RS485 Networks – Multiple Protocols
RS485 defines the physical layer of a serial communication network. It specifies electrical signaling characteristics, wiring rules, and how devices share a common transmission medium. What RS485 does not define is the communication protocol that runs on top of it.
In theory, RS485 can be used with almost any protocol. In practice, however, careful coordination is required because the RS485 trunk is a shared medium. If two or more devices transmit at the same time, message collisions occur and both transmissions are corrupted.
The principles discussed here apply to both half-duplex and full-duplex RS485 networks.
It is technically possible to run multiple protocols on the same RS485 trunk, but only under very specific conditions. The most important requirement is that no two devices transmit simultaneously. Without strict control of transmission timing, reliable communication cannot be guaranteed.
This requirement leads to several important consequences that system designers must understand before attempting to mix protocols on a single RS485 network.
Poll / Response Protocols
Multiple protocols can coexist on the same RS485 trunk if—and only if—they are poll/response type protocols. Examples include:
- Modbus RTU
- Rockwell DF1
- Other master-slave serial protocols
In these systems, a single master device initiates all communication. Slaves never transmit unless explicitly polled. This prevents collisions and ensures deterministic access to the bus.
Token-Passing Protocols
Token-passing protocols, such as BACnet MS/TP, operate very differently. In these systems, multiple devices may act as masters, but only the device holding the token is allowed to transmit.
Because the token-passing mechanism tightly controls when each device may talk, it is not compatible with other protocols sharing the same RS485 trunk. Introducing another protocol would break the timing rules and result in collisions or loss of token integrity.
As a result, if a token-passing protocol is present on an RS485 trunk, no other protocol may be used on that same trunk.
Master Control Requirements
When running multiple poll/response protocols on a single RS485 network, there must be one—and only one—master controlling all communications.
The master is responsible for:
- Determining which protocol to use for each poll
- Controlling transmission timing
- Managing response timeouts
- Ensuring no overlap between protocol frames
Without a single coordinating master, it is impossible to prevent collisions on the RS485 bus.
Practical Implementation Considerations
If you have full control over an RS485 port, you may use that port to transmit a poll using one protocol, wait for the response, and then transmit a poll using a second protocol. This approach can work reliably when implemented carefully.
However, if two or more uncoordinated applications attempt to share the same RS485 port—each unaware of the other—then running multiple protocols is not possible. In such cases, collisions are unavoidable.
In real-world systems, the safest and most maintainable approach is usually to dedicate each RS485 trunk to a single protocol or to use protocol gateways to isolate and translate between networks.
Summary
- RS485 defines the physical layer, not the protocol
- Multiple protocols may share an RS485 trunk only if they are poll/response
- Token-passing protocols cannot coexist with other protocols
- A single master must control all communications
- Uncoordinated applications must never share an RS485 port
Understanding these constraints is essential for designing stable, predictable RS485-based automation systems.