PLC5 and Ethernet collisions

Noticed on the PLC5 webpage there are a number of collisions reported. Reset the counters and can watch it rise.

This PLC5 is only communicating with a supervisory L71(EN2T). The EN2T is not reporting any collisions. The L72 is both reading and writing to the PLC5 through multiple messages. Both are connected to our facilities' VLAN.

Not a networking expert here but I understand collisions are not good and should not be seeing any. ??

I also noticed that all our PLC5's, on the network, are set on 10M/half. Not sure the reasoning here other than its older Ethernet technology?? IT did move this one to another switch port earlier this year due to a network infrastructure update. Included new copper run.

Temporarily set this one to 100M/Auto and it seemed to talk ok but was still getting collisions.

What do I need to look at to help reduce collisions?

Thanks!

Disconnect the PLC5 from the network temporarily and ping its IP address.


You shouldn't get a response with the PLC5 offline, if you do something else has the same IP address.

Most PLC-5E and 1785-ENET modules have an "AUI" connector with an Ethernet transceiver installed: most are RJ-45 style twisted-pair Ethernet, but you can also buy coaxial and fiber optic transceivers.

All of those will be 10 Mb/Half Duplex devices. Only the PLC-5E Series F controllers have an upgraded Fast Ethernet daughtercard with an RJ45 connector directly on the front of the controller. Those will support 10/100 Mb, Half/Full duplex, and auto-negotiation.

You can usually connect an older PLC-5E with its 10/Half port to a switch that auto-negotiates without seeing any serious effects. Many switches will periodically attempt to re-negotiate for a 100 Mb or full-duplex link, which leads to an occasional packet loss, but is usually not a big deal because TCP/IP retries so fast and reliably. I still prefer to hard-set the ports to which a PLC-5E or SLC-5/05 connects to 10/Half.

>I understand collisions are not good and should not be seeing any. ??

Click to expand...

In the dusty ancient days at the beginning of the Ethernet era, collisions were literally how the network functioned. Devices listened for a clear link, then transmitted when they wanted to, and listened to see if the transmitted data and the received data was the same, which proved no other device had also taken the opportunity to transmit at the same instant (a collision). When the message heard was garbled compared to the message transmitted, that was considered a collision and each device backed off for a random period of time and tried again.

So a half-duplex device like a PLC-5E will always see some collisions, because there will be broadcast packets and ordinary switched traffic that arrives at the same time as it is trying to transmit. The problem is when it sees "too many" collisions, which can indicate a defective or noisy link. And of course the definition of "too many" can be complicated.

Can confirm Kens reply - I however have never had good luck leaving switchports in Auto Negotiate on an PLC5 in Hdx mode. I always set them for 10 hdx and have no issues when they try to overachieve _