Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Wed, 24 Feb 2010 12:11:03 +0100

Hello,

here is a trivial SNL program I wrote in order to reproduce a misbehaviour 
that occurs once in a while in one of our production state machines.

Two state sets communicate via a state variable "x" which is assigned to a 
bo record and monitored by the program. The master sets x to one, then waits 
for the slave to reset it to zero. The slave waits for x to become one, then 
resets it to zero. This all happens in an infinite loop.

Since there is only one place where x gets set to one (in the master), and 
one place where it gets reset to zero (in the slave), the assertions in the 
calls to check() should never fail.

Your IOC should load this simple database:

  record(bo,"x") {
  }

and this sequencer program:

program SeqTest

%{
#include <assert.h>

static int check(x) {
    assert(x);
    return 1;
}
}%

int done = 0;
int start = 1;

int x;
assign x to "x";
monitor x;

ss Master {
    state idle {
        when (check(!x)) {
            printf("Master: work\n");
        } state work
    }
    state work {
        entry {
            printf("Master: start slave\n");
            x = start;
            pvPut(x);
        }
        when (!x) {
            printf("Master: slave finished\n");
        } state idle
    }
}

ss Slave {
    state idle {
        when (x) {
            printf("Slave: start work\n");
        } state work
    }
    state work {
        entry {
            printf("Slave: enter state work\n");
        }
        when (check(x)) {
            printf("Slave: work done\n");
            x = done;
            pvPut(x);
        } state idle
    }
}

In my tests, the IOC will run for a while but inevitably crash with

STESTIOC1C: ../seqtest.st:7: test: Assertion `x' failed.
[1]    8507 abort      ./st.cmd

This looks like a bug in the sequencer to me. I am testing with 
base-3.14.8.2 and seq-2.0.12.

(BTW, changing the pvPut calls to SYNC makes no difference.)

Cheers
Ben
-- 
"...And you know what incurable diseases do: they invite the quacks and 
charlatans in, who in this case take the form of Software Engineering 
gurus." (Dijkstra, EWD1305)

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System