RS-485 - Master-slave Arrangement

Master-slave Arrangement

Often in a master-slave arrangement when one device dubbed "the master" initiates all communication activity, the master device itself provides the bias and not the slave devices. In this configuration, the master device is typically centrally located along the set of EIA-485 wires, so it would be two slave devices located at the physical end of the wires that would provide the termination. The master device itself would provide termination if it were located at a physical end of the wires, but that is often a bad design as the master would be better located at a halfway point between the slave devices. Note that it is not a good idea to apply the bias at multiple node locations, because, by doing so, the effective bias resistance is lowered, which could possibly cause a violation of the EIA-485 specification and cause communications to malfunction. By keeping the biasing with the master, slave device design is simplified and this situation is avoided.