RS-485 2-Wire Connection Diagram
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Wire an RS-485 half-duplex network correctly with differential pair A/B lines, 120-ohm termination resistors, fail-safe bias resistors, and proper grounding for reliable industrial communications.
RS-485 (TIA/EIA-485) is a balanced differential serial communication standard designed for multi-drop, long-distance industrial networks. Unlike RS-232 which references signal voltages to a single ground, RS-485 encodes data as the voltage difference between two conductors — designated A (non-inverting, also called D+ or +) and B (inverting, also called D− or −). A logic 1 (MARK) is represented when A is more positive than B by at least 200 mV; a logic 0 (SPACE) is represented when B is more positive than A by at least 200 mV. This differential scheme gives RS-485 excellent immunity to common-mode noise.
A 2-wire (half-duplex) RS-485 network uses a single twisted pair for both transmit and receive directions, sharing the bus using a direction-control signal. All nodes connect in a daisy-chain (bus) topology — not star or ring — with exactly one 120 Ω termination resistor at each physical end of the bus. The characteristic impedance of typical RS-485 twisted pair cable is 100–120 Ω, so terminating with 120 Ω minimises reflections that cause data corruption at high baud rates.
Bias resistors are mandatory for correct idle-state behaviour. When no driver is actively transmitting, the A and B lines float to an indeterminate voltage. The receiver's input threshold is ±200 mV, so a floating line — which may be near 0 V differential — is ambiguous and causes the receiver to output a stream of noise. Fail-safe biasing uses a resistor from A to supply (pulling A high) and a resistor from B to ground (pulling B low), ensuring a defined MARK (logic 1) state on the idle bus. Typical bias resistor values are 470 Ω to 1 kΩ, applied only once on the network — at the master or at one end node.
The signal reference conductor (sometimes called the signal common or shield ground) must be connected between all devices to keep common-mode voltage within the RS-485 receiver's tolerance (typically −7 V to +12 V). Without it, ground potential differences between distant nodes cause common-mode voltage to exceed the receiver input range, corrupting or destroying the devices. Use a three-conductor cable (A, B, and signal common) or run a separate reference conductor alongside the twisted pair.
RS-485 supports up to 32 unit loads (UL) on a standard driver, or up to 256 with 1/8 UL receivers. Maximum cable length at 100 kbaud is approximately 1200 m; at 10 Mbaud, cable length drops to around 12 m due to the baud-rate × distance product limit of the standard.
How to wire rs485 2 wire connection diagram
- Plan the bus topology RS-485 requires a linear daisy-chain bus: node 1 → node 2 → node 3 → ... → node N. Avoid star connections or T-junctions. If a star is unavoidable, keep stub lengths below 1/10 of the signal wavelength at the operating baud rate. In practice, keep stubs under 1 m for baud rates up to 115.2 kbaud.
- Select appropriate cable Use a shielded twisted pair (STP) cable with a characteristic impedance of 100–120 Ω and a capacitance of less than 50 pF/m. Common choices include 24 AWG (0.2 mm²) or 22 AWG (0.35 mm²) STP cables purpose-made for RS-485. Avoid using generic non-twisted or flat cable — untwisted conductors pick up noise and create impedance mismatches.
- Connect A and B lines at every node At each device, connect the cable's A conductor to the device's A (D+, non-inverting) terminal and the cable's B conductor to the device's B (D−, inverting) terminal. Daisy-chain — run to the first device, then from its terminals run a new segment to the next device, continuing to the last node.
- Install termination resistors at both physical ends only Solder or fit a 120 Ω resistor (1% metal-film, 0.25 W) across the A and B terminals at the first node (one bus end) and at the last node (other bus end). Do not install termination resistors at any intermediate nodes — this is one of the most common wiring mistakes.
- Install fail-safe bias resistors at one location At the master or one bus end, connect a resistor from A to the positive supply (pull-up) and a resistor of equal value from B to ground (pull-down). Typical values are 560 Ω to 1 kΩ. This ensures a defined idle-bus state. Verify the chosen bias resistor value still meets the 375 mV minimum differential margin required at the furthest receiver.
- Connect the signal reference conductor Connect a third conductor (or the cable drain wire) from the signal common (GND) reference of every device on the bus. Terminate this reference wire at one end only to avoid ground loops. Keep it separated from the protective earth (PE) at devices where the system uses isolated power supplies, unless the system design specifically calls for a combined reference.
- Connect the cable shield Connect the cable shield to protective earth (PE) at one end only — typically the master or the supply end — to prevent circulating ground-loop currents from flowing through the shield. Leave the shield floating or connected via a capacitor (e.g., 10 nF) at the other end to provide RF bypass without creating a DC ground loop.
Specifications
| Standard | TIA/EIA-485-A (RS-485) |
|---|---|
| Signal type | Differential, half-duplex (2-wire) or full-duplex (4-wire) |
| Differential voltage threshold | ±200 mV minimum at receiver input |
| Common-mode input range | −7 V to +12 V (standard transceivers) |
| Termination resistance | 120 Ω (matches typical cable characteristic impedance of 100–120 Ω) |
| Maximum nodes (standard 1 UL transceivers) | 32 per bus segment |
| Maximum cable length at 100 kbaud | Approximately 1200 m |
| Maximum baud rate (short cable) | Up to 50 Mbps (cable length dependent) |
Safety warnings
- RS-485 transceivers operate at signal voltages that are generally safe (3.3 V or 5 V logic), but are often connected to industrial equipment operating at mains voltages. Always isolate mains-powered equipment and verify it is de-energised before connecting or disconnecting RS-485 wiring. Follow applicable wiring regulations (NEC/NFPA 70, BS 7671, IEC 60364) for any mains-connected work.
- Ground potential differences between RS-485 nodes in industrial environments can be tens of volts. Verify common-mode voltage between nodes before connecting equipment. If common-mode voltage exceeds the transceiver's tolerance (typically −7 V to +12 V), use optically isolated RS-485 transceivers.
- Cable shield grounding must be done with care: grounding the shield at both ends creates a ground loop that can carry large circulating currents during ground faults or transients, potentially damaging equipment or causing electric shock.
- Never connect RS-485 signal wires to untested circuits without confirming signal levels. Connecting RS-485 inputs to sources exceeding the transceiver's absolute maximum input voltage will permanently damage the IC.
- Long outdoor cable runs are susceptible to lightning-induced surges. Install surge-protective devices (SPDs) or optically isolated repeaters at building entry and exit points in accordance with IEC 62305 lightning protection standards.
Tools needed
- Digital multimeter (voltage and resistance measurement)
- Oscilloscope with differential probes (for signal integrity verification)
- Serial protocol analyser or RS-485 bus monitor
- Cable impedance tester or Time Domain Reflectometer (TDR) for long runs
- Crimping tool and RS-485 terminal block connectors
- Cable stripper and wire ferrule crimper
Common mistakes
- Installing termination resistors at every node instead of only the two physical bus ends, which overloads the driver and reduces signal amplitude below receiver threshold.
- Using a star topology instead of a daisy-chain bus, creating impedance mismatches and reflections that corrupt data.
- Omitting the signal reference conductor, causing common-mode voltage excursions that corrupt data or damage transceivers.
- Connecting the cable shield to protective earth at both ends, creating a ground loop that injects noise onto the reference conductor.
- Using untwisted or flat cable instead of twisted pair, resulting in poor noise immunity and EMI susceptibility.
- Installing fail-safe bias resistors at every node, reducing the effective pull-up/pull-down resistance and overloading the driver during transmission, causing insufficient differential voltage at far-end receivers.
Troubleshooting
- Communication works at short cable lengths but fails at longer runs
- Cause: Missing or incorrect termination, causing reflections; or cable capacitance causing signal degradation at high baud rates Fix: Verify that exactly two 120 Ω termination resistors are present — one at each physical bus end and nowhere else. Reduce the baud rate to see if reliability improves. Use a TDR or oscilloscope to observe reflections. Switch to lower-capacitance cable if necessary.
- Receiver outputs continuous noise when no device is transmitting
- Cause: Absent or insufficient fail-safe bias resistors, leaving A and B lines floating near 0 V differential Fix: Install pull-up (A to Vcc) and pull-down (B to GND) bias resistors at one bus location. Calculate the values to maintain at least 375 mV differential margin across all load conditions.
- Intermittent communication errors that worsen near electrical machinery
- Cause: Common-mode noise pickup due to missing signal reference conductor, inadequate shielding, or ground loops Fix: Verify the signal reference conductor is connected at all nodes. Check shield grounding — it must be grounded at one end only. Add ferrite beads and TVS diodes at transceiver inputs. Consider switching to optically isolated transceivers.
- Transceiver ICs running hot or failing
- Cause: Common-mode voltage exceeding the transceiver's absolute maximum input range due to large ground potential differences between nodes Fix: Measure the DC voltage between the signal commons of different nodes. If it exceeds ±7 V, replace standard transceivers with optically isolated RS-485 modules rated for the measured common-mode range.
- Communication fails when a specific device is connected
- Cause: Faulty transceiver with shorted or leaky output, loading the bus; or the device is driving the bus continuously without proper direction control Fix: Disconnect devices one at a time to isolate the faulty node. Measure differential voltage across A and B with the suspect device's transceiver driver disabled; if it remains near 0 V, the device has a leaky or shorted output stage.
Frequently asked questions
Why are exactly two 120-ohm termination resistors needed on an RS-485 bus?
Each physical end of the bus must have one 120 Ω resistor placed across the A and B conductors. This matches the cable's characteristic impedance (typically 100–120 Ω) and absorbs signal energy at the cable end to prevent reflections. More than two terminations (e.g., at every node) loads the driver excessively and degrades signal amplitude.
What is the difference between the A and B terminals on an RS-485 transceiver?
A (non-inverting, D+, or '+') is the line that sits at positive voltage during a logic 1 (MARK) state. B (inverting, D−, or '−') sits at negative voltage relative to A during a logic 1. When A is more negative than B, the receiver decodes a logic 0 (SPACE). Note: some manufacturers label their terminals in reverse — always verify against the device datasheet.
How many devices can I connect to a single RS-485 bus?
A standard RS-485 driver supports 32 unit loads (UL). One standard transceiver = 1 UL. Modern 1/8 UL or 1/4 UL receiver chips allow 256 or 128 nodes respectively on a single bus without repeaters. Beyond these limits, use an RS-485 repeater to extend node count or cable length.
Do I need a ground reference wire in addition to A and B?
Yes. The RS-485 standard requires a signal reference conductor (signal common) connecting all nodes. Without it, differences in ground potential between nodes — often several volts in industrial environments — push the common-mode voltage outside the receiver's tolerance range (typically −7 V to +12 V), causing data errors or transceiver damage.
Where should the fail-safe bias resistors be placed on the RS-485 bus?
Fail-safe bias resistors should be installed at only one location on the bus — typically at the master controller or at one bus end. Multiple sets of bias resistors add parallel resistance that reduces the effective bias and can excessively load the driver during transmission. One set per bus is the correct implementation.
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