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1. COM port
COM port (cluster communication port) that is a serial communications port, serial port for short. Serial port is a 9-pin is typically a microcomputer, there are 25-pin interface, the maximum rate 115200bps.
Usually only one motherboard external serial port, and parallel with the rear of the chassis that abalone output terminal (trapezoid) is COM1 port, the pin leading-out port COM2 generally from the motherboard. The current mainstream motherboards generally only with a serial port, even without, it will slowly be replaced by USB.
The serial bus interface standard norms and standards are RS-232.
2. UART
Universal Asynchronous Receiver Transmitter (Universal Asynchronous Receiver / Transmitter), commonly referred to as UART. Information will be transmitted in the serial communication with the parallel communication to be converted between. As the parallel input signal into a serial output signal of the chip, UART typically integrated in other communication interface coupling.
The UART is a universal serial data bus , for asynchronous communication . The bi-directional communication bus, may be implemented full-duplex transmission and reception.
3. RS232
Called standard serial port, most commonly used as a serial communication interface. By the Electronic Industries Association (Electronic Industries Association, EIA) developed by asynchronous transmission standard interface.
Typically RS-232 interface with 9 pins (DB-9) or 25 pins (DB-25) form appears, RS (recommended standard) represent recommended standard 232 is an identification number. It is generally called RS232 RS232-C
RS232 is a negative logic level, which defines the + 5 ~ + 12V is low, but a high level -12 ~ -5V.
Industrial control generally RS-232 port using RXD (2), TXD (3), GND (5) three lines.
RS-232 take the unbalanced transmission mode, i.e., a so-called single-ended communication. Because of its transmission level and the reception level difference is only approximately 2V to 3V, so poor common mode rejection, together with the distributed capacitance of the twisted pair, which is a maximum transmission distance of about 15 meters, maximum rate of 20kb / s.
4. RS485
RS-485 又名 GRAMMY-485-A, ANSI / were / are 或-485 were / are-485.
RS485 is a standard electrical characteristics of balanced digital multipoint systems in the driver and receiver define the criteria defined by the Telecommunications Industry Association and the Electronic Industries Alliance. The use of a standard digital communications network can effectively transmit signals at the remote electronic noise conditions and large environment. RS-485 network, and configured such that an inexpensive local multidrop communication links becomes possible.
From RS-422 developed on the basis of so many RS-485 and RS-422 electrical regulations similar. The balanced transmission mode are adopted, then terminating resistors are required in the transmission line and the like. RS-485 can be used with two-wire four-wire, the wire may be true multi-point two-way communication, whereas when using four-wire connection, as can only be achieved with the RS-422 point-to-many communication, i.e., only one primary ( Master) device, the rest is from the device.
Differential signal lines has a strong interference, especially for complex electromagnetic environment applied to the industrial control environment, RS-485 protocol is mainly to improve the RS-232 signal to a differential signal, thus greatly improving the characteristics of interference.
RS-485 and RS-422 as the maximum transmission distance is about 1219 m, a maximum transmission rate of 10Mb / s. Balanced twisted pair transmission rate is inversely proportional to the length, at 100kb / s rate or less, it is possible to use a predetermined maximum cable length. Only in a very short distance in order to obtain the highest rate of transmission. In general, 100 m long twisted maximum transmission rate of only 1Mb / s.
An RS485 bus can see what how many devices in parallel chips, and with the quality of the cable and associated, the more nodes, the transmission distance, the more severe electromagnetic environment, the higher the selected cable requirements.
Support 32 nodes chips: SN75176, SN75276, SN75179, SN75180, MAX485, MAX488, MAX490
支持64个节点数的芯片：SN75LBC184
支持128个节点数的芯片：MAX487，MAX1487
支持256个节点数的芯片：MAX1482，MAX1483，MAX3080～MAX3089
参考：https://blog.csdn.net/derryzhang/article/details/5004761
5. RS232 与 RS485 对比
抗干扰性：RS485 接口是采用平衡驱动器和差分接收器的组合，抗噪声干扰性好。RS232 接口使用一根信号线和一根信号返回线而构成共地的传输形式，这种共地传输容易产生共模干扰。
传输距离：RS485 接口的最大传输距离标准值为 1200 米（9600bps 时），实际上可达 3000 米。RS232 传输距离有限，最大传输距离标准值为 50 米，实际上也只能用在 15 米左右。
通信能力：RS-485 接口在总线上是允许连接多达128个收发器，用户可以利用单一的 RS-485 接口方便地建立起设备网络。RS-232只允许一对一通信。
传输速率：RS-232传输速率较低，在异步传输时，波特率为 20Kbps。RS-485 的数据最高传输速率为 10Mbps 。
信号线：RS485 接口组成的半双工网络，一般只需二根信号线。RS-232 口一般只使用 RXD、TXD、GND 三条线 。
电气电平值：RS-485的逻辑"1"以两线间的电压差为+（2-6） V 表示；逻辑"0"以两线间的电压差为-（2-6）V 表 示 。在 RS-232-C 中任何一条信号线的电压均为负逻辑关系。即：逻辑"1"，-5- -15V；逻辑"0 " +5- +15V 。