RS-485 — Physical Layer from the Ground Up

What is RS-485

RS-485 (also known as EIA-485) is an electrical standard for serial transmission. It does not define a protocol — only what the signal looks like on the wire. Modbus RTU, Flowbus, PROFIBUS and many other industrial protocols all run over RS-485.

Key feature: differential transmission. Instead of one signal line referenced to ground, RS-485 uses a pair of wires, A and B. The receiver measures the voltage difference between them. This effectively rejects common-mode interference (from VFDs, motors, etc.).

Bus topology

RS-485 is a linear bus — devices connected in series, one after another:

Master ──┬── Slave 1 ──┬── Slave 2 ──┬── [Terminator 120Ω]
         │             │             │
[Term.]  │             │             │
 120Ω    A/B          A/B           A/B

Do not create branches (star or tree topology) — signal reflections cause communication errors, especially at higher speeds.

Termination

There must be a 120 Ω resistor between lines A and B at both ends of the bus. Missing termination is the most common cause of unstable communication — everything works on a short cable but falls apart on a longer one or with more devices.

Many devices have a built-in termination switch (jumper or DIP). Enable termination only at devices on the ends of the line, not in the middle.

Bias resistors

When the bus is idle (no one is transmitting), lines A and B can float in an undefined state. Bias resistors force a defined state:

• Line A pulled to +5V through ~560–680 Ω
• Line B pulled to GND through ~560–680 Ω

Usually fitted only on the master or on one device on the bus. Too many bias resistors load down the line.

Grounding — GND matters

RS-485 is differential, but the receiver needs a common reference potential with the transmitter. Without a common signal ground, the potential difference between device grounds can go outside the receiver's input range (±7V) and damage the circuitry.

In practice: always run a third wire as a common signal ground (not to be confused with PE — protective earth).

USB–RS485 converters

To connect a computer to an RS-485 bus, you need a converter. Popular chips:

Cheap converters with no chip markings can have problems with automatic TX/RX direction switching — they show up as a chopped first frame or echo.

Transmission speeds

The standard doesn't mandate specific baud rates. In industrial practice you'll most often see:

• 9600 bps — default for many Modbus devices
• 19200 bps
• 38400 bps
• 115200 bps — maximum for most cheap USB converters

On long cables (>100 m) stick to lower speeds (9600–19200). General rule: baud rate × cable length [m] ≤ 10^8.

Pitfalls

1. A and B swapped — the standard defines A as "−" and B as "+" in the Mark state (logic 1). Some manufacturers label it the opposite way. If communication doesn't work, swap A↔B.

2. Common ground — missing a third wire for common ground is the most common installation oversight.

3. Branches — every branch (stub) longer than a few centimeters causes reflections. If you must branch, use an RS-485 repeater/hub.

4. Too many terminators — if you enable termination on every device, the total resistance drops too low and loads the driver.