CompactLogix Ethernet gateway problem

Hi,

I had an issue with a compactlogix PLC last night and I could use a little advice if anybody has seen this before.

I was asked to change the IP address of a 1769-l32e and local HMI which were set for a private network to settings which would allow connection to the factory network.

The settings I was given to put in were :-
IP 172.16.4.14
sunbet mask 255.255.240.0
gateway 172.16.255.254

After getting these settings in via linx and establishing comms ok with the HMI everything looked good, however after powercycling the machine the PLC lost it's CIP connection to the HMI and linx.

I could still ping the IP address ok but could not connect and the NS light was solid red indicating a duplicate ip which was not the case.
(even disconnecting the PLC from the network and rebooting it would cause the same conditions.)

The only way I could reconnect to the PLC was to run bootp and reset the IP address to what it was already set to.

I was quite surprised to be able to see the boot p request because bootp and DHCP were both switched off in the PLC configuration.

I eventually managed to stop this happening by removing the gateway address completely.

Any suggestions guys?

OK, so searching the tech files seems to suggest that there may be a comms issue if the gateway is not seen as valid. and looking at the addresses they gave me it looks like the gateway address is outside of the range of the subnet mask. I think this is probably the issue.

It's a bit odd that I didn't get any error messages trying to set it and that it was happy to work until it rebooted.

I am always dubious about PLCs and IP Address/Subnet/Gateway as I have had issues with Rockwell PLCs in the past, especially with the challenging networks and subnets I have here.

I would like to think that software would challenge a setting that was incorrect, or are there too many variables for it to check them all?

I have an issue with an S7 not talking to my new INAT OPC server at the moment, and think that the subnest mask is incorrect, but being Friday afternoon, I do not want to try any changes at this late hour......

The gateway address is wrong for your subnet mask.

With your subnet mask the valid IP addresses for the network would be:
172.16.0.1 through 172.16.15.254

You gateway is unreachable as it must fall somewhere within that range. Check with the folks that gave you those settings and verify the gateway with them.

OG

EDIT: Well I should have read your second post a little closer before replying.

JohnCalderwood said:

I am always dubious about PLCs and IP Address/Subnet/Gateway as I have had issues with Rockwell PLCs in the past, especially with the challenging networks and subnets I have here.

I would like to think that software would challenge a setting that was incorrect, or are there too many variables for it to check them all?
....

Click to expand...

It's math. If the math doesn't work, the PLC isn't going to work. Software will challenge an invalid setting, but "incorrect" settings are tough without knowing all the parameters for your network.

OG

Thanks, yes you are right, that was the problem.
I was surprised by the nature of the fault though, the port was working fine after the changes and was communicating with the hmi fine until a reboot then it wouldn't talk to anything. I would have expected a bad gateway address to stop any connections working via the gateway. But it stopped talking to anything, even devices on the same subnet.

Yes, I would think the subnet communications would still work. But perhaps after the reboot it saw the bad gateway and just threw it's hands up and said "I quit".

I don't recall ever seeing anything about a subnet shutting down because of a bad gateway address before. But, there's always a first time I suppose.

OG