With RS-485 communication, multiple devices share the same pair of data lines.
In a daisy-chain (bus) topology:
One master controls the communication.
Several slave devices are connected along the same cable.
All devices share the same differential data lines: A (–) and B (+), and usually a common ground (GND).
Daisy chaining is the recommended and standard connection method for RS-485 networks because it keeps wiring simple and minimizes signal reflections when done correctly.
RS-485 daisy-chain wiring and topology
Key components
Master device
The controller of the network. It sends commands and initiates communication.Slave devices
Devices that receive messages from the master and, when allowed, send responses back.
They all connect to the same A, B, and GND lines.Data lines A (–) and B (+)
RS-485 uses differential signaling:Data is represented by the voltage difference between A and B.
All devices must connect A to A, and B to B, throughout the entire bus.
Ground (GND)
While not strictly required by the RS-485 standard, it is strongly recommended to:Connect GND between all devices.
Improve signal reference and noise immunity, especially over long distances.
Linear (bus) topology
In a correct RS-485 daisy-chain:
All devices are wired in line along one continuous cable:
A (–) line from master to slave 1 to slave 2 … to last slave.
B (+) line from master to slave 1 to slave 2 … to last slave.
GND line shared between all devices.
The bus is linear from one end to the other.
Step-by-step: how to wire a daisy-chained RS-485 bus
Start at the master
Connect the master’s:
A (–) terminal to the cable’s A conductor.
B (+) terminal to the cable’s B conductor.
GND to the cable’s ground conductor.
Run a single main cable
Use one continuous length of cable from the master to the last slave.
Avoid unnecessary junctions and branches.
Connect each slave in sequence
At each slave device:
Connect A (–) in to the bus A line.
Connect B (+) in to the bus B line.
Connect GND to the shared ground.
If the device has separate “in” and “out” terminals, simply bridge in→out so the bus passes through.
Terminate the bus at both ends
Place a 120 Ω termination resistor:
At the first device at one end of the bus.
At the last device at the opposite end of the bus.
Only the two physical ends of the cable should be terminated.
Check polarity and grounding
Ensure A is A and B is B across all devices (no swaps).
Ensure GND is properly connected across all devices, especially for long runs.
How communication works on a daisy-chained RS-485 bus
In a daisy-chained RS-485 network:
The master sends messages on the shared A/B lines.
All slave devices receive every message because they are all on the same bus.
Each slave:
Checks whether the message address or ID matches its own.
Ignores messages not meant for it.
When a device needs to respond:
It only transmits when:
The master requests data, and
The protocol or master logic allows that specific slave to talk.
Only one device must transmit at a time to avoid collisions.
This bus behavior ensures:
Ordered, predictable communication.
Efficient use of a single pair of data lines.
Practical design limits and recommendations
Termination resistors
Use a 120 Ω resistor at each end of the RS-485 cable.
Do not place termination at intermediate devices.
Proper termination:
Reduces signal reflections.
Improves signal integrity, especially for long cables and high baud rates.
Number of devices
A standard RS-485 network typically supports up to:
32 devices total (1 master + up to 31 slaves) with standard load transceivers.
You can increase this number by:
Using low-load (1/4 unit load) RS-485 transceivers.
Adding RS-485 repeaters to split the bus into segments.
Always check your device and transceiver specifications for the exact supported node count.
Cable type
Use twisted pair cable for A and B:
Minimizes electromagnetic interference (EMI).
Preserves the integrity of the differential signal.
For longer distances or noisy environments, consider:
Shielded twisted pair.
Proper grounding and shield termination according to your installation rules.
Line length
As a general guideline:
RS-485 can support up to 1,200 m total cable length.
The actual maximum distance depends on:
Baud rate (higher speed → shorter maximum distance).
Cable quality and type.
Environmental noise.
If you need very high baud rates or very long runs, you may need to reduce speed, use better cable, or add repeaters.
Common wiring mistakes to avoid
Below are typical incorrect wiring patterns that cause unstable RS-485 communication.
Other problem you may encounter
1. Swapping A and B on some devices
Problem:
A (–) and B (+) are reversed on one or more devices.
That device will not receive or transmit correctly.
It can also disturb the bus for other devices.
Correct approach:
Double-check that:
All A (–) terminals are connected to the same conductor.
All B (+) terminals are connected to the other conductor.
Follow the labeling of each device; note that some manufacturers use different naming (e.g. D+ / D–) but they must still match consistently.
2. No shared reference (GND)
Problem:
Only A and B are connected, with no shared ground.
At shorter distances it may still work, but at longer runs:
Noise and ground potential differences can cause data errors.
Correct approach:
Connect a GND reference between all devices along the bus.
Follow safety and grounding rules for your installation, especially across buildings or long outdoor runs.
By following a linear bus topology, using correct termination, and avoiding stars and long branches, your RS-485 daisychained network will be much more stable, easier to debug, and more reliable over time.