Would it be possible to loop the 4-20 ma output through a 4-20 ma input and
alarm it in the ain block? Another way might be to add a low value resistor
in the loop and monitor a voltage drop across it with an FBM 02.This would
take an extra input for each loop. I do not know what type of isolation
problem, if any, you might run into. I would wire it to a different FBM. If
this would work it would save you from
having to install a position transmitter on the valve.
Loyd Greer
---- Original Message -----
From: Rick Rys <[EMAIL PROTECTED]>
To: Foxboro DCS Mail List <[EMAIL PROTECTED]>
Sent: Thursday, May 10, 2001 6:52 PM
Subject: RE: Open-loop detection on analog output cards
> Let's assume you have an FBM 04 with ECB type 2, and a simple AIN, PID(A),
> AOUT analog control loop. Next you remove the wiring at the FBM terminal
> for the 4-20ma output for this channel.
>
> The CP is good, the fieldbus is good, and the FBM is good. ECB Failsafe is
> not enabled. The field wiring is bad (open), however, and the valve is
> getting 0ma and thus going to it's failure state. You will likely be
> getting some alarms soon enough, but I understand why you might want to
get
> an alarm immediately. This might be possible with intelligent
transmitters,
> but it is my understanding that the FBM04 is not smart enough to identify
a
> single open loop output. Inputs would be caught by out of range, but I
> don't think there is any way to detect a bad output short of a valve
> position transmitter with some type of deviation alarming scheme. In the
> AOUT block the BAO when set to 1 has alarming looking at BAD (BO193AX
Block
> Book). BAD is set from the OPSTAT of the ECB block (with some logic I
could
> not find), but the ECB is only setting the BAD based on communication
> related errors (see ECB Block description) and not disconnected output
> wires. You can look at BAD, OPSTAT, or ECBSTA to verify that nothing is
> showing as bad. Maybe someone else could verify this behavior as I didn't
> check it on real hardware.
>
> Rick Rys
> R2 Controls
>
>
>
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