When using PLCs , power workers will encounter many communication protocols and interfaces. Do you understand the most basic PLC serial communication and basic communication interfaces?
1. What is serial communication?
Serial communication is a very common device communication protocol on computers (do not confuse it with Universal Serial Bus or USB). Most computers contain two RS-232-based serial ports. Serial communication is also a common communication protocol for instrumentation equipment; many GPIB-compatible devices also have RS-232 ports. Furthermore, serial communication protocols can also be used to acquire data from remote data acquisition devices.
2. Use of serial communication
Serial communication uses three wires: (1) ground, (2) transmit, and (3) receive. Because serial communication is asynchronous, the port can send data on one wire while receiving data on another. Other wires are used for handshaking, but are not mandatory.
The most important parameters for serial communication are baud rate, data bits, stop bits, and parity. These parameters must match for two ports to communicate.
a. Baud rate: This is a parameter that measures communication speed. It represents the number of bits transmitted per second. For example, 300 baud means 300 bits are transmitted per second. When we refer to clock cycles, we are referring to baud rate. For example, if the protocol requires a 4800 baud rate, then the clock is 4800 Hz. This means that the sampling rate of serial communication on the data line is 4800 Hz. Typically, telephone line baud rates are 14400, 28800, and 36600. Baud rates can be much higher than these values, but baud rate is inversely proportional to distance. High baud rates are often used for communication between instruments placed very close together.
b. Data Bits: This parameter measures the actual number of data bits used in communication. When a computer sends a packet, the actual data is not always 8 bits; standard values are 5, 7, and 8 bits. How it's set depends on the information you want to transmit. For example, standard ASCII code is 0–127 (7 bits). Extended ASCII code is 0–255 (8 bits). If the data uses simple text (standard ASCII), then each data packet uses 7 bits. Each packet refers to one byte, including start/stop bits, data bits, and parity bits. Because the actual number of data bits depends on the chosen communication protocol, the term "packet" refers to any communication scenario.
c. Stop bit: Used to indicate the last bit of a single packet. Typical values are 1, 1.5, and 2 bits. Since data is timed on the transmission line, and each device has its own clock, slight asynchrony between two devices is possible during communication. Therefore, the stop bit not only indicates the end of transmission but also provides the computer with an opportunity to correct clock synchronization. The more bits used for the stop bit, the greater the tolerance for different clock synchronizations, but the data transmission rate will also be slower.
d. Parity bit: A simple error detection method in serial communication. There are four error detection modes: even, odd, high, and low. Of course, it's also possible to have no parity bit. For even and odd parity, the serial port sets a parity bit (the bit after the data bits) to ensure that the transmitted data has an even or odd number of logical high bits. For example, if the data is 011, then for even parity, the parity bit is 0, ensuring that the number of logical high bits is even. For odd parity, the parity bit is 1, resulting in 3 logical high bits. The high and low bits don't actually check the data; they are simply set to logical high or logical low for parity. This allows the receiving device to know the state of a bit, giving it a chance to determine if there is noise interfering with communication or if the transmitted and received data are out of sync.
3. Commonly Used PLC Basic Interfaces
a. What is RS-232?
RS-232 (ANSI/EIA-232 standard) is a serial connection standard used on IBM PCs and compatible machines. It can be used for many purposes, such as connecting mice, printers, or modems, and can also be used to connect industrial instruments. Due to improvements in drivers and wiring, in practical applications, RS-232 transmission lengths or speeds often exceed the standard values.
RS-232 is limited to point-to-point communication between a PC serial port and devices. The maximum distance for RS-232 serial communication is 50 feet.
b, What is RS-422?
RS-422 (EIARS-422-AS Standard) is the serial port connection standard for Apple's Macintosh computers.
RS-422 uses differential signals, while RS-232 uses signals with an unbalanced ground reference. Differential transmission uses two wires to send and receive signals, offering better noise immunity and a longer transmission distance compared to RS-232. This superior noise immunity and longer transmission distance are significant advantages in industrial environments.
c, What is RS-485?
RS-485 (EIA-485 standard) is an improvement on RS-422 because it increases the number of devices allowed, from 10 to 32, and defines the electrical characteristics for the maximum number of devices to ensure sufficient signal voltage. With the ability to support multiple devices, you can establish a device network using a single RS-422 port. Its excellent noise immunity and multi-device capability make RS-485 the preferred serial connection for establishing distributed device networks connected to PCs, other data acquisition controllers, HMIs, or other operations in industrial applications.
RS-485 is a superset of RS-422, therefore all RS-422 devices can be controlled by RS-485. RS-485 can communicate serially over cables exceeding 4000 feet.
