Approved in 1983, RS-485 (TIA/EIA-485) remains a foundational physical-layer standard for industrial, medical, and embedded systems. Its longevity comes from one strength: robust differential signaling that survives noise, distance, and harsh electrical environments. This guide distills the core rules engineers need to design reliable RS-485 hardware.
⚙️ Electrical Standard and Core Characteristics #
RS-485 defines electrical behavior only. It specifies driver and receiver characteristics for balanced multipoint links, but leaves protocols and connectors to higher layers.
Key parameters
- Signaling: Differential (A/B pair)
- Supply: Typically single 5 V (modern parts may support 3.3 V)
- Common-mode range: –7 V to +12 V
- Bus loading: 32 unit loads (UL) per segment
- Maximum data rate: 10 Mbps (short distances)
- Maximum distance: ~1200 m / 4000 ft (≈100 kbps)
These limits are tightly coupled: speed, distance, and noise margin trade off against each other.
🔀 Topology: Bus Discipline Matters #
RS-485 is designed for a linear bus (daisy-chain). Star or tree layouts introduce impedance discontinuities that cause reflections and eye closure.
Duplex options #
| Mode | Wires | Characteristics |
|---|---|---|
| Half-duplex | 1 twisted pair | Most common; transmit and receive are time-shared |
| Full-duplex | 2 twisted pairs | Simultaneous TX/RX; higher cost, simpler firmware |
In half-duplex systems, firmware must explicitly control the driver enable (DE) signal to avoid bus contention.
📡 Signal Integrity: Termination and Stubs #
At higher speeds or longer distances, RS-485 behaves as a transmission line. Proper termination is non-negotiable.
Termination strategies #
- End termination
A single 120 Ω resistor at each physical end of the bus, matching cable impedance. - Split termination
Two 60 Ω resistors with a center capacitor to ground, improving common-mode noise rejection.
Only the two extreme nodes should be terminated. Intermediate nodes must not add termination.
Stub length rule #
Stubs act like unterminated transmission lines. Keep them short: $$ [ L_{stub} \le \frac{t_r \cdot v \cdot c}{10} ] $$ Where (t_r) is the driver rise time and (v) is the cable velocity factor. In practice, keep stubs under a few centimeters for high-speed designs.
🧲 Fail-Safe Biasing and Idle State Control #
A floating RS-485 bus is undefined when all drivers are disabled. Fail-safe biasing forces the receiver into a known logic state during idle, open-circuit, or shorted conditions.
- Integrated fail-safe: Modern transceivers provide ~10–50 mV of internal bias.
- External biasing: Pull-up and pull-down resistors can raise the idle differential to >200 mV, improving noise immunity in industrial environments.
External biasing should be applied once per bus, typically near the master or power entry point.
🔌 Grounding, GPD, and Isolation #
The most common RS-485 failure is not EMI—it is ground potential difference (GPD) between distant nodes. Even a few volts can exceed the common-mode range and destroy transceivers.
Best practice: galvanic isolation #
- Signal isolation: Digital isolators or isolated RS-485 transceivers
- Power isolation: Isolated DC-DC converters
Isolation breaks ground loops and allows systems to tolerate hundreds or thousands of volts of potential difference, turning a fragile link into an industrial-grade one.
🧾 Practical Design Checklist #
- Cable: 22–24 AWG shielded twisted pair, 120 Ω characteristic impedance
- PCB layout: Route A/B as a tight, length-matched differential pair
- Node count: Use 1/8-UL transceivers to scale up to 256 devices
- Protection: Add TVS diodes for ESD and surge at the connector
- Topology: One bus, two terminations, minimal stubs
🧠 Closing Notes #
RS-485 endures because it rewards discipline. When engineers respect bus topology, termination, biasing, and isolation, the result is a communication link that runs quietly for decades. Most “mysterious” RS-485 bugs can be traced back to breaking one of these fundamental rules.
Design it like a transmission line—not a UART wire—and RS-485 will not disappoint.