PLC output Hold Last State

klcheng1

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Nov 2007
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I have SLC5/05 with BSN module link to Remote IO via 1794-ASB. The ASB is set to "Hold Last State" for its output. When the RIO cable is unplug from the ASB, the output does hold its last state. BUT when the CPU is power-off, it doesn't hold last state, all output goes to OFF. How to overcome this problem? TQTQTQTQ!
 
Do you have any code that clears the I/O image used in the process?
For example, on First Scan, Mov 0 into B16:12, Length 10 Words?
It is common in everyday programming (depending on application of course) to make the program user friendly by making the system start in a controlled state, thus clearing all states and making a non-energy system start properly. In some cases, you actually want the "outs" to remain in the energized state, for specific conditions only....
 
Hold last stage via M file config

Hi,
TQ for your reply!
For your question, no, I do not have program to reset output.
I beleived is something that to do with the M file setting as attached. Just to double confirm if this is the problem.
What do you think??
 
Personally, I would review the code to see what happens at start-up, because the RIO will hold its state if the comms go down, as directed. Communciation loss is different than a warm boot, from Off-Prog-Run mode.
Might involve more code checking, to see what is exactly making the state change.
 
My concerns is more on Run to Off (power off the CPU) reaction rather than during power up reaction on the RIO output. The requirement is to hold the RIO output in the last state before the PLC CPU is down due to power failure or processor fault. The ASB dip s/w has set to "hold last state". Will this help if we did not modify the M-file and when the processor faulted or power-off? As my observation, by leaving the BSN M-file as default, and set the 1794-ASB dip s/w to hold last state...when power failer to the CPU, the ASB's related output will still goes to ZERO.
 
I must be mis-interpreting the question. If the processor goes down in any case, I can't understand why you would allow outputs that are controlled by the PLC to remain energized. In any system that I have worked with, there is usually an MCR that controls safety issues like this.....
Maybe clarify this if this is what you are talking about. You want to have outputs hold ON, when power is interupted from your PLC?
 
YES.
To hold the output as the last state just before the PLC CPU is gone. some application needs some pumps/valves to continue to operate if the plc is gone. in this case we can actually hold the status using hardwire (relay self holding) mathod. In my case is too many output to modify for my system (around 500 o/p) which needs me to modify the whole MCC panel! if possible I just need to set some swithes or some s/w configuration to overcome this mess...
 
I think you are into something that I would never do. The only thing that I could think of is hardwiring these "pumps" as external driven systems, not controlled by the PLC, as what you currently are doing. I have never heard of keeping the outputs energized if the power fails in my industry. We use UPS and Backup Generators for this type of scenario, but if you want to hold the state in the PLC, this is different.

Maybe there are some other people on this forum that could help,
 
KLcheng, I agree with SNK. If the PLC is "gone" there is no method that I know of to keep the PLC ouputs ON. Your best hope may be to keep the PLC from losing its power. Use a small Uninterruptible Power System (UPS) to run the PLC and control circuits.
 
Another test would be to put PLC in program mode and then back to run mode and compare the results with power cycling.

Are you turning power off to just the PLC or to the entire panel?

This may not be related to what is happening in your case but it is something I have seen: The main disconnect is turned off to a panel. The 24v power supply to the inputs and the power supply for the PLC both power down. However the 24V input power supply dies faster than the PLC power supply. While the PLC is in run mode and unaware that power is going down all inputs go to off state. The PLC code reacts to certain of these inputs going low and turns the output off just before it dies itself. On power up the output is comes back up into its last state which was off. Nothing to do with power up as it would first appear but actually happening durnig power down. This can be confusing since it apprears to work correctly if only PLC is power cycled or run mode cycled. Only appears when entire panel is powered down.

I have seen this is two cases... One was movement of data when a dial went through 0 degrees (all inputs off) which would occur at power down no matter where the dial was and data would be off by one nest when power restored. The other was a seal in rung where an input going low was a condition to unseal... the output should have kept itself on but the input going low unsealed as power went down.
 
The reason why I need to keep the output ON even losing the control of the PLC, is to maintain the water feed to the factory which can not affort to stop for longer than 10 secs. The client is concerns if power failure (to be exact: power flick/dip/or short power disturbance) is happen at CPU only. The RIO is installed at local panels which has it own power source. They hope the PLC will continue to control(change duty etc) those pumps once the power resume to normal at CPU's panel.
I agree just to install a UPS and solve the problem. But I still want to find out why the output is not able to hold at it last state...I will make some changes in the Mfile of the BSN module, do a test and will inform the result soon.
 
Another option in future may be: use 2 state control for pumps...each pumps has one start and one stop output command. Each command will latch a hard relay at panel to hold the command. So we have one relay for start and one relay for stop at panel. After PLC command to start, the pump will start and will keep going regardless of PLC start signal, until PLC issue a stop command to cut the circuit at the panel and stop the pump. any comment?
 

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