PC controller interfacing notes 
===============================


 RS232 at 19200 bits per second, no parity 1 stop bit and RTS/CTS control
 
 The packet format is
 
 <preamble=0x3b>
 <packet len>
 <source device #>
 <destination device #>
 <message ID>
 <message data bytes>
 <checksum byte>
 
 
 A sample packet to query the firmware version  
 
 <0x3b> preamble
 <0x03> packet length after source device ID is 3 message bytes
 <0x0d> source ID used for an external connection
 <0x10> destination device motor controller
 <0xfe> request firmware version
 <0xeb> checksum
 
 A sample packet to change the C20 focus is
 
 <0x3b> preamble
 <0x04> packet length
 <0x0d> source ID for external connection  
 <0x12> destination to focusser controller as device 18 (0x12)
 <0x24> command to move motor in positive direction at rate specified by data
 <0x05> motor rate (0 to 9, 0 to stop)
 <csum>
 
 
  
 The bytes are
 
 <destId> = destination for packet: 
   0x01 main board
   0x03 dummy source number 
   0x10 azm controller
   0x11 alt controller

 <msgId> = message 
   0x01 = get position with 24 bit response
     <data1> degrees
     <data2> minutes
     <data3> seconds
   0x02 = goto fast, accepts 24 bit signed data, rate = 9, acknowledges
   0x04 = set position accepts 24 bit signed data as above, acknowledges
   0x06 = set positive guide rate, accepts rate as 24 bit data or 16 bit data
     0xffff sidereal
     0xfffe solar
     0xfffd lunar
   0x07 = set negative guide rate as above
   0x0b = start leveling process in GPS systems, acknowledges
   0x0c = start recording PEC data and acknowledge
   0x0d = start or stop PEC playback, accepts 8 bit data, acknowledges 
     0x00 stop playback
     0x01 start playback
   0x10 = set positive backlash, accepts 8 bit data in range 0 to 99
   0x11 = set negative backlash
   0x12 = level done, returns 8 bits
     0x00 not done
     0xff done
   0x13 = slew done, returns 8 bits 
     0x00 not done
     0xff done
   0x15 = PEC record done, returns 8 bits
     0x00 not done
     0xff done
   0x16 = PEC record stop no data and no acknowledgement
   0x17 = goto slow, accepts 24 bit signed data, acknowledges
   0x18 = query if motor is at index, used by PEC, 8 bit response
     0x00 not at index
     0xff at index 
   0x19 = seek index, no data or acknowledgement  
   0x24 = move positive, 8 bit data rate rate 0 to 9
   0x25 = move negative, 8 bit data rate rate 0 to 9
   0xfc = get approach, returns 8 bit data 
     0x00 positive
     0x01 negative
   0xfd = set approach, accepts 8 bit data as above, returns acknowledgement



 <data#> = message data set to zero if not used

 <responseBytes> = number of bytes to echo as a response
   Recommended handling is to wait for a response but ignore the echo 



 The command format for a connection through the PC interface is    
 
   where

   0x50 is a code requesting that the data pass through to the motors     
   msgLen is how many bytes including msgId  and valid data bytes     
   destId is the AUX command set destination number                    
   msgId  is the AUX command set message number                        
   data1-3 are up to 3 bytes of message data                           
   responseBytes is the number of bytes to echo back in response       

 For example, the command to get data from the focus motor is          

   focusCmd[] = { 0x50, 0x01, 0x12, 0x01, 0x00, 0x00, 0x00, 0x03 }     

 These pass-through commands are used in                               

   GetFocus   (CDK20 only) 
   Focus      (CDK20 only) 
   StartTrack 
   StopTrack  
   StartSlew  
   StopSlew