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Hey All,

Trying to resolve an issue that's got me thinking outside the box.
Looking for opinions as to whether my idea is acceptable.

We have a machine that I recently upgraded from relays to PLC.
I'm using a V570 with all remote I/O,
With the exception of an encoder that comes straight to a V-200-E1B snap I/O.

Our management is on a Data Collection kick and they want runtime info spoon fed to them. I've accommodated their needs by writing things to a data table, and retrieve it to excel using DataXport. All is good so far...

But the machine has setup changes a dozen times a day where the operator must power down and LOTO. I cannot control nor predict when the machine is powered down, and obviously I cannot talk to the PLC with no power.

You all with me so far?

So, my thought is to supply power to the PLC only, from a source before the lockout disconnect.

Everything else is safely locked out, 480v to motors, control power to other devices, etc. just power to the PLC for communications

I'll have a relay that monitors power after the disconnect, and when it turns off it will give an input to the local snap I/O where I'll switch to an HMI display with a black backround and the phrase "Power is Off" dancing around like a screen-saver.

Anybody out there see a safety issue with this?



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  • MVP 2023

I work with chemical delivery systems every day and we always have a separate relay for the equipment and the PLC. The equipment gets powered down whenever an interlock alarm occurs, but the PLC stays powered on. The EMO on the panel does power down the PLC, as well as the equipment power. This meets strict regulations and you should be able to do what you want to do with no issue.

That said, I have to ask, why did you choose an IO-DI16 plus IO-RO16 instead of a single IO-D16A3-RO16, which would be cheaper and use less space on the DIN rail?

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Thanks for the input Flex....

It seemed a reasonable thing to do, but sometimes I do catch myself doing "old school" stuff that just wont fly under todays rules and regulations.

Choice of modules? it's what I keep on the shelf for other panels I've built here....


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  • 4 weeks later...

Well, I got a chance to play with this over the weekend, mounted a separate breaker & power supply before the lock-out disconnect that only powers the V570 and the snap I/O module.

Power to the remote I/O stuff shuts off with the lock-out disconnect. but, I find that when I power it back up, the remote I/O does not connect comms back to the V570.

I've found that I can bang SB 300 (PLC Reset) on power up of the rest of the panel and reset the V570, which then connects to the remote I/O.

But, I was wondering if there was a better way to re-establish comms between the V570 and remote I/O?


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  • MVP 2023

I always run the I/O module power on the same circuit as the PLC. Why would you power down the I/O and keep the PLC running? That's just asking for problems.

The way we do things:

K1 Power relay: Powers entire cabinet, shuts off when EMO pressed. PLC and I/O modules are on K1 power.

K2 Power Relay: Powers output relays and other misc. Turns off when an interlock alarm occurs. When it is shut off everything powers down except PLC & PLC I/O modules. It can also be turned off and on from the PLC HMI.

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  • MVP 2023

And also why not periodically log your data table to the SD anyway as a secondary backup?  You could also have them do a "power is going off" HMI push to do a "manual" write to the card, which might well separate logging data better for a different setup.  Tables on the card could be named according to results from a setup entry screen once each setup changeover is complete, and they could be run on a FIFO basis.

I know it is overkill, but I always like to have 2 different sources of crucial data. 



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