How to turn off a bit when power goes out?

alive15

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Good afternoon,

I have a motor running on a machine and whenever I cycle power on the PLC, the motor stops and comes back on when the PLC turns back on. Is there a way to keep the motor off, even when the PLC comes back on after losing power? Thanks.
 
My first question to ask is are you using latching instructions? In any of the flavors of RSLogix, it will be Output Latch and Output Unlatch (OTL and OTU I think). These will produce the effect that you are experiencing, in that the output will remain energized even though power is cycled to the PLC. If you don't want this effect, Try using a simple holding contact, just as if it were a magnetic motor starter.
 
First off - when the power drops the MCR better drop off, and no power can get to the motor starter until the MCR is turned back on. And I find it good practice to put a CR-Enable relay controlled by the PLC, so if the PLC turns off, faults or is put in Program mode the MCR will be interrupted.

Second, for that to happen the program seems to be programmed with latching outputs, either reprogram to OTE's that get turned off by the PLC on the pre-scan, or check the First Scan bit and OTL any latched bits or outputs.
 
SandwichMagic said:
My first question to ask is are you using latching instructions? In any of the flavors of RSLogix, it will be Output Latch and Output Unlatch (OTL and OTU I think). These will produce the effect that you are experiencing, in that the output will remain energized even though power is cycled to the PLC. If you don't want this effect, Try using a simple holding contact, just as if it were a magnetic motor starter.
Click to expand...

Thanks for your reply, Yes, I am using latch and unlatch outputs. The issue is even if I don't use OTL and OTU, my HMI will still "toggle / latch" the output back on. And I need to keep it on toggle/ latch for the entire shift basically. Is there not an option to not "hold the last state" or something along those lines?
 
You can use the first scan bit (S:FS) to clear any bits you choose when the PLC powers up.

Do you know WHY this output is coming back on - latched output or retentive bit with no logic to clear it? I'd be wary of latched output bits in most circumstances. I've seen situations where power down occurred simultaneous with an HMI button being pressed. For this reason, I'd strongly recommend using the S:FS to clear any HMI pushbutton bits which may be accidentally set.
 
Aabeck said:
First off - when the power drops the MCR better drop off, and no power can get to the motor starter until the MCR is turned back on. And I find it good practice to put a CR-Enable relay controlled by the PLC, so if the PLC turns off, faults or is put in Program mode the MCR will be interrupted.

Second, for that to happen the program seems to be programmed with latching outputs, either reprogram to OTE's that get turned off by the PLC on the pre-scan, or check the First Scan bit and OTL any latched bits or outputs.
Click to expand...

Thanks for your reply, the issue with this is I actually have 3 motors, so even if the MCR was dropped when power is off, and then the MCR comes back on after the power is back on, all 3 motors will still start at the same time, thus causing possible overcurrent and blown fuses, which is what I am trying to avoid.
 
RonJohn said:
You can use the first scan bit (S:FS) to clear any bits you choose when the PLC powers up.

Do you know WHY this output is coming back on - latched output or retentive bit with no logic to clear it? I'd be wary of latched output bits in most circumstances. I've seen situations where power down occurred simultaneous with an HMI button being pressed. For this reason, I'd strongly recommend using the S:FS to clear any HMI pushbutton bits which may be accidentally set.
Click to expand...

Thanks for your reply, this idea seems very interesting. So if I place the first scan bit on the rung that unlatches all 3 motors, then you are saying this will fix the problem with the motors starting on when the power comes back on? Is this 100% guarantee?
 
alive15 said:
The issue is even if I don't use OTL and OTU, my HMI will still "toggle / latch" the output back on.
Click to expand...
Guess I was typing while you were posting. I'd recommend using the HMI to set your pushbutton bit and the PLC to clear the pushbutton bit. I have found HMI toggle operations cause nothing but grief.

alive15 said:
And I need to keep it on toggle/ latch for the entire shift basically. Is there not an option to not "hold the last state" or something along those lines?
Click to expand...
Sorry, but I'm not understanding what you mean. Hopefully you can clarify.
 
So, you can certainly enable the drives by the HMI. But I would use seal-in logic instead of latches, and make someone positive confirm they want the drives to restart. You can use this positive confirm to trigger internal sequencing logic to restart the drives in a controlled fashion.
 
alive15 said:
Thanks for your reply, this idea seems very interesting. So if I place the first scan bit on the rung that unlatches all 3 motors, then you are saying this will fix the problem with the motors starting on when the power comes back on? Is this 100% guarantee?
Click to expand...

Without being able to see your logic and assess it, there are no guarantees! Feel free to post a PDF or zipped copy of your logic and we'll be able to offer some better advice.

It sounds like clearing your HMI pushbuttons is the main issue given your earlier post. What others have said about MCR & avoiding latches for outputs is also good advice.
 
RonJohn said:
Guess I was typing while you were posting. I'd recommend using the HMI to set your pushbutton bit and the PLC to clear the pushbutton bit. I have found HMI toggle operations cause nothing but grief.


Sorry, but I'm not understanding what you mean. Hopefully you can clarify.
Click to expand...

So the motor has to be running for the entire shift, the only way it can turn off is if someone turns it off on the HMI itself or the power goes out. Essentially, the input button the the HMI (automation direct, cmore) will be on toggle mode, so when toggled on, the output will be latched on, and when toggled off, the output will be unlatched. The issue I have is 9 times out of 10, the input button for the motor will be in "toggle on" mode, so the output will be latched in. So when the power is cycled on the PLC, it will still be latched in, thus causing the motors to start at the very beginning, which is what I want to avoid.
 
alive15 said:
Thanks for your reply, Yes, I am using latch and unlatch outputs. The issue is even if I don't use OTL and OTU, my HMI will still "toggle / latch" the output back on.
Click to expand...


The way that you are describing it sounds like your HMI is the one thats actually doing the latching. I reccomend only having the PLC do this. Just make whatever button turns on the motors momentary, and then do the latching in your ladder logic.
 
SandwichMagic said:
The way that you are describing it sounds like your HMI is the one thats actually doing the latching. I reccomend only having the PLC do this. Just make whatever button turns on the motors momentary, and then do the latching in your ladder logic.
Click to expand...

Yes, exactly right, my HMI is the one doing the latching. Unfortunately, I need to keep it this way. At the end of the day, the motors will be turned off, and then the next day, the first operator will come and manually turn on the motors through the HMI. They won't have the PLC logic in front of them to toggle on and off.
 

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