All PLC mainframes have a standard configuration with two built-in communication interfaces: one RS232 and one RS485. The RS232 interface is mainly used for uploading and downloading programs or for communicating with a host computer or touch screen, while the RS485 interface is mainly used to build a network using the RS485 protocol to achieve communication control.
1. RS232 Interface The RS232-C interface connector generally uses a 9-pin plug of model DB-9. It only requires 3 interface lines, namely "transmit data", "receive data" and "signal ground" to transmit data. The definition of its 9 pins is shown in Figure 1.
Figure 1 RS232-C interface connector definition
In the RS232 specification, a voltage value between +3V and +15V (generally +6V) is referred to as "0" or "ON". A voltage value between -3V and -15V (generally -6V) is referred to as "1" or "OFF". On a computer, the RS232 "high potential" is approximately 9V, while the "low potential" is approximately -9V.
RS232 operates in full-duplex mode, its signal voltage is derived from ground, and it can transmit and receive data simultaneously. In practical applications, the transmission distance using an RS232 interface can reach 15 meters. However, RS232 only has single-station functionality, meaning it supports one-to-one communication.
2. RS485 Interface: RS485 uses two signal lines, positive and negative, as the transmission line. A voltage difference of +2V to 6V between the two lines represents logic "1"; a voltage difference of -2V to 6V between the two lines represents logic "0".
RS485 operates in half-duplex mode, and its signal is obtained by subtracting the signal levels of the positive and negative lines. It is a differential input method with strong common-mode interference immunity, meaning it has good noise immunity. In practical applications, its transmission distance can reach 1200 meters. RS485 has multi-station capability, i.e., one-to-many master-slave communication.
In serial communication, data is usually transmitted between two stations. According to the direction of data transmission on the communication line, there are three basic transmission modes: simplex, half-duplex and full-duplex, as shown in Figure 2.
Figure 2 Simplex, half-duplex and full-duplex communication
Simplex communication uses a single wire, and the sender and receiver of the signal have a clear directionality. In other words, communication only occurs in one direction.
If the same transmission line is used as both a receiving and transmitting line, data can be transmitted in both directions, but the two communicating parties cannot send and receive data simultaneously. This transmission method is called half-duplex. In half-duplex mode, the transmitter and receiver at each end of the communication system are time-division multiplexed onto the communication line via a transmit/receive switch to switch directions.
When data transmission and reception are handled by two separate transmission lines, both parties can send and receive data simultaneously; this is called full-duplex communication. In full-duplex mode, each end of the communication system has a transmitter and a receiver, allowing data to be transmitted in both directions simultaneously. Full-duplex communication eliminates the need for direction switching.
Serial communication can be divided into two types: synchronous communication and asynchronous communication. In synchronous communication, all characters are grouped together, allowing them to be transmitted one after another. However, a synchronization character must be added at the beginning of each group, and blank characters are used to fill in any gaps when no information needs to be transmitted, as synchronous transmission does not allow for gaps. In asynchronous communication, the transmission interval between two characters is arbitrary, so data bits are used before and after each character as separators. Comparatively, at the same transmission rate, synchronous communication is more efficient than asynchronous communication because the proportion of non-data information is smaller in synchronous communication.
However, from another perspective, synchronous transmission requires both parties to coordinate using the same clock. This clock determines the position of each bit of information during synchronous serial transmission. Therefore, in synchronous transmission, the clock signal must be transmitted simultaneously with the data. In asynchronous transmission, the clock frequency of the receiver and the clock frequency of the sender do not need to be exactly the same; they only need to be close, i.e., within a certain allowable range. Asynchronous communication is widely used in data transmission, and the standard data format for asynchronous communication is shown in Figure 3.
Figure 3 Asynchronous communication data format
As shown in Figure 3, asynchronous communication transmits characters one by one, with each character transmission always beginning with a start bit and ending with a stop bit. There is no fixed time interval requirement between characters. Each transmission consists of a start bit, followed by 5 to 8 data bits, then a parity bit (which can be odd, even, or omitted), and finally a 1-bit, 1.5-bit, or 2-bit stop bit. Following the stop bit is a variable-length idle bit. Both the stop bit and the idle bit are set to high level, ensuring a falling edge at the start of the start bit to indicate the beginning of data transmission.
Disclaimer: This article is a reprint. If it involves copyright issues, please contact us promptly for deletion (QQ: 2737591964 ) . We apologize for any inconvenience.
