Hi Pros,
Every applications so far that we programmed did not require us to use a PLC. We had enough process power in the UR and all the IO's that we needed. I saw many post on DoF referring to PLC. What are you criteria when you decide whether or not you need a PLC. This adds cost to the projects and we simply don't want to buy it when we don't need it.

When we need additional IOs only, we use the WAGO units. We have also used the WAGO PLC with the additional IOs. Both are much less expensive then traditional PLC. The UR can handle around 50 additional inputs and 50 additional outputs using the WAGO units through the Modbus TCP.

@Beachy
thanks a lot and that makes sense since the IOs are quite limited on the UR. Anyone else using really a PLC to control the robotic cell and probably having the robot as a slave in the cell?

We have not used a PLC in any of our applications. I highly recommend the WAGO I/O expansion over modbus TCP. I have tried the IO from Automation Direct and it works well and is almost half as much with two caveats. If you need very high speed we had issues but to be honest we didn't try any other IO, we went straight to the robot and it solved our issue. Also, we had a strange issue where sometimes we'd lose the connection and couldn't restore without a reboot. Seemed to occur only when someone pressed the emergency stop button but still something to think about. 

Back to the original question, I have seen an AB PLC used with the robot because the equipment that we were integrating into was ran by the AB and was an existing line that we added a robot to when I was still at SFEG. This was before UR added the Ethernet IP or prominent so it was basically a input/output handshake between the robot and the machinery. Similar to how we wire into older Haas mills.