Experimental Physics and
Industrial Control System

_pos is an absolute drive field.  We write to that from CA clients.  It writes to the _rel record with PP.

_rel is a transform record that computes the relative move amount.  It forward links to the _lim field, and sends the computed relative move to the _trunc record, which eventually sends it to the device.

_lim is a limit checking record.  It compares _pos to software limit values and sets high and low limit flags if they are exceeded.

grecord(ao,"$(P)$(S):$(D):pM$(M)_pos") {
    field(DESC,"PicoMotor Position in EGUs")
    field(DTYP,"Soft Channel")
    field(SDIS,"$(P)$(S):$(D):pM$(M)_movn.VAL  NPP NMS")
    field(FLNK,"0")
    field(OUT,"$(P)$(S):$(D):pM$(M)_rel.A  PP MS")
    field(PREC,"3")
}

grecord(transform,"$(P)$(S):$(D):pM$(M)_rel") {
    field(DESC,"Convert EGUs to rel steps")
    field(FLNK,"$(P)$(S):$(D):pM$(M)_lim.PROC")
    field(CLCD,"(A>B)?(A-B):(A<B)?(-1*(B-A)):0")
    field(CLCE,"NINT((1/C)*D)")
    field(CLCF,"A")
    field(CLCH,"G=0?E:G=1?((-1)*E):E")
    field(INPA,"0")
    field(INPB,"$(P)$(S):$(D):pM$(M)_prev.VAL  NPP MS")
    field(INPC,"$(P)$(S):$(D):pM$(M)_res.VAL  NPP MS")
    field(INPG,"$(P)$(S):$(D):pM$(M)_dir.VAL  NPP MS")
    field(OUTC,"0")
    field(OUTD,"0")
    field(OUTE,"0")
    field(OUTF,"$(P)$(S):$(D):pM$(M)_prev.VAL  PP MS")
    field(OUTG,"0")
    field(OUTH,"$(P)$(S):$(D):pM$(M)_trunc.A  PP MS")
    field(PREC,"6")
    field(CMTC,"user input EGUs/step")
    field(CMTD,"rel change in EGUs")
    field(CMTE,"rel change in steps")
}

grecord(transform,"$(P)$(S):$(D):pM$(M)_lim") {
    field(CLCD,"A>=C?1:0")
    field(CLCE,"A<=B?1:0")
    field(INPA,"$(P)$(S):$(D):pM$(M)_pos.VAL  PP MS")
    field(INPB,"$(P)$(S):$(D):pM$(M)_pos.DRVL PP MS")
    field(INPC,"$(P)$(S):$(D):pM$(M)_pos.DRVH  PP MS")
    field(OUTD,"$(P)$(S):$(D):pM$(M)_hi.VAL  PP MS")
    field(OUTE,"$(P)$(S):$(D):pM$(M)_lo.VAL  PP MS")
}

Note that the _lim record appears to contain a serious logic error.  The INPA, INPB, and INPC links to the _pos record are PP.  This is almost certainly not what was intended.  They should probably be CP so the record processes whenever those POS fields change.

However, with this logic error the database has worked fine, including with 7.0.1.

However, with 7.0.2 it no longer works.  What happens now is that the _pos and _rel records process twice in rapid succession.  _pos -> _rel -> _lim -> _pos ->_rel

The second processing of _rel computes a relative distance of 0 counts, because _prev now equals _pos.  This calculation happens before the previous relative move command is sent to the controller, with the net result being that every move command sent to the controller is now for 0 counts and it never moves.

I am able to fix the problem by changing the PP links in _lim to CP links, as I think was intended.

My question is why did it ever work, and what has changed in 7.0.2 that caused it to stop working?

record(ao,"_pos") {
    field(OUT,"_rel.A PP")
    field(TPRO, 1)
}

record(calc,"_rel") {
    field(FLNK,"_lim")
}

record(calc,"_lim") {
    field(INPA,"_pos PP")
    field(INPB,"_pos.DRVL PP")
    field(INPC,"_pos.DRVH PP")
}

ca:anj@tux: dbProcess of '_pos'
ca:anj@tux: dbProcess of '_rel'
ca:anj@tux: dbProcess of '_lim'
ca:anj@tux: dbProcess of Active '_pos' with RPRO=1
ca:anj@tux: dbProcess of Active '_pos' with RPRO=1
ca:anj@tux: dbProcess of Active '_pos' with RPRO=1
scanOnce: dbProcess of '_pos'
scanOnce: dbProcess of '_rel'
scanOnce: dbProcess of '_lim'
scanOnce: dbProcess of Active '_pos' with RPRO=0
scanOnce: dbProcess of Active '_pos' with RPRO=0
scanOnce: dbProcess of Active '_pos' with RPRO=0

ca:anj@tux: Process _pos
ca:anj@tux: Process _rel
ca:anj@tux: Process _lim
ca:anj@tux: Active _pos
ca:anj@tux: Active _pos
ca:anj@tux: Active _pos

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