RS232 vs RS485 vs RS422 Serial Cable Guide:
Differences, Wiring, and How to Choose
RS232, RS485, and RS422 are three serial communication standards that have been in use for decades - and are nowhere close to disappearing. This guide explains what each standard does, how they differ electrically and practically, and how to choose the right cable.
💡 Key Takeaways
- RS232 is single-ended, point-to-point, limited to ~15 meters and 115.2 kbps. It is a legacy standard widely used for PLC programming and CNC machines.
- RS485 uses differential signaling, supports up to 1,200 meters and 32–256 devices on a single bus. It is the dominant standard for industrial multi-drop networks (Modbus).
- RS422 uses differential signaling with the same distance/speed as RS485, but is limited to 1 transmitter and 10 receivers. Used for long-distance point-to-point links.
- The key decision factor is topology: point-to-point short distance → RS232; point-to-point long distance → RS422; multi-drop bus → RS485.
- Cable selection, shielding, and termination are critical for RS485/RS422 reliability - most "random data errors" trace back to cable or termination problems.
📋 Contents
01 · What Are RS232, RS485, and RS422?
All three are electrical standards published by the Telecommunications Industry Association (TIA) and the Electronic Industries Alliance (EIA). They define the physical layer of serial communication - voltage levels, signal polarity, driver and receiver characteristics, and cable limits. They do not define the communication protocol (that is handled by higher-level standards like Modbus, NMEA 0183, or proprietary device protocols).
- RS232 (TIA/EIA-232): The original serial standard, first published in 1960. Designed for connecting a computer (DTE) to a modem (DCE) over a short cable.
- RS422 (TIA/EIA-422): Published in 1975 as a higher-speed, longer-distance successor to RS232, using differential signaling instead of single-ended.
- RS485 (TIA/EIA-485): Published in 1983 as an extension of RS422, adding multi-point (multi-drop) capability - multiple devices sharing a single bus.
Note: The "RS" prefix originally stood for "Recommended Standard." The current official designations use TIA/EIA numbering, but the RS names remain universal in industry usage.
02 · RS232 Explained: Specs, Pinout, and Limits
RS232 uses single-ended signaling: each data signal is referenced to a common ground wire. This makes it simple to implement but vulnerable to electrical noise and limited in distance.
Electrical Characteristics
| Parameter | Value |
|---|---|
| Signal type | Single-ended (unbalanced) |
| Logic 1 (Mark) | −3V to −15V |
| Logic 0 (Space) | +3V to +15V |
| Undefined zone | −3V to +3V |
| Max data rate | 115.2 kbps (standard); up to 1 Mbps at very short distances |
| Max cable length | 15 meters (50 feet) at 19.2 kbps |
| Topology | Point-to-point: 1 driver, 1 receiver |
| Duplex mode | Full-duplex (simultaneous TX and RX) |
DB9 Pinout (DTE Side)
| Pin | Signal | Direction (DTE) | Function |
|---|---|---|---|
| 1 | DCD | Input | Data Carrier Detect |
| 2 | RXD | Input | Receive Data |
| 3 | TXD | Output | Transmit Data |
| 4 | DTR | Output | Data Terminal Ready |
| 5 | GND | - | Signal Ground |
| 6 | DSR | Input | Data Set Ready |
| 7 | RTS | Output | Request to Send |
| 8 | CTS | Input | Clear to Send |
| 9 | RI | Input | Ring Indicator |
Practical Limits: RS232's 15-meter distance limit is not arbitrary - it results from the cable capacitance specification (2,500 pF maximum load). At 30 meters, signal integrity becomes unreliable. At 50 meters, it is effectively unusable. Always check device documentation for required hardware flow control (RTS/CTS) handshakes.
03 · RS485 Explained: Specs, Wiring, and Multi-Drop
RS485 was designed to solve the two main limitations of RS232: distance and device count. It achieves this through differential signaling - data is encoded as the voltage difference between two wires (A and B), rather than the voltage of one wire relative to ground. This makes RS485 highly resistant to electromagnetic interference.
Electrical Characteristics
| Parameter | Value |
|---|---|
| Signal type | Differential (balanced) |
| Differential voltage | ±1.5V to ±6V |
| Max data rate | 10 Mbps (at 12m) |
| Max distance | 1,200 meters (4,000 feet) at 100 kbps |
| Max devices | 32 unit loads (up to 256 with high-impedance receivers) |
| Termination | 120 Ω at each end of the bus |
Speed vs. Distance Trade-Off
RS485 cannot deliver maximum speed and maximum distance simultaneously.
| Distance | Practical Max Data Rate |
|---|---|
| 12 m | 10 Mbps |
| 100 m | 2 Mbps |
| 500 m | 500 kbps |
| 1,200 m | 100 kbps |
2-Wire vs. 4-Wire Configuration
2-wire (half-duplex): One twisted pair carries data in both directions, but only one device can transmit at a time. The most common configuration for Modbus RTU.
4-wire (full-duplex): Two twisted pairs (one for TX, one for RX) allow simultaneous bidirectional communication without turn-around delay.
04 · RS422 Explained: How It Differs from RS485
RS422 and RS485 use the same differential signaling technology and share identical distance and speed specifications. The difference is in the bus topology.
| Feature | RS422 | RS485 |
|---|---|---|
| Drivers (transmitters) | 1 only | Up to 32 (or 256) |
| Receivers | Up to 10 | Up to 32 (or 256) |
| Multi-master | No | Yes |
| Duplex | Full-duplex (always 4-wire) | Half or full-duplex |
| Bus direction control | Not needed (1 driver) | Required (driver enable) |
Choose RS422 when: You need long distance and noise immunity, communication is one-to-many, full-duplex is required, and only one master transmits.
Choose RS485 when: Multiple devices need to transmit data on the same bus (like Modbus or BACnet).
A common point of confusion: many industrial devices labeled "RS422/RS485" support both standards because the electrical characteristics are compatible. The difference is in how the bus is managed at the protocol level.
05 · RS232 vs RS485 vs RS422: Side-by-Side Comparison
| Feature | RS232 | RS422 | RS485 |
|---|---|---|---|
| Signaling | Single-ended | Differential | Differential |
| Max distance | 15 m (50 ft) | 1,200 m (4,000 ft) | 1,200 m (4,000 ft) |
| Max data rate | 115.2 kbps | 10 Mbps | 10 Mbps |
| Topology | Point-to-point (1:1) | Point-to-multipoint (1:10) | Multi-drop bus (32–256) |
| Duplex | Full | Full | Half or Full |
| Noise immunity | Low | High | High |
| Common connector | DB9, DB25 | DB9, screw terminal | DB9, screw term, RJ45 |
| Termination required | No | Yes (120 Ω) | Yes (120 Ω) |
| Wires needed (min) | 3 (TX, RX, GND) | 5 (TX±, RX±, GND) | 3 (A, B, GND) for half-duplex |
Quick Decision Logic
├── Distance < 15m? → RS232 (simplest, lowest cost)
├── Distance 15m–1,200m, full-duplex? → RS422
└── Distance 15m–1,200m, half-duplex OK? → RS485
Need to connect 3+ devices on one bus?
└── RS485 (only option)
06 · Cable Types: Straight-Through, Null Modem, and Shielded Twisted Pair
RS232 Cables
Straight-through cable: Connects DTE (computer, PLC) to DCE (modem, converter). Pin 2 to Pin 2, Pin 3 to Pin 3. This is the standard RS232 cable.
Null modem (crossover) cable: Connects two DTE devices (computer to computer, PLC to PLC). The transmit and receive lines are crossed - Pin 2 (RXD) connects to Pin 3 (TXD). May also cross handshake lines.
How to tell which you need: If both devices are DTE, a null modem is needed. If the manual says "standard serial cable", use straight-through.
RS485 / RS422 Cables
RS485 and RS422 require shielded twisted pair (STP) cable. The twisted pair geometry is essential for differential signaling to reject external noise.
- Shield type: Foil shield + drain wire (minimum); braid + foil for high-EMI.
- Wire gauge: 24 AWG (up to 300m), 22 AWG (300m–1,200m).
- Impedance: 100–120 Ω characteristic impedance.
- Jacket: PVC (general), PUR (oil/outdoor), LSZH (enclosed spaces).
Do not use flat ribbon cable or unshielded cable for RS485/RS422 runs longer than a few meters.
07 · RS485 Wiring and Termination: Getting It Right
RS485 termination errors are the single most common cause of unreliable serial networks in industrial installations. Follow these topology rules:
- Daisy-chain only: Wire device to device in series. Star topology violates the bus rules and causes signal reflections.
- Stub length: Keep drops from the bus to each device under 30 cm (12 inches).
- Termination: Place a 120-ohm resistor between A and B at each physical end of the bus (only at the 2 endpoints).
Common Termination Mistakes
| Mistake | Symptom | Fix |
|---|---|---|
| No termination resistors | Intermittent errors at high speed/distance | Add 120 Ω at both ends |
| Termination at every device | Excessive current draw, reduced voltage swing | Remove - keep only at 2 endpoints |
| Star wiring (not daisy-chain) | Random data corruption, some devices fail | Rewire as linear bus |
| Missing ground wire | Common-mode voltage >7V, devices offline | Add dedicated ground between devices |
08 · USB to Serial Converters: What to Know
Modern computers no longer include RS232 serial ports, making USB-to-serial converters essential for connecting to legacy equipment. For RS485/RS422, they also handle protocol conversion.
Converter Chipset Matters
- FTDI (FT232R, FT2232H): Excellent reliability, stable drivers across OS. Preferred for industrial use.
- Prolific (PL2303): Moderate reliability, driver issues common on newer Windows. Avoid for mission-critical.
- CH340/CH341: Good budget option, acceptable for non-critical bench testing.
Galvanic Isolation
For industrial applications, choose USB-to-serial converters with galvanic isolation (typically 1.5 kV or 2.5 kV). Isolation protects the PC from voltage spikes, ground loops, and high common-mode voltages that are common in industrial environments.
09 · Applications by Industry
Industrial Automation
RS485 with Modbus RTU is the dominant serial protocol. PLCs, VFDs, power meters, and temperature controllers communicate over RS485 buses spanning entire factory floors. RS232 is used for PLC programming ports.
Building Management Systems (BMS)
HVAC controllers, lighting systems, and energy meters use RS485 with BACnet MS/TP protocol. A single bus can connect dozens of controllers across multiple floors.
Marine Electronics
NMEA 0183 uses RS422 for instrument data transmission. GPS receivers, depth sounders, and autopilots transmit navigation data over RS422 serial links.
Access Control & Security
Card readers, door controllers, and alarm panels commonly use RS485 for device-to-controller communication across large facilities.
CNC & Motion Control
CNC machines use RS232 for program upload/download (G-code transfer) and RS422 for real-time feedback signals (encoder data) where distances exceed RS232 limits.
Energy & Utilities
Solar inverters, battery management systems, and smart meters use RS485 Modbus. A monitoring system can poll hundreds of inverters across a solar farm.
10 · What to Check Before Buying Serial Cables
- Identify the protocol first: Check the device manual for the standard (RS232, RS485, or RS422). "Serial cable" is ambiguous.
- Straight-through or null modem? For RS232, determine if you are connecting DTE-to-DCE (straight) or DTE-to-DTE (null modem crossover).
- Shielding for RS485/RS422: In any industrial environment, specify shielded twisted pair (STP). Unshielded cable is only for bench testing.
- Wire gauge vs. distance: For RS485 runs over 300 meters, specify 22 AWG or thicker to prevent excessive voltage drop.
- Connector gender and pin assignment: DB9 male and female are not interchangeable. Verify pinout as some devices use non-standard mappings for RS485 over DB9.
- Cable jacket material: PVC for indoor cable trays; specify PUR or LSZH for outdoor, oil-exposed, or high-temperature environments.
Frequently Asked Questions
Can I connect RS232 directly to RS485?
No. RS232 uses single-ended signaling at ±3V to ±15V; RS485 uses differential signaling at ±1.5V to ±6V. Connecting them directly will not work and may damage equipment. Use an RS232-to-RS485 converter.
Is RS485 faster than RS232?
Yes. RS485 supports up to 10 Mbps, while RS232 is limited to 115.2 kbps in standard implementations. RS485 also maintains reliable communication at much longer distances.
What is the main difference between RS422 and RS485?
RS422 allows only 1 transmitter and up to 10 receivers. RS485 allows up to 32 transmitters and receivers (or 256 with high-impedance chips) on the same bus. If multiple devices need to transmit, RS485 is required.
Do I need a ground wire for RS485?
Yes. Even though RS485 uses differential signaling, a ground reference wire is strongly recommended to prevent common-mode voltage from exceeding the receiver's limits.
What is a null modem cable?
A null modem cable crosses the transmit and receive lines so two DTE devices (like two computers or PLCs) can communicate directly without a modem in between.
Can I run RS232 at 30 meters if I use high-quality cable?
Not reliably. The 15-meter limit is based on cable capacitance. High-quality cable extends it slightly, but beyond 20 meters you should switch to RS422/RS485 with a converter.
Final Thoughts
RS232, RS485, and RS422 continue to serve industrial communication needs that Ethernet does not always justify - short configuration links, sensor polling, legacy equipment integration, and long-distance serial buses where the simplicity and low cost of two-wire communication outweigh the bandwidth advantages of Ethernet.
The standard you choose determines the cable type, connector, wiring topology, and termination requirements. RS232 for short, simple, point-to-point links. RS422 for long-distance, one-to-many, full-duplex connections. RS485 for multi-drop bus networks that are the backbone of industrial automation and building management.
Premier Cable manufactures RS232, RS485, and RS422 serial cables in standard and custom configurations - DB9, DB25, screw terminal, and RJ45 connector options with shielded twisted pair construction. Custom cable lengths, jacket materials, and connector combinations are available for OEM and project-specific requirements.
Source Industrial Serial Cables
Need custom null modem cables, shielded RS485 twisted pairs, or reliable FTDI USB-to-serial converters for your project? Contact our engineering and sales team for a rapid quote.
