What Is a Serial Communications Interface?

The serial communication interface is referred to as the serial port (RS-232-C), also called the serial communication interface. The interface is a standard interface for the computer to communicate with other equipment. The computer has at least two serial ports COM1 and COM2.

Serial communication interface

The serial communication interface is referred to as the serial port (RS-232-C), also called the serial communication interface. Interface is a type of information transmitted between a computer and other equipment

1.RS-232-C is

2. RS-485 bus, RS-485 serial bus standard is widely used when the communication distance is tens of meters to thousands of meters. RS-485 uses balanced transmission and differential reception, so it has the ability to suppress common mode interference. Coupled with the high sensitivity of the bus transceiver, it can detect voltages as low as 200mV, so the transmission signal can be recovered from kilometers away.

RS-485 adopts half-duplex working mode, and only one point can be in the transmitting state at any time. Therefore, the transmitting circuit must be controlled by the enable signal. RS-485 is very convenient for multi-point interconnection, and can save many signal lines. Application RS-485 can be connected to form a distributed system, which allows up to 32 drivers and 32 receivers to be connected in parallel.

In the past, PCs communicated with smart devices

The full name of the RS-232C standard (protocol) is the EIA-RS-232C standard, where EIA (Electronic Industry Association) represents the American Electronics Industry Association, RS (ecommeded standard) represents the recommended standard, 232 is the identification number, and C represents the latest RS232 Revised (1969). Prior to this, there were RS232B and RS232A. . It specifies the connection cables and mechanical, electrical characteristics, signal functions and transmission processes. Common physical standards are EIA RS-232-C, EIA RS-422-A, EIA RS-423A, and EIA RS-485. Only EIA RS-232-C (abbreviated as 232, RS232) is introduced here. For example, the COM1 and COM2 interfaces currently on the IBM PC are RS-232C interfaces. 1. Electrical characteristics EIA-RS-232C specifies electrical characteristics, logic levels and various signal line functions.

On TxD and RxD: Logic 1 (MARK) = -3V to -15V Logic 0 (SPACE) = +3 to + 15V On RTS, CTS, DSR, DTR, and DCD control lines: The signal is valid (ON, ON State, positive voltage) = + 3V + 15V Invalid signal (open, OFF state, negative voltage) =-3V -15V Figure 1 The above specification illustrates the definition of logic level in the RS-323C standard. For data (information code): the level of logic "1" (pass number) is lower than -3V, the level of logic "0" (empty number) is + 3V; for control signals; the ON state is The effective level of the signal is higher than + 3V, and the off state (OFF) means that the level of the invalid signal is lower than -3V, that is, when the absolute value of the transmission level is greater than 3V, the circuit can effectively check it out, between Voltages between 3 and + 3V are meaningless. Voltages lower than -15V or higher than + 15V are also considered meaningless. Therefore, in actual work, the level should be between ± (3 to 15) V.

EIA-RS-232C and TTL conversion: EIA-RS-232C uses positive and negative voltages to indicate the logic state, which is different from the TTL requirement to indicate the logic state at high and low levels. Therefore, in order to be able to connect with the TTL device of the computer interface or terminal, the level and logic relationship must be changed between the EIA-RS-232C and the TTL circuit. The method to achieve this transformation can be discrete components or integrated circuit chips. At present, IC conversion devices such as MC1488 and SN75150 chips are widely used to complete the conversion from TTL level to EIA level, while MC1489 and SN75154 can achieve the conversion from EIA level to TTL level. The MAX232 chip can perform TTL EIA bidirectional level conversion. Figure 1 shows the internal structure and pins of 1488 and 1489. MC1488 pins (2), (4,5), (9,10) and (12,13) are connected to the TTL input. Pins 3, 6, 8, and 11 are output terminated to EIA-RS-232C. MC1498's pins 1, 4, 10, and 13 are connected to the EIA input, and pins 3, 6, 8, and 11 are connected to the TTL output. The specific connection method is shown in Figure 2. The left side of the figure is the main chip UART in the microcomputer serial interface circuit. It is a TTL device. The right side is the EIA-RS-232C connector, which requires EIA high voltage. Therefore, all output and input signals of RS-232C must pass through MC1488 and MC1498 converters respectively, and then can be sent to the connector or sent in from the connector after level conversion.

Figure 2 2. Mechanical characteristics of the connector:

Connectors: Because RS-232C does not define the physical characteristics of the connector, it appears

DB-25, DB-15 and DB-9 connector types have different pin definitions. The two types of connectors are described below. (1) DB-25: PC and XT use DB-25 type connector. The DB-25 connector defines 25 signal lines, which are divided into 4 groups: 9 voltage signals for asynchronous communication (including signal ground SG) 2, 3, 4, 5, 6, 7, 8, 20, 22 20mA current 9 loop signals (12, 13, 14, 15, 16, 17, 19, 23, 24) 6 empty (9, 10, 11, 18, 21, 25) 1 protected area (PE) as The ground terminal (1 pin) of the device and the shape of the DB-25 connector and the signal line assignment are shown in Figure 3. Note that the 20mA current loop signal is only provided by IBM PC and IBM PC / XT machines. It is no longer supported for AT machines and beyond. Cable length: When the communication rate is lower than 20kb / s, the maximum physical distance directly connected by RS-232C is 15m (50 feet). Explanation of the maximum direct transmission distance: The RS-232C standard stipulates that if the MODEM is not used, the maximum transmission distance between DTE and DCE is 15m (50 feet) when the symbol distortion is less than 4%. It can be seen that this maximum distance is given on the premise that the symbol distortion is less than 4%. In order to ensure that the symbol distortion is less than 4%, the interface standard specifies in the electrical characteristics that the load capacitance of the driver should be less than 2500pF.