Re: [foxboro] system error in sequences

  • From: Jerry Hidahl <Jerry_Hidahl@xxxxxxxxxxxx>
  • To: foxboro@xxxxxxxxxxxxx
  • Date: Mon, 30 Aug 2010 08:48:09 -0500

Dirk,

The next best thing to connecting your off-station references to a
parameter in your sequence block is to use a parameter in a different block
in the same compound. The next best thing after that is a parameter in a
different compound, but in the same station. It could be another sequence
block or a calc-type block.

The key factor is when the value is updated within the station where your
block is running. Connections are updated every time before a block is
executed, but C:P.B references to different stations are only updated when
a line of code is executed. Whenever an off station reference occurs, the
code execution is suspended until the answer returns. If a reference is in
the same station, the value is just fetched or stored to a memory location,
and the code drives on.

When we upgraded our kettles last year to FCP270s and consolidated some CPs
we had to add a WAIT statement for a sequence to work correctly.
Previously, a key handshake reference was in a different CP, so the
execution always suspended. After we consolidated, the handshake stopped
working until we forced the sequence to wait.

Adding connections to off station parameters won't always be the best
solution. You have to weigh the cost of the added overhead of updating
connections every scan, versus slowing the HLBL execution down with C:P.B
references.

Jerry Hidahl
Process Control Engineer
Port Neches Performance Products
Huntsman Corporation

In reference to:

Hi Chuck,
Thanks for the advice, For these (small) sequences I already changed the
code using user labels, but I have many sequences where there are too many
C:B.P references to use user labels,  I'll go through the sequences and
change the network calls using variables where needed.

Rgds,

Dirk

 
 
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