SFELPHOTON-1650:shapepath.py: stop-and-go struggeled with sync for first 3 measurements

python/shapepath.py

@@ -960,6 +960,9 @@ class ShapePath(MotionBase):
         twait_=round((tmove+twait)/fel_per)*fel_per-tmove
         if twait!=twait_:
           _log.warning(f'adjust twait({twait}) to {twait_} to match multiple of fel-cycles')
+        # syncPlc:
+        # Sys.Udata[0] 0->1 'fast counter: counter up ech pluse from 0 to pt2tp_felpulse
+        # Sys.Udata[0] 0->1 'slow counter: increments by one after 'pt2tp_felpulse' pulses
         syncPlc=f'''\
 close all buffers
 disable plc 2
@@ -981,6 +984,15 @@ enable plc 2
         if comm: comm.gpascii.send_block(syncPlc, verb&0x08)
         prg+=f'''\
   //mode 5: grid pvt motion
+  //Sys.Udata[0] fast counter
+  //Sys.Udata[1] slow counter
+  //Sys.Udata[2] last slow counter value
+  while(Coord[1].DesTimeBase==0){{}}
+  Sys.Udata[0]=0 // resets fast counter
+  Sys.Udata[1]=0 // resets slow counter
+  Sys.Udata[2]=0
+  while(Sys.Udata[2]==Sys.Udata[1]){{}} //initial wait
+  Sys.Udata[2]=Sys.Udata[1] //save last slow counter value
   pvt{tmove} abs
   L1=0 //slow loop x
   L0=0 //fast loop y