General information
　　Serial connector communication of two of the most basic ways: synchronous and asynchronous serial communication serial communication. Serial interface by electrical standards and protocols to include RS-232-C, RS-422, RS485 etc. RS-232-C, RS-422 and RS-485 are only on the characteristics of the electrical interface provided, without regard to connectors, cables, or Protocol.
RS-232
　　Also known as a standard serial port, one of the most common serial communication interface. It was in 1970 the United States electronic industry association (EIA), Bell System computer terminal, modem manufacturers and manufacturers to develop standard for serial communication. Its full name is "data terminal equipment (DTE) and data communications equipment (DCE) interface standard for serial binary data exchange between". 　　Traditional RS-232-C standard 22 wires, using standard core 25 d-type plugs (DB25), later reduced to 9 core d-socket (DB9), now 25 block core plug has been rarely used in the application. RS-232 unbalanced transmission mode, the so-called single-side communication. Due to their send level and receive level difference between 2V 3V-only, so its common-mode rejection differential, coupled with the distributed capacitance on the twisted, its maximum transmission distance is about 15 m, maximum rate of 20kb/s. RS-232 is for the point to point (that is, a pair of ship, receive-only device) communication and design, drive load it as a 3~7k ω. RS-232 for local communication between the devices.
RS-422
　　Standard designation is "electrical characteristics of balanced voltage digital interface circuit", it defines the interface circuit features. Typical RS-422 are four-wire interface. There are a signal ground, total 5 wires. The DB9 connector pin definition. Because the receiver with high input impedance and sending ability to drive stronger than RS232 driver, allowing multiple receive nodes on the same transmission lines up to 10 nodes. A master device (Master), the rest is from the device (Slave), communicate between devices, so RS-422 support bi-directional communication. Receiver input impedance of 4k, began the largest load capacity are 10x4k+100 ω (terminating resistor). 　　RS-422 four-wire interface as a separate send and receive channels, so you don't have to control the data, any signal that must be exchanged between the various devices are software (XON/XOFF shaking hands) or hardware (single pair of twisted-pair cable). RS-422 the maximum transmission distance of 1219 meters, maximum transfer rate of up to 10Mb/s. Its balanced twisted-pair cable and transfer rate is inversely proportional to the length of, 100kb/s rate, can achieve maximum transmission distance. Only in a very short distance in order to achieve the highest rate of transmission. 100 m long double strand of the maximum transfer rate is only available on 1Mb/s.
RS-485
　　RS-422 developed on the basis of which is coming from, so many electrical requirements and RS-485 similar to RS-422. Such as balance transfer, are required in transmission line termination resistors. 　　RS-485 can used second-line and four line way, second-line system can implementation real of more points bi-directional communications, and used four line connection Shi, and RS-422 as only implementation points on more of communications, is only has a main (Master) device, remaining for from device, but it than RS-422 has improved, regardless of four line also is second-line connection way bus Shang can more received 32 a device. 　　RS-485 and RS-422 are different is that the common mode output voltage is different, between +12V and RS-485 is -7V and RS-422 between -7V to +7V, RS-485 minimum input impedance receiver range 12K, RS-422 are 4k ω; due to RS-485 meet the specifications of all RS-422, so the drive can be used in RS-422 RS-485 in the network application. RS-485 and RS-422, the maximum transmission distance of about 1219 meters, maximum transfer rate of up to 10Mb/s. Balanced twisted-pair cable and transfer rate is inversely proportional to the length of, 100kb/s rate below, may only be used provided the longest cable length. Only in a very short distance in order to achieve the highest rate of transmission. UTP 100-meter maximum transfer rate is only 1Mb/s.