pic
 talks pic protocol to safety

safety
 rtprio
 passes on all pic traffic
 safety incl polarity, no collisions, curvature, etc.
 converts instructions to/from text
 takes instructions only from multiplex
all real-time stuff so clients can be confident of no misthings ?
 on-event for point changing, stopping, etc. ?
 specify intended path and speed profile for each train ?
  - list of points, in order, with setting
      this is good enough to be in time for shinkansen on inner loop
      inner loop is			 3589mm
      minus longest section for slop	 -657mm (X10)
      minus length of the train		-1118mm
					-------
      spare				 1813mm
	or at 1m/s			 1813ms
      minus safety delay		 -500ms
					-------
					 1313ms

  optimistic path change
   - optimistic change request recorded on some point
   - when we find it we check
	  is point free
	  is another set request recorded
		  if so check whether we'll be out of the way
	  plot new path for this train

 deadline scheduler for point change and cdu

 for points autochanging
  lay train an extra 600ms or so
 

commsn between realtime and multiplexer
  each command produces `ack' or `error'
  multiplexer counts responses for flow control

multiplex
 talks to all clients
 access control
 message mux/demux incl filtering