adccommunitymod (AutomationDirect) asked a question.
Created Date: September 10,2010
Created By: archerynut
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Hello All. We just began implementing PLCs into some of our upcoming projects. I have played a little with some but this is the first from scratch. We are close to putting our PLC into action and would like some help on a question. We are using the model DL-06 and wondering if there is an output already setup for a PLC Failure Alarm? If there isn't, I have an idea on how to create one. Just wondering if this would work. We would jumper one of the inputs directly from our 24v DC power source negative (sinking I/O). Then we designate an output. When the power is on and PLC is operating correctly the output will be active. If we lose power the output would open. Now if we had a PLC failure can I assume the output would open? The output of the PLC controls a relay with in turn is interfaced with our building automation system where the PLC Failure Alarm will flag. Any other ideas?
Created Date: September 10,2010
Created by: JoeW
Its hard to do directly, because obviously you can't control the PLC when it fails. We sometimes interlock two PLCs with transistor outputs using a "heartbeat " i.e. a flashing output linked to an input of the other PLC.
Created Date: September 10,2010
Created by: bartb102
You have the easy answer, using an input to turn on an output. Generally (but not guaranteed) outputs turn off when the PLC fails or goes to program mode. This would not meet standards for personal safety circuits.
I 've used an output to drive a stop motion time delay relay- each time the output changes state the stop motion delay is reset. If the output stays on too long or off too long the relay times out & turns off it's output relay. Solid state outputs would be preferable to relay outputs. this way the detector is independent of the PLC. You can also use a delay on operate & a delay on release relays as detectors. As PLC outputs can fail on or off you need both.
I 've got several plants where PLC output relays have been clicking on & off every 5 seconds for over 20 years. Quite a bit abuse for a mechanical contact. In my application I don't need instant response to a failure so I can wait for the stop motion delay.
Created Date: September 13,2010
Created by: JoeW
I 've got several plants where PLC output relays have been clicking on & off every 5 seconds for over 20 years. Quite a bit abuse for a mechanical contact. In my application I don't need instant response to a failure so I can wait for the stop motion delay.
I wouldn't trust a relay to last very long under those circumstances. I'm sure they could, but we have had relays clicking on/off every second go bad in under 7 months.
Created Date: September 14,2010
Created by: bartb102
Agreed, not desirable for mechanical contacts. Newer facilities use solid state outputs. I am amazed that the mechanical contacts have lasted as long as they have.
Created Date: September 14,2010
Created by: JimHoward
When I did projects a while back for the USPS (US Postal Service) for their conveyor systems, the USPS had a requirement for "an independent PLC failure alarm contact ". What we came up with, and was accepted by the USPS was a simple plug-in Off-Delay Timing Relay.
Let's say the Off-Delay Timing Relay is set to ~2 seconds. A solid-state output (no clicking relay!) from the PLC energizes every 1.5 seconds (for maybe .2 seconds), resetting the Off-Delay Timing Relay.
As long as the PLC is alive, the Off-Delay Timing Relay never times-out. If the PLC fails, the Off-Delay Timing Relay times-out - use the contacts in the Off-Delay Timing Relay as your alarm.
Created Date: September 15,2010
Created by: bartb102
I'd suggest either the on/off time delays or a stop-motion relay (actually two timers in one). Output contacts can fail on so the delay off relay may not detect all failures.
Created Date: September 10,2010
Created by: archerynut
Hello All.
We just began implementing PLCs into some of our upcoming projects. I have played a little with some but this is the first from scratch. We are close to putting our PLC into action and would like some help on a question.
We are using the model DL-06 and wondering if there is an output already setup for a PLC Failure Alarm? If there isn't, I have an idea on how to create one. Just wondering if this would work. We would jumper one of the inputs directly from our 24v DC power source negative (sinking I/O). Then we designate an output. When the power is on and PLC is operating correctly the output will be active. If we lose power the output would open. Now if we had a PLC failure can I assume the output would open?
The output of the PLC controls a relay with in turn is interfaced with our building automation system where the PLC Failure Alarm will flag.
Any other ideas?