Overview: How to wire RS-485 in non-isolated systems
RS-485 is widely used in industrial and building-automation systems because it offers robustness, simplicity, and strong noise immunity. However, engineers often encounter communication problems not because of the protocol itself, but because of incorrect wiring, especially in non-isolated RS-485 systems. For this reason, this article explains, in practical terms, how to connect A/B, Common, and Shield when a system does not use galvanic isolation.
A and B: The Differential Pair in RS-485 Wiring
The A and B lines form a differential signal pair. In practice, devices transmit data as the voltage difference between A and B, not relative to ground. Therefore, always route A and B as a twisted pair and use a linear (daisy-chain) topology. At the same time, avoid star connections and long stubs, as they degrade signal integrity. Additionally, install termination resistors (typically 120 Ω) only at the two physical ends of the bus, not at every node.
Common (Signal Ground): Mandatory for How to wire RS-485 in non-isolated systems
In a non-isolated RS-485 system, all transceivers share the same electrical reference. For this reason, every node must connect the Common wire to GND (0 V). In this role, the Common conductor keeps bus voltages within the allowed common-mode range of the receiver. Otherwise, differences in ground potential between devices cause unstable communication, corrupted data, or even permanent damage.
Important clarifications:
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- Common is not protective earth (PE)
- Common is not the cable shield
- Common is a signal reference and must remain continuous across all node
Shield: Noise Protection and Grounding Rules
The cable shield provides EMI and noise suppression, rather than a signal or ground return path. To prevent ground loops, connect the shield at one point only. Best practice: connect the shield to chassis or protective earth (PE). If PE is unavailable, connect the shield to GND, still at only one point. Never use the shield as Common.
Examples of Devices and How to wire RS-485 in non-isolated systems
In many cases, industrial and IoT devices use non-isolated RS-485 interfaces, which makes correct wiring especially important. For example, typical devices include:
- TCW242 – non-isolated RS-485 industrial IoT module
- TCW210-TH – Temperature and Humidity data logger with RS-485 interface
- TCG140-4 – 4G LTE IO module for remote monitoring and control
Wiring summary
In a non-isolated RS-485 system:
- Connect A and B to all nodes
- Connect Common to GND at every node
- Connect the shield to chassis or earth at one point only
Most RS-485 problems that engineers attribute to “noise” actually result from missing Common connections or incorrectly grounded shields.