Performing a Serial Loopback Test on the COM2 Port of a Touch Panel Computer

Issue Details

I want to perform a Loopback Test on the COM2 port of a Touch Panel Computer to verify correct operations. Is it possible?

Solution

It is possible to perform a Loopback Test on the COM2 port from a TPC even when the pinout is different. You just need to ensure that the TX and RX terminals of equal sign are connected together. 
 

Pin	RS-422	RS-485
1	TX-	D-
2	TX+	D+
3	RX+	-
4	RX-	-
5	GND	GND
6	-	-
7	-	-
8	-	-
9	-	-

Therefore, if the port is set to RS-422, you need to connect pins 1 to 4 and 2 to 3. These connections on a female RS-422/485 DE-9 plug are shown in red in the figure below. 

Additional Information

If the port is set to RS-485, the pins assignment shows a 2 wire mode, for which a Loopback Test is not possible by definition. However, if you wire the pins in the same way required for RS-422, the Loopback Test can be executed successfully anyway. 

Once the appropriate pins have been connected, you can follow the How to Do a Serial Loopback Test tutorial to verify if the COM2 port is working properly. 

Note: The COM2 port from the TPC-2230 can be configured using the SW2 jumpers. The other models need to be configured in the BIOS. More detailed instructions can be found in the corresponding user manuals from the Related Links section. 

Note: If an RS-422/485 Null Model cable is used, pin 4 must be replaced with pin 8. Please refer to What is the 485 Null-Modem Cable Pinout? for further details. 

Note: The COM2 port from the TPC-2230 can also be configured as RS-232 as explained in the User Manual. For this mode, there are no additional considerations required than those explained in the How to Do a Serial Loopback Test tutorial.