how to prove working of RS 232 full modem,RS 422 working PC to PC and LOOP BACK

Hello there I am a newbie trying to prove the working of RS 232 Full modem and also one RS 4开发者_运维知识库22( RX,TX,RTS,CTS)

These 2 ports are on a custom designed board and I need to prove they are working.

I am able to confirm the working at register level but I need to prove the working using softwares like Minicom or any other custom program.

How can I prove the working of these ports from one PC to a different PC using DB 9 connections and LOOP BACK too

Can someone help me with this? Do I need to use any extra hardware to prove the working of this in Linux?

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The most common type of serial port test is probably a loopback test. Create a test fixture that connects output pins of the port to the input pins (TX->RX, RTS->CTS, etc). If you do not have matching input pins for every output pin, you will need to do a three-way connection.

After you create the loopback, you will need to write software that exercises the pins. If TX and RX are connected, you can send a byte and verify that it was echoed back. For the control pins, toggle them and make sure the other side of the connection saw the transition. Make sure you exercise every pin of the serial port.

Note that you should run a TX->RX data loopback at multiple baud rates. It is possible for there to be a signal integrity issue in the design that only shows up at higher bauds rates. It is also possible for there to be a bad signal connection on the board that is masked by inductance and capacitance at higher baud rates. Therefore, it is a good idea to run a data loopback at the slowest baud rate, the fastest baud rate, and 1-2 in the middle.

Another thing you should do is a baud rate accuracy test. This will prove the clock driving the UART is running at the right frequency, and being divided properly. Transmit X amount of bytes at a certain baud rate and verify they arrived in the expected amount of time. To get an accurate number, you will need to bypass any buffering in the OS serial port driver (e.g. use direct register I/O), and make sure to accommodate for any start/stop bit overhead (see comments below).

However, a loopback test is not exhaustive. It only proves the device can talk to itself. The device may still have some flaw (e.g. voltage levels) that cannot be detected locally. So, you should also run some tests with external hardware. Cable your board to another system and run a test (e.g. with minicom). Make sure they can talk to each other.

Even an external communication test can miss something. You can still have poor signal quality from your board, but it happens to be good enough for the other device. To accurately verify the signal quality/timing, you need an oscilloscope.

While you are running communication tests, connect the scope probes to the various signals and verify the signal integrity. Make sure that voltage levels are valid, that you see clean low/high bit transitions, and that the timing of the data pin appears correct for the specified baud rate. (A scope can be a more accurate way to measure baud rate than the software-based method described earlier.)