Outputs during Program Mode MicroLogix1400

Hello there,
Is there a way to control what state the outputs will be during program Mode?

I sometimes run into program changes I have to do offline and when I download the new file the outputs turn off. In order to prevent hard stops on equipment I override the output by wiring 24V directly to the output.

I was wondering if there wouldn't be a way to tell the PLC to keep the last Output state when entering Program Mode.

Thanks

No, all the outputs will turn off while the PLC is not in RUN mode. I don't know of any way to avoid this.

I believe the ML1400 support online editing in Remote Run. With an online edit in rem run the outputs would maintain their state. If you did a full download then no, the outputs would all transition off during the download.

_Dock_ said:

I believe the ML1400 support online editing in Remote Run. With an online edit in rem run the outputs would maintain their state. If you did a full download then no, the outputs would all transition off during the download.

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Yes, this is true. Certain things cannot be edited while online, like rungs that contain MSG instructions and data file creation or resizing. This is why I try to design my projects with lots of extra ladder and data files and set up my MSG instructions for as many conceivable needs in advance each driven by a single bit instruction. Then at runtime, I can do online editing to cover 90% of the future needs.

John93,

before doing any offline edits, place the machine in manual and manually place the machine in the home position. upload the program to the plc, make your edits, then download the program. you should then be in the same state when you go into run mode.


you may want to get a copy of NFPA79 - electrical standard for industrial machinery. look at the requirements for machine startup / estop conditions.


james

Yeah, you should be able to toggle your output back on (Ctrl + T on Rs Logix 5000, or you have to right-click and select toggle on 500), then when you download, that output will be on. But if your program is telling it to turn back off, it will be off.

You may be able to force something on or off in offline mode, I haven't tried that myself, but just an idea. And of course be sure it's safe for these outputs to turn on, machine is safe, no one in the way, etc.

Before you go offline with the project, hit save, then upload values of all your i/o (on 5000 it's called uploading tag values, not sure on 500, maybe save image or something like that). This should keep the last known state of all your i/o. Then make changes offline, then download. It will download whatever values it had when you saved the project.

alive15 said:

Yeah, you should be able to toggle your output back on (Ctrl + T on Rs Logix 5000, or you have to right-click and select toggle on 500), then when you download, that output will be on. But if your program is telling it to turn back off, it will be off.

You may be able to force something on or off in offline mode, I haven't tried that myself, but just an idea. And of course be sure it's safe for these outputs to turn on, machine is safe, no one in the way, etc.

Before you go offline with the project, hit save, then upload values of all your i/o (on 5000 it's called uploading tag values, not sure on 500, maybe save image or something like that). This should keep the last known state of all your i/o. Then make changes offline, then download. It will download whatever values it had when you saved the project.

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NO, you are wrong.

Each and every Allen-Bradley processor performs a "Pre-Scan" when going into Run mode, to protect machinery and lives from exactly the scenario you have just suggested.

During the Pre-Scan, the user program is scanned, with outputs disabled, and ALL Non-Retentive Data is reset to zero. All outputs, will be turned off, all TON and TOF timers will be reset, etc. etc.

It will be the first "proper" scan of the user's code which will determine which outputs are turned On, and, if the programmer has done his job properly, this must not cause any "exposed" machinery to restart without all interlocks, safety devices checked etc., and it is usually good practice to require operator intervention to re-start sequences ....

daba said:

NO, you are wrong.

Each and every Allen-Bradley processor performs a "Pre-Scan" when going into Run mode, to protect machinery and lives from exactly the scenario you have just suggested.

During the Pre-Scan, the user program is scanned, with outputs disabled, and ALL Non-Retentive Data is reset to zero. All outputs, will be turned off, all TON and TOF timers will be reset, etc. etc.

It will be the first "proper" scan of the user's code which will determine which outputs are turned On, and, if the programmer has done his job properly, this must not cause any "exposed" machinery to restart without operator intervention.

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I'm not sure what you are talking about, I'm saying if you turn your output off, or perhaps force it off, in your offline program, and re-download the program into the PLC, your output will be off.

I've tested this myself with motors and lights, I would turn them "on" in remote run mode, then I go offline, then I turned all my output bits off, then whenever I download it, my motors and lights would shut back off due to the change in values from my new program.

Same the opposite way, I'll be in remote run, with my motor and light off; then I'll go offline and change tag values to "1" to turn them on. So then, when I download, my motor and light are on immediately after download completes.

Same with timers, I can change the .acc value to be whatever I want in offline, then when I download the program into the plc, that new .acc doesn't reset to 0, it's the same value I typed in.

This was on L32E compact Logix plc v20 or 24.

I agree with daba, this is a bad practice.

the first scan he is referring to is correct.
when the machine is placed into run mode, the outputs are turned off. the inputs are read and updated in the input table. the logic is then solved 1 rung at a time. left to right, top down.
at the end of the program, the outputs are then updated to the output table and turned on/off.
NFPA79 details how machinery is supposed to work when the e-stop is pushed, power restored, and program downloaded.
i'm doing this from memory, someone correct me if I am wrong.
other than resetting the safety circuits on any machine, no component may move with human intervention when the plc is or is not connected when entering run mode.
in my opinion, if you have to force outputs on/off to keep something from moving when the program is downloaded and placed into run mode, the machine is not designed correctly. no offence meant, I have never programmed a machine in such a way that forcing I/o on/off was required in order to keep it from moving when you go into run mode. that is clearly a safety violation in any plant I have worked at.
james

Sure, it's a bad practice when the machine is operational and live, but during testing and start-up phases, it's a very useful trick, especially if it's in the shop and not on the assembly line or production area.

If the OP is asking about best practice, then this is not the route to go. From my understanding, he is asking if it's possible, so I am saying "yes, it is possible".

No, what he is asking is not possible. Hes asking to maintain outputs in an on state in program mode or during a download. No amount of toggling or forcing will allow this.

alive15 said:

...
he is asking if it's possible, so I am saying "yes, it is possible".

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But it is not possible to download without having the Micrologix turn off outputs when you go to program mode. His jumper wire method is still going to be necessary to avoid interruptions.

All this assumes that the OP understands his machinery well enough to assess the risks of having the machine "hot wired" briefly compared to performing a safe shutdown of the equipment before doing these changes ... the latter being the preferred choice from the standpoint of most of us reading this when it comes to the safety of people and equipment ...

With that said, there are times when keeping machinery running is the safer choice when interruptions can create larger hazards.

I would say that if you need to keep something running, or powered on when the PLC goes into download or program mode, then a safe way would be to use an external "override" switch, possibly key operated.

And I would certainly NOT be putting volts onto the output leg of a solid-state output module with wire links under ANY circumstances.

Or you could upgrade to ControlLogix, where you CAN tell the outputs to go either OFF, ON, or RETAIN LAST STATE.

Even Analog modules have that functionality too.

EDIT : If a process is SO important to you that you cannot stop it, then you ought to look into building proper redundancy systems, as offered by Logix5000 systems.