The half-duplex optical module is also called BIDI optical module, which refers to both transmission and reception by a transmission line, but only one direction of data transmission at a time, that is, data is received after the data is sent. What's Simplex? Simplex means allowing. Half-duplex mode is a balance between simplex and full-duplex modes, providing dual communication but with some compromises in speed and performance. They can be transmitted in both directions at the same time when they are transmitted by two different transmission. The RS-485 bus standard is one of the most widely used physical layer bus interface in protection, control, and monitoring applications. The RS-485 interface could be half duplex of full duplex. FireFly optical modules are currently available in.