PLC-5 Losing Its IP Address

msinclair

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Join Date
Sep 2003
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US
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398
Short of cycling power, what might cause a PLC-5 (5/20C) to lose its IP address?

I know on power up, the PLC must query the BootP server to get its IP address. Is there anything else?

We had a PLC lose its address this afternoon during a power sag (about 1 second). Our system is set up so that the PLC should never have seen this sag - it is on a UPS that should have kept power to it for almost 30 minutes! Despite the UPS, I suddenly had no Ethernet communication with this PLC, and I had to restart the BootP server to fix it (long story why that wasn't running . . . )

We will certainly be verifying the operation of the UPS the next time the line is down, but in the meantime if anyone has any thoughts, I would appreciate hearing from you.

Thanks,

Marc
 
Did the PLC-5 keep it's program during the loss of the IP address, and is the battery good ?

I think the PLC-5's action is an indication that your UPS didn't function as desired.

The only time I've seen 1785-ENET (because you said it's a 5/20C I assume that's what you're using) go to BootP mode unintentionally was a system on which the IT department decided to enforce IP address leases and send a "Force Renew" command to every BOOTP/DHCP client that had been using it's IP address for more than 7 days. The PLC-5 dutifully renewed it's IP address with the BOOTP server... and joined the office IP domain, rendering it mute to the production subnet.
 
Check to see that the UPS is an online type. If it is an offline type, there will be a momentary changeover time before going to battery. Also check that the UPS batteries have been discharge tested regularly. They do gain a memory sometimes and can also be failing if not regularly tested.
I have seen this type of event before in a power station where the PLCs momentarily dropped offline when the UPS changed over. The PLCs were SLC500s. In this case it caused a blackout.
beerchug
 
Thanks to you both for the replies. My guess was the UPS - I just wanted to see if there was anything else that might be the cause.


Ken - in answer to your questions:

Yes, we are using the sidecar 1785-ENET.

Yes, the battery is good. We change them regularly, and tested this one fairly recently. The BATT diagnostic light was not illuminated.

Yes, the PLC retained its program - simply restarting the BootP server and cycling power to the PLC was all I did to restore normal operation.

The PLC is on a separate control network with 4 other PLC's and a SCADA node - nothing else (so there wasn't a "Forced Renew"). We are reconfiguring portions of this line, and this is the only PLC that has not been updated to disable BootP (so the fact that ONLY this PLC lost its address isn't significant).

Thanks again for the replies,

Marc
 

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