It doesn't sound like a bad CT unit because you are able to communicate with both units via 232. I'm pretty sure the driver commands are the standard strings so it doesn't make sense that the CT unit could be the problem. Make sure both CT units have their firmware updated and make sure none of the driver settings are the same. Even if you are not using IP control it might be possible you could have a conflict if both drivers were set to the same IP address. I had an unrelated issue in the beginning with the RS232 port being the same as another driver RS232 port even though I wasn't using that port at the time. It seems like the issue is in the command output or the driver setup.