Serial Ports Explained: A Complete Guide to Serial Communication

A serial port, also known as a serial interface, serial communication interface, or COM port, is an interface used for serial bit-by-bit data transmission.

A serial port (Serial Port) is an important interface for data communication between a computer and external devices. The terms ‘serial,’ ‘COM,’ and ‘RS-232’ are often used interchangeably. Although modern technologies such as USB and Ethernet have become widespread, serial ports (especially RS-232 ports and RS-485) are still widely used in fields such as industrial automation, embedded systems, and medical devices.

‘Serial port’ is the generic term referring to any port that transmits data in a serial fashion. However, the term is almost exclusively used to refer to a COM port (typically RS-232, occasionally RS-485/422). USB and Ethernet ports also transmit data in a serial fashion, but these are not referred to as serial ports. This guide provides a comprehensive overview of how serial ports work, their types, protocol standards, application scenarios, and answers to common questions.

Serial Port Basics

1. What Is a Serial Port?

A serial port (also called a COM port or comm port) is a communication interface that transmits data bit by bit over a single wire. Unlike parallel communication, serial communication uses just 1-2 data lines (TXD for transmit, RXD for receive) plus a ground (GND), making it ideal for long-distance data transfer.

Key Features:

Supports point-to-point communication (e.g., RS-232) or multi-device networks (e.g., RS-485).
Operates in simplex, half-duplex, or full-duplex modes.
In Windows, serial ports are typically labeled COM1, COM2 (hence the term COM ports).

2. How Serial Communication Works

Serial communication relies on start bits, data bits, and stop bits for synchronization:

Start bit (1 bit) – Signals the beginning of data transmission.
Data bits (usually 8 bits) – The actual transmitted data.
Stop bit (1-2 bits) – Indicates the end of data.

Communication Modes:

Simplex: One-way communication (e.g., sensor → controller).
Half-duplex: Two-way communication, but not simultaneously (e.g., RS-485).
Full-duplex: Simultaneous two-way communication (e.g., RS-232).

3. Serial vs. Parallel Communication

Feature	Serial Port	Parallel Communication
Wires Used	1-2 (+ ground)	Multiple (e.g., 8/16-bit)
Range	Long (RS-485: 1200m)	Short (typically <5m)
Noise Immunity	High (differential signaling)	Low (prone to interference)
Typical Use	PLCs, industrial control, serial devices	Old printers, IDE hard drives

Types and Standards of Serial Ports

Serial Ports

1. Physical Connector Types

Common serial port connectors include:

DB-9 (DE-9):
- The most common RS232 port interface (9-pin, found on older PCs).
- Pinout: TXD (transmit), RXD (receive), GND (ground).
DB-25:
- Older 25-pin connector, rarely used today.

2. Protocol Standards (RS-232/RS-485/RS-422)

(1) RS-232 (EIA-232)

Key Traits:
- Point-to-point, full-duplex.
- Voltage range: ±3V to ±15V (good noise immunity).
Limitations: Short range (≤15m), low speed (typically 115.2kbps).

(2) RS-485 (EIA-485)

Key Traits:
- Differential signaling (excellent noise immunity).
- Supports multi-device networks (up to 32 nodes).
- Range: Up to 1200 meters (ideal for industrial automation).

(3) RS-422

Similar to RS-485 but limited to one master, multiple slaves (no bus contention).

Technical Details of Serial (COM) Ports

1. Electrical Characteristics

Baud Rate:
- Common values: 9600, 19200, 115200 bps (higher = faster but shorter range).
Data Format:
- Typically 8N1 (8 data bits, no parity, 1 stop bit).

2. Configuration & Management

How to Switch Protocols (RS-232/422/485)?
- Hardware jumpers (e.g., USB-to-RS232 port adapters).
- Software settings (e.g., terminal programs).
Windows COM Port Setup:
- Device Manager → Ports (COM & LPT) → Properties → Adjust baud rate.

Applications of Serial Devices

1. Industrial Automation

PLC Systems: RS-485 networks for sensors (temperature, pressure).
CNC Machines: Legacy equipment often uses RS232 ports.

2. Embedded Development

Microcontrollers (STM32/Arduino): Firmware uploads via UART (serial).

3. Medical Equipment

Older devices (e.g., ECG machines) may still rely on COM ports.

Serial Port vs. Other Interfaces

Feature	Serial Port	USB	Ethernet
Speed	Slow (115.2kbps)	Fast (5Gbps)	Very Fast (1Gbps+)
Range	Long (RS-485: 1200m)	Short (5m)	Medium (100m)
Noise Immunity	High (industrial-grade)	Moderate	Moderate
Cost	Low	Medium	High

Conclusion:

Industrial use: RS-485 (noise immunity, long range).
Consumer electronics: USB or wireless.

Serial Port Configuration for RCOMPT Industrial Panel PCs

RCOMPT Industrial Panel PCs are designed for robust performance in automation, manufacturing, and IoT applications. Below are the serial port (COM) configurations across different models, along with other key interfaces for seamless integration:

Panel PC Interface

Model	Power (DC)	USB Ports	COM Ports	LAN	HDMI	VGA	Wireless	Audio	Expansion
Standard	12V/5A (5525)	2×USB 3.0 + 2×USB 2.0	2×RS232	2×LAN	1×HDMI	1×VGA	WiFi (2.4G+5.8G), BT 4.0	Line In	-
Compact	12V/4A (5521)	1×USB 3.0 + 1×USB 2.0	2×RS232	1×LAN	1×HDMI	-	WiFi (2.4G), BT 4.0	Line In	1×TF (SD) + SIM Slot
Enhanced	12V/5A (5525)	1×USB 3.0 + 3×USB 2.0	2×RS232	1×LAN	1×HDMI	1×VGA	WiFi (2.4G+5.8G), BT 4.0	Line In	-

Industrial Monitor Interface

Model	Power (DC)	Video Outputs	Audio	Data Ports
Basic	12V/4A (5521)	1×HDMI + 1×VGA	Line In	1×DVI + 1×USB-B
Slim	12V/4A (5521)	1×HDMI + 1×VGA	-	1×USB-B

Key Features of RCOMPT Serial Ports:

Dual RS232 Ports: Support legacy devices (e.g., PLCs, barcode scanners).
Industrial-Grade Reliability: EMI shielding, wide-temperature operation (–20°C to 60°C).
Flexible Connectivity: Combine with USB, LAN, or wireless for hybrid systems.

Typical Use Cases:

Factory Automation: PLC communication via RS232/RS-485.
Data Logging: Serial sensors → Panel PC → Cloud (via WiFi/LAN).
HMI Integration: Touchscreen control + legacy device support.

For detailed wiring diagrams or protocol settings, consult the [RCOMPT Technical Manual].

Frequently Asked Questions (FAQ)

Q1: Is a COM port the same as a serial port?

Yes. COM port is Windows’ naming convention (e.g., COM1, COM2).

Q2: Why is RS-485 still used in modern systems?

Noise immunity, multi-device support, and long-range capability (critical for industrial apps).

Q3: How to troubleshoot serial communication issues?

Verify baud rate, data bits, stop bits match.
Test cables (use a multimeter).
Check RS-485 termination resistors (120Ω).

Penny

Web Content Writer

4 years of experience

This article is edited by Penny, the website content writer of COMPT, who has 4 years working experience in the industrial PCs industry and often discusses with colleagues in R&D, marketing and production departments about the professional knowledge and application of industrial controllers, and has a deep understanding of the industry and products.

Please feel free to contact me to discuss more about industrial controllers. sales@gdcompt.com

Post time: Jul-29-2025

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