关于232，422接口

232，3跟线，发送，接收，地。由于两端的地的电平有差异，因此，需要使用大于5伏的电压。实际是12伏。标准里最大速率只到19.2k，虽然实际会使用到57.6和38.4k。

422，4根线，发送的差分±，接收的差分±。通过差分信号线形成回流，因此，可以采用更低的电压，最大差分电压±6伏。通信距离，1200米支持90kps，12米支持10Mbps。

RS-422 specifies differential signaling, with every data line paired with a dedicated return line. It is the voltage difference between these two lines that define the mark and space, rather than, as in RS-232, the difference in voltage between a data line and a local ground. As the ground voltage can differ at either end of the cable, this required RS-232 to use signals with voltage magnitudes greater than 5 volts. Moving to dedicated return lines and always defining ground in reference to the sender allowed RS-422 to use 0.4 V, allowing it to run at much higher speeds.

Conservative maximum data rates with 24AWG UTP (POTS) cable are 10 Mbit/s at 12 m to 90 kbit/s at 1200 m as shown in the figure A.1.

磨砂CP-134-U，支持4路422和485，其中1和2路支持232。