Synchronous EIA-449 Null Modems

The high-speed serial interface for Sbus (HSI/S) has a 37-way EIA-449 port. This type of port uses balanced transmission to provide improved speed and distance characteristics; therefore, there are two pins for each signal.

When both sides of the connection provide a clock signal, pins 17 and 35 (Terminal Timing) and pins 8 and 26 (Receive Timing) are crossed-over, as shown in Figure B-8.

Figure B-8 Synchronous EIA-449 Null Modem (both sides supply clock)

When one side of the connection provides a clock signal, pins 17 and 35 (Terminal Timing) on one side are connected to pins 8 and 26 (Receive Timing) on both sides, as shown in Figure B-9.

Figure B-9 Synchronous EIA-449 Null Modem (one side supplies clock)