Sculy relay logic for sticky bits

stuck bit usually means burnt input/output or memory error, if there's no forcing or logic error. that would cause plc to raise fault, so i don't really see the point in detecting stuck bit...

jandrul said:

I have found some logic in a PLC (90's vintage) controlling our hot oil furnace which attempts detect a stuck bit ! I think the theory was that bits could get stuck much like a relay. Anybody out there seen this before ? Anybody have issues with stuck bits ? This logic is currently bypassed as it uses an external relay for the test which ironically fails too often.

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Yes, but only bits that are accessed from a remote device. I commonly write code to avoid "stuck button syndrome", so that if comms are lost or if only one packet gets lost (the one to turn of an HMI button bit for example), then the program ensures that no unexpected or neverending action takes place.

If the "stuck bit" isn't written by an external device then all of this is unnecessary, since bits in a PLC can't get "stuck" on or off all by themselves.

you know what, I have seen this problem in an old S5-95u. The 24v power supply started to fail - The way it failed caused a floating voltage on the negative 24v which was grounded, consequently causing the logic to fail sporadically. I was called to site to investigate some strange goings on with a palletiser and monitoring online found inputs remaining at logic 1 with no 24v on the input; and vice versa...I found the problem because the HMI went up in smoke about 1 hour after getting to site......

randomFire said:

I found the problem because the HMI went up in smoke about 1 hour after getting to site......

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yes, that white smoke always helps when troubleshooting...

In safety systems this is very normal.
furnace at sea (for lloyds etc) need this to be sure it is safe.

jandrul said:

I have found some logic in a PLC (90's vintage) controlling our hot oil furnace which attempts detect a stuck bit ! I think the theory was that bits could get stuck much like a relay. Anybody out there seen this before ? Anybody have issues with stuck bits ? This logic is currently bypassed as it uses an external relay for the test which ironically fails too often.

Click to expand...

Yes, this is common (probably required based on your reading of NFPA) in Burner Management Systems and other Safety Systems.

Use a Solid State Relay in place of the mechanical relay and you won't have to bypass the logic, BTW.