T3-LB Direct Ethernet Connection

I have integrated a T3-LB with a device running Ubuntu 20.04. It appears that the device does not work when connect directly to the ethernet port of the server. I was able to resolve pinging and polling data by using a switch between the server and the T3-LB. Is there any autonegotiation for the ethernet port of the T3? I previously used a T3-TB and did not have this issue. Was this a change made in firmware somehow?

The T3 devices also don’t have auto crossover detection and it sounds like the same applies to your Linux server. You could try with a simple hub in between the two devices or you could use a crossover cable.

I had figured it did not. Just curious how a previous integration was successful, whereas this integration was not. The only thing that changes on the Linux server was the operating system (Ubuntu 18.04 to Ubuntu 20.04).

We can use a crossover cable in the future. Thank you for the quick reply.

Hard to say what might be the difference you’re experiencing there… hopefully the crossover cable fixes it. We’re standing by to dig into it more if you have problems.


I wanted to follow up on this. I was able to make 2 cables: 1 - Straight patch cable (568B at both ends) and 1 - Crossover cable (568A at one end and 568B) at the other.

In the first image, I was able to pull data from the T3-BB connected with the patch cable to the ethernet ports of the server and T3.

The second image show the ping intermittently dropping with a patch cable, and then failing for destination host with the crossover cable.

The final image show the modbus polling failing with the crossover cable.

In the field, we were using a T3-LB connected with a patch cable directly between ethernet ports. However we were not able to ping or modpoll the device. We installed a small, unmanaged switch between the two and this was resolved. Both devices are using the same firmware version 53.9.

We are just trying to ensure that we do not need a switch for production deployments.

Our T3 controllers don’t have the auto crossover detection but most laptops and PCs will have it and can connect with either a straight through or a crossover cable.

It sounds like your server doesn’t have auto detection like our T3 and (should) work with a crossover cable and direct connection. You’re showing that it doesn’t however, and there’s not much that could be going wrong here except a bad crossover cable. Would you be able to test out the cable on some other hardware there. Maybe we can set up a test here and see if we can repeat your results, what hardware are you using for the server.