Communication with PLC5 (from compactlogix)

Hello,

I am trying to read data over Ethernet from a PLC5 ethernet enabled processor to my compactlogix processor. I understand that it's necessary to use the msg instruction and I believe that I have it set up correctly.

The problem is that I am unable to see the PLC5 in rslinx, nor do I recieve a response when pinging the processor from my laptop. What could be the issue? I am able to ping the PLC5 from an existing programming PC, I have set my laptop to have the same Subnet mask as the programming PC. I am able to get a response from a ping to the programming computer via my laptop.

I have noticed that in RSLogix 5 in the IO Configuration that channel 2 has a subnet mask of 255.255.255.254

The programming computer has a subnet mask of 255.255.255.0

How is it that these two are communicating? What is stopping my laptop from seeing this PLC5?



Thanks for the help,
Will

Whomever set up that programming laptop and PLC-5E was specifically configuring only those two devices to talk to one another.

You can read all about Subnet Masking in any TCP/IP tutorial (and I'm not an expert), but the short summary is that the PLC-5 will only be able to communicate with an IP address whose last octet is the PLC-5's address +1.

Example:

PLC-5 IP Address: 192.168.1.2
PLC-5 Subnet Mask: 255.255.255.254

Look at the last octet in binary:


So the last octet of the only IP address the PLC-5 can communicate must be different from the IP address of the PLC-5 itself by +1, so the programming laptop must be 192.168.1.3.

The most common solution for this would be to change the last octet of the PLC-5's subnet mask to zero, so it could communicate with any of the hosts on the 192.168.1.xxx subnet (254 available).