Notes on implementing the Dual DAC output per TCS Axis on the PMAC

1. Each telescope axis has 2 motor requiring 2 pmac DAC per axis. We will 
   assign the pmac DAC outputs as so:

       HA:  DAC1,DAC3    ; default for motor 1 and 3
      DEC:  DAC2,DAC4    ; default for motor 2 and 4

   The general scheme is to have a PLC program run every servo cycle
   with a simple algorithm:

      dac0 = PID output from PMAC motor
      dac1 = motor 1 for axis
      dac2 = motor 2 for axis
      base1 = backlash value for dac1.
      base2 = backlash value for dac2.

      ; HA : dac0 neg for west; pos for east  
      ;      dac1 is west (negative)
      ;      dac2 is east (positive)
      if( dac0 < 0 )
         dac1 = -base1 + dac0;     ; go west
         dac2 =  base2;            ; backlash
      else
         dac1 = -base1;            ; backlash
         dac2 =  base2 + dac0;     ; go east

      ; DEC: dac0 neg for north; pos for south  
      ;      dac1 is north (negative)
      ;      dac2 is south (positive)
      if( dac0 < 0 )
         dac1 = -base1 + dac0;     ; go north
         dac2 =  base2;            ; backlash
      else
         dac1 = -base1;            ; backlash
         dac2 =  base2 + dac0;     ; go south

   Based on some engineering data, the following is determined:

      The west  motor DAC range is 0-negative, base is about -0.31
      The east  motor DAC range is 0-positive, base is about  0.35

      The north motor DAC range is 0-negative, base is about -0.30
      The south motor DAC range is 0-positive, base is about  0.43

      A value of 0.30 for the base seems reasonable.

      We use separate base values (base1&2) to allow differenct base values for each 
      motor, and to include a bias value to handle any calibration. Each PMAC card
      needs it own values (for example if you want 0.30 volts on each value).
      The D/A conversion can vary by 0.05 volt on each DAC.

2. PMAC implementation notes:

   The dual_dac program will run as PLC0. 
   Set I05 to 3 to enable foreground & background PLC.
   Set I08 to 0  to run PLC0 every servo cycle.
   DPR Variable Open buffer will provide the storage space need for 
     the program. See mem_map.txt for details.

   To cal base value: Base_volts * 2^16/20v.
   For example,
       0.3v :  0.3v * 2^16/20 v = 983
       1.0v :  1.0v * 2^16/20 v = 3276.8

   PMAC pdac value scaled by << 8, ie: +/10 V = +-32767 * 256.
   Plc0  shift the cmd by 8 (cmd/256), so internal it is working
     with 16-bit data (range -32768 to 32767). When writing the result
     to the DAC memory locate the results (adc1,adc2) are * 256 to
     scale back to pmac's format.
   The base value can be set by changing the mvariables:
      M8071=983                ; m1_base1
      M8072=983                ; m1_base2
      M8081=983                ; m2_base1 
      M8082=983                ; m2_base2 
   The software performs the above step at startup, see pm_initialize().
   The change the default values.

   Ixx02 pmac defaults are:

      ixx02 default  DAC    Use in dual DAC code
      ----  ------   ---    -------------------------------
      I102  $078003  DAC1   Motor1 DAC reassigned to WEST (neg)
      I202  $078002  DAC2   Motor2 DAC reassigned to NORTH (neg)
      I302  $07800B  DAC3   Motor3 DAC reassigned to EAST (pos)
      I402  $07800A  DAC4   Motor4 DAC reassegned to SOUTH (pos)

   Motor Ixx02 Command Output Address will be modified to reference mx_adc0 in DPR.
      I102 = $0f80    ; default is DAC1, address $78003
      I202 = $1000    ; default is DAC2, address $78002

   plc0 will
         read motor command values, m1 at y:$0f00, m2 at y:$0f01
         write adjusted DAC0 values to DAC1&2.            
         write values to DPR for feedback

2. Switching back to single output mode, and other plc notes

   2.1 Dual -> single

      disable plc 0         ; disable plc 0 
      I102 = $78003 
      I202 = $78002

   2.1 Single -> dual

      I102 = $0f80    ; memory location for plc0. default is $78003
      I202 = $1000    ; memory location for plc0. plc0. default is $78002
      enable plc 0    ; enable plc 0