Selection of the signal pair is via internal jumpers. Isolation is an important consideration if components of a system use different power sources, have noisy signals or must operate at different ground potentials. The Model 281 requires no external power to operate.
All power is derived from the signals applied to the ports.
The Model 281 will operate if only TD, RD and Grounds are connected. The DCE port is implemented with a DB25 male connector while the DTE port uses a DB25 female connector. The case is aluminum and is not connected to either port.