Re: Tips for doing conditional logic in EPICS records

[Mark Davis via Tech-talk <tech-talk@aps.anl.gov>]

    On 4/27/2020 12:16 PM, Ralph Lange via Tech-talk wrote:

On Mon, 27 Apr 2020 at 18:04, Mooney, Tim M. via Tech-talk <tech-talk@aps.anl.gov> wrote:

One way to do conditional execution in an EPICS database is by using the SELM and SELN fields of a seq record.  Set SELM to "Specified", and write a number to the SELN field. If you want more than one of several things to happen, you can write "Mask" to SELM, and write a bitmap value to SELN, specifying which things to do.

+1

The combination of a calcout record (evaluating conditions) writing to a seq record in "Mask" mode (executing actions) is extremely powerful. As actions can include delays and changing the calcout's fields, that setup is good enough for a small state machine. (You will also win any district level obfuscation contest.)

Cheers,
~Ralph

    The above comments by Tim and Ralph were the inspiration for the
    solution I decided on:
    
    NOTE:  Some of the PP/NPP/CP/MS/NMS/etc values may need tweaking
    yet...)

record(ai, "I_RSET")  {
  field(DESC, "current set readback")
  field(PREC, "3")
  field(ADEL, "0.01")
}

record(bi, "_updateRSET")  {
  field(VAL,  "1")
  field(PINI, "YES")
}

record(scalcout, "_psInReset")  {
  field(DESC, "Is PS in reset state?")
  field(INAA, "CMD_STAT CP MSS")
  field(CALC, "AA = 'R'")
}


# Compute value for seq record bit-mask to specify which actions to perform
record(transform, "_RSET_UpdateLogic")
{
  field(INPA, "_psInReset CP MSS")
  field(INPB, "_updateRSET CP MSS")
  field(INPC, "ON_RSTS CP MSS")

  # if (psInReset == 1)  { I_RSET = 0;  updateRSET = 0; }  (actions 1 and 2: Mask=0x03)
  field(CLCJ, "(A=1) ? 3 : 0")

  # else  if (_ON_RSTS_ == 1)  { I_RSET = I_CSET;  updateRSET = 0; }  (actions 3 and 2: Mask=0x06)
  field(CLCK, "(J!=0) ? 0 : (C=1) ? 6 : 0")

  # else  if (updateRSET)  I_RSET = 0;  (action 1: Mask=0x01)
  field(CLCL, "(K!=0) ? 0 : (B=1) ? 1 : 0")

  # Write the resulting bitmask and trigger the desired actions
  field(CLCM, "J | K | L")
  field(OUTM, "_RSET_UpdateActions.SELN")
  field(FLNK, "_RSET_UpdateActions")
}


# The list of actions used by the RSET_UpdateLogic record which determines
# what (if any) change needs to be made to the RSET value.  Which actions to
# performed is specified by writing a bitmapped value to the SELN field (1 bit
# for each action).

record(seq, "_RSET_UpdateActions")  {
  field(SELM, "Mask")

  # Mask bit 0x01:  Set I_RSET = 0
  field(DOL1, "0")
  field(LNK1, "I_RSET PP")

  # Mask bit 0x02:  Set updateRSET = 0
  field(DOL2, "0")
  field(LNK2, "_updateRSET PP")

  # Mask bit 0x04:  Set I_RSET = I_CSET
  field(DOL3, "I_CSET NPP")
  field(LNK3, "I_RSET PP")
}

    The above approach is modeled on the more procedural approach
    typically used in high level languages (given a set of conditions,
    perform all the necessary actions):
    
    if (expression1) 
       do a bunch of stuff ...
    else
       if (expression2)
         do a different bunch of stuff ...
      else
        do yet a different bunch of stuff ...
    
      
    However, a more data-driven approach (compute the next state
    for each variable) could produce the same effect with just a
    transform record:
    
       A = (set of nested if/else conditions and expressions)
       B = (possibly overlapping set of nested if/else
      conditions and expressions)
    
    Having (very successfully) used the later approach heavily in most
    the embedded controller code I wrote for years, this approach
    definitely has some advantages.  However, given the vast difference
    in the tools used to express it in this case  (EPICS records rather
    than C code), I think the more verbose approach is easier to
    comprehend, debug, and maintain.  But your mileage may vary.
    
    
    Mark

Mark Davis NSCL/FRIB Control Systems Software Engineer davism50@msu.edu

On 4/27/2020 12:16 PM, Ralph Lange via Tech-talk wrote:

On Mon, 27 Apr 2020 at 18:04, Mooney, Tim M. via Tech-talk <tech-talk@aps.anl.gov> wrote:

One way to do conditional execution in an EPICS database is by using the SELM and SELN fields of a seq record.  Set SELM to "Specified", and write a number to the SELN field. If you want more than one of several things to happen, you can write "Mask" to SELM, and write a bitmap value to SELN, specifying which things to do.

+1

The combination of a calcout record (evaluating conditions) writing to a seq record in "Mask" mode (executing actions) is extremely powerful. As actions can include delays and changing the calcout's fields, that setup is good enough for a small state machine. (You will also win any district level obfuscation contest.)

Cheers,
~Ralph