Bridging the Gap: Essential PLC Serial Communication Knowledge for Electrical Professionals

Bridging the Gap: Essential PLC Serial Communication Knowledge for Electrical Professionals

Serial Communication Basics

Serial communication is a common protocol for device - to - device data exchange. Unlike USB, most computers have two RS232 - based serial ports. Serial ports are also widely used in instrumentation and remote data - acquisition devices.

Key Parameters of Serial Communication

For effective serial communication, the following parameters must match across ports:

Baud Rate: Measures data - transmission speed in bits per second. For example, 300 baud equals 300 bits per second. Baud rate directly affects the sampling frequency on the data line. High baud rates, which facilitate short - distance, high - speed communication, are inversely proportional to transmission distance.

Data Bits: Represent the actual data in transmission. Standard values are 5, 7, or 8 bits. For instance, standard ASCII uses 7 bits, while extended ASCII uses 8. The choice depends on the specific data - transmission requirements.

Stop Bits: Mark the end of a data packet, typically with values of 1, 1.5, or 2 bits. They help devices synchronize clocks during communication. More stop bits increase tolerance for clock - timing differences but reduce the data - transmission rate.

Parity Bit: A simple error - checking method. Options include even, odd, high, low, or none. Even and odd parity set the parity bit to ensure an even or odd number of logical high bits in the transmitted data. High and low parity simply set the bit to logical high or low, allowing receiving devices to detect potential communication interference or desynchronization.

Common PLC Interfaces

RS - 232: The RS - 232 standard is common on IBM - PC and compatible systems. It supports diverse applications like connecting mice, printers, modems, and industrial instruments. While RS - 232 enables point - to - point communication between PC serial ports and devices, its maximum transmission distance is 50 feet.

RS - 422: RS - 422, used in Apple Macintosh computers, employs differential signaling. Unlike RS - 232's single - ended ground - referenced signaling, RS - 422's differential transmission uses two lines for sending and receiving signals. This enhances noise immunity and extends transmission distance, making it suitable for industrial settings.

RS - 485: RS - 485 improves on RS - 422 by supporting up to 32 devices and defining electrical characteristics for maximum - device - count scenarios to ensure adequate signal voltage. It allows for the creation of device networks using a single RS - 422 port. With strong noise - immunity and multi - device support, RS - 485 is ideal for industrial applications like building distributed device networks connected to PCs, data - collection controllers, HMIs, and other operational systems. RS - 485 is a superset of RS - 422, enabling control of all RS - 422 devices with transmission distances exceeding 4,000 feet.