In serial communication, data is sent one bit at a time over a signal line, so in order for the receiving end to receive the data accurately, the sending end must know the sending data for each bit.在RS232中，同步已经定义了<通信和异步通信标准。 In RS232, synchronous and asynchronous communication standards have been defined. For peripherals used for measurement or control, the previously mentioned full-duplex communication and asynchronous communication are usually used.
Synchronous communication
This method sends and receives data synchronized with a clock generated by another device or synchronized with a clock generated by itself. Communication is based on a synchronization signal added to each bit from the sending end, which has good data transmission efficiency, but the disadvantage is that the transmission process becomes complicated.
Asynchronous communication
This method sends and receives data, synchronized with both parties’ own clocks. Normal communication cannot occur if the transmission rate settings do not match, in other words, both the sender and receiver initially agree on how many bits per second to transmit, and then each creates a synchronization signal at a frequency that matches that transmission rate. In asynchronous communication, data is sent and received one at a time on a data line, so if the communication condition settings of both parties do not match initially, normal communication cannot occur. It is a normal setting method to match the settings on the computer (controller) side with those on the peripheral device side.
Transmission rate
Specify the number of bits sent per second, in bps (bits per second), select from 300, 600, 1200, 2400, 4800, 9600, 19200, etc., by matching the settings and timing, the data delimiter corresponds, and can Send and receive data normally. So add a start bit to each data item (1 byte) to get the correct timing.
Stop bit length
This sets the length of the bits that indicate the end of data. Usually choose 1 digit, 1.5 digit or 2 digits. The length of the start bit is fixed to 1 bit, so this setting is not required.
Data bit length
It specifies the number of bits making up each item of data, which depends on the device used, but usually 7 bits are specified for alphanumeric characters and symbols, and 8 bits are specified for 1-byte binary data.
Parity settings
This feature looks for errors in the data and can be selected from Even Parity (EVEN), Odd Parity (ODD) or No Parity (NONE).
Parity details
On the sending side, a parity bit “1” or “0” is added to the data so that even numbers are even and odd are odd number of “1” data bits. At the receiving end, “if the number is even, then the number will count as 1” data bits, and the data is judged as correct and odd as odd.
flow control
When sending and receiving data between devices, data may be lost if sent when the receiver is not in a receiving state, so it is important to check the status of the other party during the communication. Flow control is a feature that maintains communication reliability. A signal is sent from the sender to the receiver stating “data is being sent” and the receiver receives the signal and reads the data from the signal line. It then sends a reply to the sender stating: “Data received”. In other words, data can be transmitted while both parties check the sending and receiving of data.
XON/XOFF flow control
This is a control method that sends an “XOFF code” to the sender to request a temporary interruption of sending when the remaining free space in the receiver’s buffer becomes less. When enough space is available, an “XON code” is sent to request the sender to restart sending.
As an alternative to sending XON/XOFF codes in software flow control, control lines (RTS or DTR) are automatically opened or closed. RTS signal and CTS signal or DTR signal and DSR signal must be connected to each other.
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