Delta RMC101 User Manual
Page 491
Serial (RS-232/422/485) 5.5
Communications
5-241
The RS-422 and RS-485 diagrams above show biasing and termination included. Termination
and biasing can be left out of networks at the expense of maximum cable distance and noise
immunity. See RS-422/485 Termination and Biasing for details.
Multi-Drop
Only RS-485 supports multi-drop. Multi-drop is the connecting of multiple slaves with a single
master. Slaves should be chained together. Neither a star topology nor a chain with long stubs
(wires from the main chain to the device) should be used. These topologies will cause excessive
ringing on the network and unreliable data transmission.
The following diagram shows a typical multi-drop chain. Notice that termination should only be
located at the extreme ends of the network:
The number of devices that can be connected to the network is dictated by the number of unit
loads that each represents. According to the TIA/EIA-485-A specification, there can be a
maximum of 32 unit loads connected to a single network. Each RMC represents ¼ unit load for a
total of 124 RMCs on the network, assuming the host is a unit load.
The following diagram shows one host with two RMC controllers in a daisy-chained two-wire RS-
485 configuration: