Rockwell plc digital outputs statuses at power up

Kataeb · May 12, 2021

Kindly, what is the status of the digital outputs, while waiting for the plc to complete its startup (when we power off then power on the supply)?
Does the output stay :
Off
On
Last value before shut down

The plc is: Logix5580E Controller With 10 Mbytes Memory , Rockwell , 1756-L83E

Thank you

Nova5 · May 12, 2021

Depends on the logic command status.

If a OTL was used last used an no OTU in a power up routine to unlatch, they usually they will come back latched.

If OTU was last used and no OTL hits it then it should be unlatched.

IF its a OTE, it depends on the rung commanding it.

Allen Bradley warns about this, that outputs CAN come back as their previous status and so using OTL and OTU is generally not recommended for that reason specifically.

It REALLY depends on how your site logic is programmed. I've gone through mine and hit every single motor HMI bit with a OTU on the first scan. That makes all of them unlatch on power up. To help prevent the motors from firing up on a PLC restart from power failure, ect.

PROPERLY programmed IMHO they would all be off. Protected/Controlled by self collapsing latches. Use a pulse to start, then the output holds itself running.

I have some field controls that are physical switches that hold in position, on powerup I put a protective rung in that if on first scan they are in the run position the motor will not start until the switches are placed to off. Then you can flip them to run to start the motor without issue.

plvlce · May 12, 2021

Nova5 said:
Depends on the logic command status.

If a OTL was used last used an no OTU in a power up routine to unlatch, they usually they will come back latched.

If OTU was last used and no OTL hits it then it should be unlatched.

IF its a OTE, it depends on the rung commanding it.

Allen Bradley warns about this, that outputs CAN come back as their previous status and so using OTL and OTU is generally not recommended for that reason specifically.

It REALLY depends on how your site logic is programmed. I've gone through mine and hit every single motor HMI bit with a OTU on the first scan. That makes all of them unlatch on power up. To help prevent the motors from firing up on a PLC restart from power failure, ect.

PROPERLY programmed IMHO they would all be off. Protected/Controlled by self collapsing latches. Use a pulse to start, then the output holds itself running.

I have some field controls that are physical switches that hold in position, on powerup I put a protective rung in that if on first scan they are in the run position the motor will not start until the switches are placed to off. Then you can flip them to run to start the motor without issue.

The question was about the state of outputs 'while waiting for the plc to complete its startup', i.e. before logic is being run.

My suspicion would be that the answer is either 'off' or 'configurable'.

Back in the SLC days transistor outputs would have a transient pulse when voltage was applied to the output common (see this thread for some discussion), but I don't know if that is true for Logix as well.

Nova5 · May 12, 2021

The question then is.. what is the boot time of a ControlLogix processor before logic begins processing. Unlike a PC it is not minutes before they become operational. Its very quick. So could you even have time to turn it on and grab a meter before it begins processing?

Jim3846 · May 12, 2021

The state of IO outputs are shown on a tab of the card other than run mode. Its a choice given to the programmer.

daba · May 13, 2021

On power-up, all outputs would be OFF until data gets sent to the output modules by the controller.

The data will not get sent until the controller has executed ....

All POST operations. This is the longest time, taking several seconds, and it depends on the model of controller.
Made connections to the output modules.
Performed a "pre-scan" of the user's logic code, putting all non-retentive (e.g. OTE) data into the OFF state.
Commenced scanning the user's code, where the state of the outputs will be determined by the user's logic.

Note that the output data gets sent to output modules asynchronously to the program scan, so the first update to output modules will be after the program has started executing, not before.

But also be aware that Logix5000 outputs (individually) can be "configured" to be OFF, ON, or "HOLD LAST STATE" in two eventualities.

1. The controller powers-up in Program Mode, which will be unlikely unless it was powered off in Program Mode.

2. The module loses communications with the controller. This does not happen unless it gets a connection, then loses it.

daba · May 13, 2021

Adding to my last post...

An output module will not assert any outputs "high" until it either....

1. Sees a "connection" to a controller that is in program mode, in which case the outputs will go to the configured program mode states.

2. Receives output data from a running controller, in which case the outputs will go to the desired state, as dictated by the data received.

3. "Loses" the connection to the controller, in which case it must have previously been "connected", in which case the outputs will go to the configured "lost comms" states.

I am nearly 100% certain that no outputs can possible come on unless that important "connection" is made. All of the above assume the connection is established, and that the output module is aware of the controller's mode, or that a previously established connection no longer exists.

Anything else would be manufactured suicide ....

Rockwell plc digital outputs statuses at power up

Kataeb

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