ARP tables in Logix PAC's

Does anybody know if it’s possible to clear the ARP table in a CompactLogix PAC without cycling power or waiting for it to clear itself? From what I’ve been able to find in Google-land the answer is no but before I go to plan “B” (cycling power to the processor whenever I need to change to a new device to talk to) I thought I’d give the brain pool a chance. I’m building a setup that will be used to test our Protocol to I/O modems in which part of the process is testing all of the I/O points on a given remote as well as calibrating the analog outputs. This involves communicating to a protocol master from the CompactLogix issuing commands to the specific I/O remote. The problem comes in when I change from one Protocol master to another. They both have the same IP address but different MAC addresses and as it is now it takes anywhere from 10 to 20 minutes for the ARP table to dump the first master and find the second one. Long story short is that I can cycle power on the PAC but if there is a way to use logic to clear the table then I can automate it and make it… well let’s just say “production technician proof” (all due respect given). OK, so I spent a couple of hours trying to figure out why I couldn’t change from one master to another until I realized that the ARP table was getting in the way.

I am a novice in this area, but you may be able to alter the connection timeout properties for the ethernet port. I have done this with a 5/05 but I am not sure if this exists and where to look for it with Logix 5k controllers. I also do not know if this will resolve your issue. In my case, the default value of 30 minutes was causing me a similar headache. I changed it to 1 minute for my testing purposes.

I think you can disable and re-enable the connection and that will wipe the table. Same problem years ago and we just changed the target IP. it would be nice if we have access to clear the table.

Not sure how to delete the arp cache on the embedded device unless they give us a button to do it. However, if you go through a router that you can flush, that would solve your issue as the Logix will use the MAC of the gateway all the time; what is behind the GW will change so you would be moving the problem one step away from the embedded device to a device that does what you need. Many routers will allow this table management. Even if it doesn't, if it boots faster than 10-20min (many reboot in seconds, especially the consumer grade stuff), you could just reset it. However, a device to do this might cost 20$ or so anyway to have ability to delete entries. NAT could probably do something similar.

It's possible sometimes also to get the entry to be updated with a gratuitous ARP, but that would be very version specific and in practice does not work often (for me, anyway).

I know Schneider products and their IOScanner ModbusTCP service issues a new ARP everytime the connection is enabled, consistent with someone else's recommendation to try enabling/disabling the connection. Don't recall what Rockwell does, assume this is NOT EtherNet/IP Class 1/Implicit IO? Either explicit with TCP, or raw sockets?

Did pulling the cable work from the Logix device do anything? Sometimes, on link change, they may flush everything. Worth a shot, if you haven't tried.

Update:
I was able (with off line help from Ken) to achieve my goal although not as “elegant” as I would have liked. When I need to change devices (that have the same IP address) I send a reset command to the local Ethernet port via a message instruction. When the port resets it resends a gratuitous ARP which associates the IP address with the new device and I’m off and running again. The drawback is that the HMI that I created to control the process (I used Advanced HMI, great software) loses its connection as well but that would have happened if I cycled power to the PAC so this is still a better solution. More importantly I learned something about PAC’s that I didn’t know (that you can issue reset commends to the Ethernet port).
Thanks for the conversation.

OK so share the info. I assume it is a SSV.