It is 485, but in this case the termination resistor was not the issue - it was another HMS device. Specifically, the IXXAT SG connects via 2 wire RS485 to a Modbus serial to TCP gateway which provides network isolation into the internal power plant SCADA network. This Modbus RTU to TCP gateway in turn talks to several Modbus TCP slaves re-writing the requests coming from the IXXAT (RTU master) as TCP requests and re-writes the TCP responses from the slaves as RTU responses which the IXXAT gets as if the slaves were RTU and connected to the IXXAT on the RS485 bus. The Modbus RTU to TCP gateway maps different slave IDs on the serial side to the TCP/IP slave devices on the ethernet side.
Because we re-located the panel back to my lab I had to configure some hardware to stand in as the slaves the system normally talks to in the plant. I used two PLCs for this and an older HMS AnyBus SG device; after running tyhe RTU to TCP gateway in debug mode I saw that the HMS slave was not responding to the requests from the master fast enough (or at all in some cases) so I replaced it with a PC running a Modbus slave application to stand in as the same plant device and now everything works.
This was particularly hard to troubleshoot because there is no way (that I am aware of) to see a log of the actual bytes sent and received through the serial port on the IXXAT SG device. That would have allowed me to see that the requests to that particular slave ID (mapped by the Modbus gateway to a given TCP slave) was not responding.