Hello,
This can get complicated very quickly but, I feel, the path of least resistance is to connect the Ethernet port from each device (Robot, PLC, and Computer) to an unmanaged Ethernet switch with enough ports for each device you want to connect to. In your example you would need 4 ports minimum.
The critical thing is that each device (Robot, PLC, Computer) gets its own unique IP address like Robot 1 would be 192.168.0.20 and the other one would be 192.168.0.21. Most PLC’s that I’ve used default to 192.168.0.10 so you change that to match the robots or leave it if it’s the correct default.
Then hook your central computer up to the same Ethernet switch and set your central computer’s Ethernet IP address to something that matches the first three numbers of the robots (192.168.0.XXX), so 192.168.0.5 for instance.
I see in your question you can connect to one Robot via SSH but not the other one, so you understand that process. This should straighten out your network issues.
To answer the question about what each ethernet connection does, is a lot more complicated. This all depends on how you want the PLC to communicate to the Robots. Some options are Ethernet I\P, Modbus IP, or Discreet IO connections between the Robot and the PLC.
The “No Buffer Space Error” I’m not familiar with but a quick Google search provided a link to this forum ( Limit code amount - Technical Questions / Robot Communication - Universal Robots Forum ).
I hope this helps.
Kind regards,
–Jim