count registers reset to zero?

Hi all
does anyone know what happens to the incremented count in
the PLC-5 Allen-Bradley 1771-VHSC card when a power glitch
occurs? Operators are telling me the count is resetting to
zero, although the way its programmed, you normally have to
enter a reset from the Wonderware computer.


Thanks,
charles

If there is any kind of glitch going on with Wonderware or the network... all kinds of interesting things can happen. Outputs can go ON or OFF, Analog Outputs can go up or down.

I had a communication cable on a Honeywell that would drop a bit every now and then... it affected different outputs and affected an analog output signal to a motor. The bit was significant so the speed change was significant. I jammed an empty plastic fuse box (10-pack) between the connector and the chassis to try to ensure a good connection... so far (3-years) it has worked... no more spooky stuff.

A few months ago I had a keyboard die on me... while it was on-line with the process. It shut the process down. While the process was running, the keyboard failed and sent some kind of "get out of run mode" signal. I don't know if it went to program mode. It was not a pretty picture. I don't know of any way to prevent that except to stay off-line... which is unacceptable.

Also... you need to be really, really sure that the program isn't stepping on the count somehow.

I don't think the VHSC has any retentive memory.
Simplest solution may be a UPS to keep the PLC & VHSC alive.

Thanks guys. i checked and rechecked the program looking for ways the counter may have inadvertandtly reset itself when a power interruption occurs, but saw nothing. I did not know the count registers did not have some type of a memory backup, but then again, why would they? I'll try installing a UPS when the machine
finishes its current production run, but i've always wondered if there is a downside to keeping a PLC powered when the large machine/process its controlling loses all power?


- charles

katratzi said:

i've always wondered if there is a downside to keeping a PLC powered when the large machine/process its controlling loses all power?

Click to expand...

You should dedicate an input to show presence of I/O power. Then, upon loss of that input, your program can set a safe state so that nothing untoward happens when mains power is restored.