RS-485 Trcvr
Translates a UART TX into a differential pair (A,B) swinging about ±2 V around a common-mode point, and recovers a logic-level RX from the difference even when up to ±7 V of common-mode noise rides on top.
A push-pull driver actively drives A high and B low for a logic 1 and inverts for a 0; a tri-state DE pin lets multiple drivers share the bus half-duplex. The receiver is a high-impedance differential comparator with ±200 mV threshold and fail-safe pull-up/down resistors so the output is defined when no driver is active. End-of-line 120 Ω terminations match the cable impedance and prevent reflections.
In plain terms
Like passing notes by lifting one hand and dropping the other rather than just lifting one — the receiver only watches the difference between hands, so a passing truck shaking both hands together is invisible.
Why designers use it
- Reject industrial noise — the differential ±200 mV threshold rides through ground shifts, motor drive transients, and welder EMI that a single-ended line couldn't survive.
- Multi-drop topology — one twisted pair links 32 (or 256 with low-load receivers) nodes, slashing wiring against star topologies in factory floors and building control.
- Long reach — at 100 kbit/s, RS-485 carries 1200 m of CAT-5; even at 10 Mbit/s it manages 12 m, far beyond UART or SPI.
- Fail-safe biasing — an idle bus reads as a defined logic level rather than triggering spurious framing errors.
Best for
- Modbus-RTU
- DMX-512
- BACnet MS/TP
Key specifications
- Differential signalling: −7 V to +12 V common-mode
- Data rate: 100 kbit/s – 50 Mbit/s
- Bus length: Up to 1,200 m at 100 kbit/s
- Nodes: 32 standard, 256 with low-load parts
- Supply: 3.3 V or 5 V
When not to use it
- Inside a single board where a few cm of trace separate ICs — the bus capacitance and termination losses waste power; use plain UART or SPI.
- When you need true full-duplex on one cable — RS-485 is half-duplex on a single pair; pull RS-422 (two pairs) for full-duplex.
Common mistakes
- Putting 120 Ω terminations on every node instead of just the two ends — the driver loads down to <30 Ω and the swing collapses below the receiver threshold.
- Forgetting to bias the bus with fail-safe resistors — when no driver is active, the floating differential reads garbage and UARTs spit framing errors during every direction reversal.
Where you will find it
- A theatre lighting rig uses a Maxim MAX485 transceiver per node to chain 96 DMX-512 fixtures off one console: the 250 kbit/s differential signal hops through XLR-3 cables for hundreds of metres and shrugs off the dimmer-rack SCR noise sitting half a metre away.
- A grain elevator's Modbus-RTU temperature monitoring loop runs 1100 m of unshielded twisted pair through a humming silo, polling 28 ADAM-4117 modules each carrying a Texas Instruments SN65HVD75 transceiver: the ±15 kV ESD-protected differential receiver keeps the link up through every lightning-induced common-mode surge.
- A solar inverter farm's SunSpec Modbus link daisy-chains 64 SMA Sunny Tripower units over RS-485 using Analog Devices' LTC2862 isolated transceiver: the integrated digital isolator severs the ground loop that would otherwise let one inverter's switching noise corrupt every neighbour's communication.
A short history
RS-485 is a balanced differential serial transmission standard: a driver creates a voltage difference between two wires rather than referencing a single wire to ground, which improves noise immunity and allows longer cable runs than single-ended signaling. RS-485 networks typically connect multiple nodes in parallel on one shared bus, supporting multipoint communication. The standard defines only electrical characteristics of drivers and receivers, not a communication protocol, so higher-level protocols often reference it for their physical layer.
Good to know
- RS-485 lets a single twisted pair carry data over 1.2 km in factory floors — the industrial backbone that survived USB and Ethernet for sensor buses.
- Most RS-485 transceivers are half-duplex; the bus-direction control pin (DE/RE) must be flipped between every read and write.
- The 120 Ω terminators at each end of an RS-485 cable absorb reflections — forget them and the bus won't pass 100 kbit/s.