If you use RS-485 to connect just one HMT130 to a master computer, enable the internal termination of HMT130 by switching switches 1 and 2 ON. Make sure that the master's end of the line is also terminated (by using the master's internal termination or with a separate terminator).
If you are connecting many transmitters to the same RS-485 bus, make sure that switches 1 and 2 are OFF and terminate the bus with separate terminators at both ends. This allows removing any transmitter without blocking the bus operation.