3 Discrete Design for Switchable Termination and Duplex Switching

System designers trying to design a common PCB with switchable termination resistor, so that same PCB can be used across all nodes in an RS-485 network, can use optical relay drivers, such as opto-MOS. Opto-MOS is a device that responds to a logic input signal and enables or disables a low resistance semiconductor switch. Optical isolation enables the bus to be at any common mode voltage independent of the reference of control signal. Image below shows this design and extends the idea to include switchable duplex – meaning with the use of two extra opto-MOS, a design can be built that can act as half duplex or full duplex, with termination enabled or disabled.

Issues with the previous design is that this occupies considerable board area and is expensive.