Industrial Communication · Serial Protocols · RS232 · RS485 · Modbus
What Is Serial Communication? A Plain-English Guide for Instrumentation Engineers
Serial communication sends data one bit at a time over a single wire. It underlies RS232, RS485, Modbus RTU, HART and almost every field instrument connection in process automation. This guide explains how it works, the key parameters you configure, and how it compares to parallel communication.
What Is Serial Communication?
Serial communication is a method of transferring data between two devices by sending one bit at a time over a single communication line. The word "serial" means one after another: the bits in a data byte are queued up and transmitted in sequence, like cars going through a single-lane tunnel.
In process plants and industrial automation, serial communication is everywhere. Every time a PLC polls a pressure transmitter over Modbus RTU, reads a flow computer over RS485, or a laptop connects to a field instrument for configuration over RS232, serial communication is what carries the data. The same principle also underlies HART, Profibus and Foundation Fieldbus at the physical level.
Figure 1: Serial communication (top) sends all 8 bits of a byte one after another on a single wire. Parallel communication (bottom) sends all 8 bits simultaneously on 8 separate wires. Serial is slower per byte but practical for long distances. Parallel is faster but only used for short distances inside equipment.
Serial vs Parallel Communication
| Parameter | Serial | Parallel |
|---|---|---|
| How data travels | One bit at a time on one wire | Multiple bits simultaneously on multiple wires |
| Number of wires | 1 data wire (minimum) | 8, 16 or 32 wires (one per bit) |
| Speed per byte | Slower (8 clock cycles per byte) | Faster (1 clock cycle per byte) |
| Distance | Long distance (metres to kilometres) | Short distance only (centimetres to metres) |
| Cross-talk noise | Very low (fewer wires) | Higher risk (many parallel conductors) |
| Cost | Lower (less cable, simpler connectors) | Higher (more cable, wider connectors) |
| Industrial use | RS232, RS485, Modbus, HART, Profibus, all field instrument connections | Inside PCBs, memory buses, printer ports (rare in process plants) |
Three Duplex Modes: How Data Direction Works
| Mode | Direction | Simultaneous? | Industrial example |
|---|---|---|---|
| Simplex | One direction only | N/A: only one direction exists | 4-20 mA transmitter sending a process value to a DCS (transmitter never receives back). Radio broadcast. |
| Half Duplex | Both directions, but not at the same time | No: one device transmits while the other listens | RS485 Modbus RTU: master sends a request, then listens; slave responds, then listens. Walkie-talkie conversation. |
| Full Duplex | Both directions simultaneously | Yes: both devices can transmit and receive at once | RS232 connection between PLC and PC (separate TX and RX wires). Telephone call. |
In process instrumentation, half duplex is the most common arrangement. RS485 Modbus RTU networks are half duplex by design: the master asks, then waits; the slave replies, then goes quiet. Only one device drives the bus at any moment.
Synchronous vs Asynchronous Transmission
For two devices to exchange serial data correctly, they must agree on timing: when each bit starts and ends. There are two approaches:
| Type | How timing works | Overhead | Industrial use |
|---|---|---|---|
| Synchronous | A shared clock signal is sent alongside the data. Both devices are locked to the same clock. Data is sent in continuous blocks or frames. | Low overhead: no start/stop bits needed per byte | SPI, I2C (short-range chip-to-chip). Profibus uses synchronous framing. Higher throughput. |
| Asynchronous | No shared clock. Each byte is wrapped in a start bit (signals "data coming") and one or two stop bits (signals "byte complete"). Both devices agree on baud rate in advance. | Higher overhead: 2 to 3 extra bits per 8-bit byte | RS232, RS485, UART, Modbus RTU. Most field instrument connections. Simple and robust. |
For most field instrumentation work, you will be dealing with asynchronous serial communication. The key is that both the transmitter and receiver must be configured to the same baud rate before communication starts. If they disagree on baud rate, every byte received will be corrupted.
Key Parameters You Configure for Serial Communication
Every serial port configuration in a PLC, DCS, or SCADA system requires these four settings to match exactly between sender and receiver:
The speed of data transfer in bits per second (bps). Both devices must use the same baud rate. Common values: 9600, 19200, 38400, 115200 bps. For Modbus RTU on RS485, 9600 or 19200 bps is typical.
The number of data bits in each transmitted character. Almost always 8 bits (one byte). Some legacy systems use 7 bits. Shorthand: "8N1" means 8 data bits, No parity, 1 stop bit.
An optional error-detection bit added to each byte. Even parity, odd parity or none. Most Modbus RTU applications use no parity (N). Parity catches single-bit errors only and is not a substitute for CRC checking at the protocol level.
The number of idle bits sent after each byte to mark its end. Either 1 or 2 stop bits. Most asynchronous serial applications use 1 stop bit. Use 2 stop bits only if the receiving hardware requires it. Typical Modbus RTU: 8N1 or 8E1.
Modbus RTU on RS485: 9600 or 19200 bps, 8 data bits, No parity, 1 stop bit (8N1)
RS232 instrument configuration port: 9600 bps, 8N1 (check device manual first)
HART over RS232 modem: 1200 bps, 8N1 (HART uses 1200 Hz / 2200 Hz FSK at 1200 bps)
These settings must match on both the field device and the host (PLC, DCS, laptop) for any communication to work.
Serial Communication Standards Used in Process Plants
| Standard | Duplex | Max distance | Max devices | Common use in plants |
|---|---|---|---|---|
| RS232 | Full duplex | 15 m | 2 only | Laptop to PLC programming cable, analyser configuration port |
| RS485 | Half duplex (2-wire) | 1200 m | 32 (standard) | Modbus RTU field instrument bus, VFD speed control, energy meters |
| HART | Half duplex (FSK on 4-20 mA) | Limited by loop resistance | 1 per loop (multidrop: up to 15) | Smart field transmitter configuration and diagnostics |
| Modbus RTU | Half duplex (uses RS485) | 1200 m | 247 device addresses | Most widely used industrial serial protocol worldwide |
Quick FAQs
What we learn today
- Serial communication sends one bit at a time on one wire. It is slower per byte than parallel but works over long distances with simple cables, which is why every field instrument connection in a process plant uses it.
- Three duplex modes: Simplex (one direction only), Half Duplex (both directions but not at the same time, like RS485 Modbus RTU), Full Duplex (both directions simultaneously, like RS232). Most industrial serial networks are half duplex.
- Four settings must match exactly between sender and receiver: Baud Rate (speed in bps), Data Bits (almost always 8), Parity (usually None), Stop Bits (usually 1). The shorthand 8N1 covers the most common configuration for industrial instruments.
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