# Modbus On Multi-drop Networks

The major advantage of using multi-drop networks, like RS-485, on the Catalyst Modbus port is the ability to interface to multiple Modbus slave devices. In contrast, with RS-232C, only a single Modbus device may be interfaced. Either of the topologies shown above may be “designed” and wired-up to the Catalyst Modbus port interface connector.

Note that each node on the network must be configured with a different Modbus node number. Additionally, care must be taken to ensure that a method exists to control each node’s TX_ENABLE line. This is usually accomplished by use of the standard RS-232 signal called RTS (Request-to-send). However, some RS-232-to-RS485/422 converter devices employ an automatic, timer-driven transmit enable feature. Please refer to catalogs for products from Black Box, B & B Electronics and others. Some of the timer-driven transmit enable control interfaces have been tested by Raco and appear to function quite adequately on a Modbus RS-485 network.