Experimental Physics and
Industrial Control System

Dirk Zimoch <[email protected]> · Fri, 15 Dec 2017 17:02:52 +0100

Hi Folks,

A colleague puzzled me with a db that shows surprising behavior.

An aSub record is triggered periodically though a CP input link. It

looks up if it needs re-initialization (the user can press a button to

request that). If yes it does an additional calculation. After

processing it resets the record that requested the re-initialization.

See the attached .db file.

Usually when the user presses the button to re-initialize, the aSub does

the additional calculation once and then goes back to normal processing.

But the calculation may may take a bit too long -- longer than the rate

of the CP input link monitors. In that case, the record never leaves

initialization mode!

I have simulated the slow calculation with the attached subroutine code

which simply waits. (I know that is is a bad idea to wait in a

synchronous subroutine. The original code does not wait but calculates.)

I think that this is an effect of lock sets because the "mode" record is

as well input being processed by a caput as well as output of the aSub

record. But is this the intended behavior?

The CP monitor finds the record busy (PACT=1) and arranges for

re-processing when the record is done. But should not the FLNK be

processed and the mode record go to 1 before the aSub runs again?

Any idea how to fix this? Is it necessary to make the subroutine

asynchonous? Making the links from and to "mode" CA links does not fix

it. Scanning the aSub itself with ".1 second" does not show the strange

behavior, it simply delays the scan thread.

Puzzle for the weekend...
Dirk

record(calc, "trigger") {
    field(INPA, "trigger")
    field(CALC, "A+1")
    field(SCAN, ".1 second")
}

record(aSub, "slow_aSub") {
    field(SNAM, "slow_aSub")
    field(INPA, "trigger CP")
    field(INPB, "mode")
    field(INPD, ".2")
    field(FLNK, "seq")
}

record(seq, "seq") {
    field(DOL1, "1")
    field(LNK1, "mode")
}

record(bo, "mode") {
    field(ZNAM, "uninitialized")
    field(ONAM, "initialized")
}

#include <stdio.h>
#include <aSubRecord.h>
#include <epicsThread.h>

void slow_aSub(aSubRecord* rec)
{
    double delay = *(double*)rec->d;
    int mode = *(double*)rec->b;

    static int i=0;

    printf("%3d: entering subroutine: mode=%d\n", i, mode);
    if (mode == 0)
    {   
        printf("%3d:   delay %.3g sec\n", i, delay);
        epicsThreadSleep(delay);
        printf("%3d:   done\n", i);
    }
    i++;
}

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System