RS-232, RS-485, and RS-422 often encounter various serial communication issues during on-site debugging, such as communication connection problems between different ports like RS-232, RS-485, and RS-422. Typically, conversion between RS-485 and RS-422 is quite convenient. However, when communicating between RS-232 and RS-422 ports, if there are no other conversion methods available on-site, and mailing could take a long time, a temporary solution can be applied. (1) Connect the R+ and T+ of RS-422 together and connect it to the GND of RS-232. Connect the R- of RS-422 to the Txd of RS-232, and connect the T- of RS-422 to the Rxd of RS-232. The principle is as follows: The electrical characteristics of the RS-232-C interface have a negative logic relationship for voltage. That is: Logic “1” is -5 to -15V; Logic “0” is +5 to +15V. The noise tolerance is 2V, meaning the receiver must be able to recognize signals as low as +3V as logic “0” and as high as -3V as logic “1”. The electrical characteristics of RS-485: Logic “1” is represented by the voltage difference between the two wires being + (2-6)V; Logic “0” is represented by the voltage difference being – (2-6)V. Therefore, when RS-232 sends “1”, the voltage difference between Txd and GND is -3 to -15V, forming a logic “1” level between R+ and R- of RS-422 (Note: the line level attenuation is minimal). When RS-422 sends “1”, the logic “1” level between T+ and T- of RS-422 is + (2-6)V, forming a logic “1” level between Txd and GND of RS-232 (Note: Since RS-422 communication uses differential levels, the attenuation over longer lines is also minimal, and the level received by RS-232 is maintained around 5 volts). The communication distance can be considered based on RS-232, which is generally less than 50 meters. I have temporarily tested this at Baota Petrochemical, and it was still good at 40 meters.
(2) RS-232 and RS-485: Connect the R+ and T+ of RS-485 together and connect it to the GND of RS-232. Short-circuit the R- and T- of RS-485, and short-circuit the Rxd and Txd of RS-232, then connect the two. Since RS-232 is full-duplex and RS-485 is half-duplex, if communication is needed, processing must be done in the communication program. However, for temporary use (like connecting a watt-hour meter to NAS-968D/E, using DEBUG to set the meter’s address), after preparing, first press and hold button 2 on the meter, send once (due to the short sending time interval), and then stop. You can see that the second string received is the address setting confirmation (At this time, you can leave RS-232’s Rxd disconnected; you will just not see the address setting confirmation string). The above is a bit of my small experience; please advise if there are any mistakes.
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