordinate of en
                                 cx, cy, cz,
                                 settings->AA_current, settings->BB_current, settings->CC_current,
                                 settings->u_current, settings->v_current, settings->w_current);
-
-  rate = std::max(0.1, (length * block->f_number));
+  if (length == 0){
+    rate = 0.1; // See https://github.com/LinuxCNC/linuxcnc/issues/2410
+  } else {
+    rate = length * block->f_number;
+  }
+
   enqueue_SET_FEED_RATE(rate);
   settings->feed_rate = rate;
-
+  printf("Length = %f, rate = %f\n", length, rate);
   return INTERP_OK;
 }

src/hal/drivers/mesa-hostmot2/modbus/mesa_modbus.c

@@ -472,7 +472,7 @@ int send_modbus_pkt(hm2_modbus_inst_t *inst){
     rtapi_u16 checksum;
     rtapi_u16 fsizes[1];
     rtapi_u8  frames;
-    int i; 
+    int i;
 
     checksum = RTU_CRC(ch->data, ch->ptr + 1);
     ch->data[++(ch->ptr)] = checksum & 0xFF;