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General Information on RS-485 Communication


General questions and best practices for RS-485 protocol including cable length and terminating resistors




Understanding best practices for installing, cabling, and terminating RS-485 networks can avoid a lot of problems with communication and hours of troubleshooting communication problems.



MS/TP stands for Master Slave Token Passing. Each device on the link is considered the master when it has the token. If it does not have immediate need to use the token, it is required to pass the token along to the next device. This is the token passing part. All devices on the link which do not currently have the token are regarded as slaves, and are expected to listen to any messages the current master may have for it. Because all devices take turns being master, the link is effectively peer to peer.

Electrically, MS/TP adheres to standard RS-485, physically wired using two-conductor shielded twisted pair cable.


RS-485 Cable Specifications

Cables used to form the RS-485 network should conform to the following specifications:

Wire Size: 22-24 AWG
Cable Type: Twisted-pair, copper wire, tinned
Shield: Braid
Nominal Impedance: 100-120
Velocity of propagation: 78%
Capacitance: <12.5 pF/ft between conductors and <22 pF/ft between the conductor connected to ground and the next conductor.


Daisy chain bus topology.

Star topology can be achieved using Infilink/B-Link repeaters.

Maximum Cable Length

Cable lengths of up to 4000 ft (1220m) daisy chained from controller to controller on one arm of a star are supported. After 4000 ft (1220m), a signal repeater (b-Link) is required to add more nodes to the network or further extend the cabling. Using a b-Link fiber repeater allows you to extend the length of RS-485 with fiber optic cable, recommended for running cable between buildings and through noisy environments.


Maximum Number of Devices

TAC's MS/TP RS-485 supports a maximum of 127 nodes overall. Each controller represents a single node on the network.


RS-485 Termination

The MS/TP field bus requires a 120 ohm terminator resistor at both ends of a daisy chain segment. The following diagram illustrates the connection of the terminator:


Also check out the SmartStruxure-RS485-Network Installation Quick-Help video on the Exchange.

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