# Difference between RS-485 and RS-422

Both RS-485 and RS-422 can be connected as multi-drop bus topology networks. Multi-drop means that nodes can be attached to the bus network in “party line” fashion. RS-422 networks, however, are only “quasi-multi-drop”. RS-422 networks use separate unidirectional twisted pairs in a twisted pair cable for transmit and receive. In contrast, RS-485 uses the same bi-directional twisted pair line for both transmit and receive.

Below is an example of a 4 node RS-485 network. Notice that each node can always “listen” to anything transmitted by any other node. Also notice that, for the “party line” scheme to work, only one node can be allowed to transmit at one time. Each node uses its TX_ENABLE line to turn on its own transmitter when it is time to transmit to the bus. Furthermore, each node must disable its own transmitter after the transmission is complete; otherwise all other nodes would be prevented from transmitting or receiving.

Example RS-485 Network

Below is an example of an RS-422 network. Notice that on one twisted pair, only one transmitter, the master, drives all the other slave receivers. On the other twisted pair, all slave transmitters are connected and may be enabled to drive the single receiver at the master. As with RS-485, only one node is allowed to transmit at one time.

Example RS-422 Master-Slave Network