How to Do a Serial Loopback Test

A loopback test allows you to send and receive data from the same serial port to verify that the port is operational. To perform this test, you need to modify your null-modem serial cable temporarily so that the transmit and receive pins are connected. If you do not have a null-modem cable, or if you do not know which serial cable to purchase for your serial card, see the Serial Cables section below. Follow the steps below to modify the appropriate cable.

RS-232 DB-9 Connector

The DB-9 connector is a 9-pin connector, as shown below. You need to insert one end of the cable into your serial port and use the other free end to make the loopback connection. The figure below shows the female DB-9 connector with pin# 2 and pin# 3 shorted. You can also use a simple paper clip to short the two connections by inserting the clip into each hole in the female connector, as shown below:

RS-485 DB-9 Connector

The DB-9 connector is a 9-pin connector, as shown below. You need to insert one end of the cable into your serial port and use the other free end to make the loopback connection. The figure below shows the female DB-9 connector with pin# 4 and pin# 8 and pin #5 and pin# 9 shorted. You can also use a simple paper clip to short the two connections by inserting the clip into each hole in the female connector, as shown below:

RS-232 DB-25 Connector

The DB-25 connector is a 25-pin connector, as shown below. You need to insert one end of the cable into your serial port and use the other free end to make the loopback connection. The figure below shows the female DB-25 connector with pin# 2 and pin# 3 shorted. You can also use a simple paper clip to short the two connections by inserting the clip into each hole in the female connector, as shown below:

RS-485 RJ-45 Connector

The RJ-45 connector is am 8-pin connector, as shown below. It is a connector very similar to the network cable connector, but the wiring scheme is different. You need to insert one end of the cable into your serial port and use the other free end to make the loopback connection. It is not easy to short pins with this connector; therefore, you should purchase the RJ-45 to DB-9 convertor cable (part #182845-01). However, if you do short the pins, short pin# 7 to pin# 3 and pin# 6 to pin #2.

Serial Cables

The type of null-modem serial cable you need depends on the type of serial card you have. Below is a list of serial cables National Instruments offers:

1. RS-232 9-pin female D-Sub to 9-pin female D-Sub

182238-01 (1 meter)
182238-02 (2 meters)
182238-04 (4 meters)

2. RS-485 9-pin female D-Sub to 9-pin female D-Sub

183283-01 (1 meter)
183283-02 (2 meters)
183283-04 (4 meters)

3. RS-232/485 10-position RJ-45 to 9-pin male D-Sub

182845-01 (1 meter)*
184428-01 (1 meter isolated cable)*

4. RS-232/485 PCMCIA to 9-pin male D-Sub

183905-0R3 (30 cm) for single port and 2-port PCMCIA-232/485 cards*
185205-01 (1 meter) for 4 port PCMCIA-232 cards*

5. RS-232 SCSI-68 to 8-port 9-pin male D-Sub

185196-01 (1 meter) for 8 port PCI/PXI-232 cards*

6. RS-485 SCSI-68 to 8-port 9-pin male D-Sub

185845-01 (1 meter) for 8 port PCI/PXI-485 cards*

7. RS-232 16-port breakout box

186936-01 (1 meter) for 16 port PCI/PXI-232 cards*

* This is only a convertor cable/box to male DB9 connector. You also need one RS-232 or RS-485 null-modem cable per port, as shown above.

Performing a Loopback Test with Windows HyperTerminal

After you modify the null-modem cable as directed in the sections above, you can perform the loopback test.

Insert the cable

Launch HyperTerminal

Launch HyperTerminal by selecting Start»Programs»Accessories»Communications»HyperTerminal.


[+] Enlarge Image

Create a Session

Create a session by giving it a name and press OK. The example below shows the name Loopback Test.



Select the COM port from the drop-down list and press OK.



Select the serial properties shown below and press OK.



You will see the terminal window as shown below. If your serial port is working and the loopback cable is properly connected, the text you type appears in the window. If you remove the cable and type text, it should not appear in this window. If this test behaves differently than described here, see the troubleshooting advice at the end of this section.

If the loopback test is successful, it indicates that your serial port is functional and you can send and receive data through that port.


[+] Enlarge Image

Troubleshooting

335 ratings | 4.52 out of 5

Read in English French | Print

Reader Comments | Submit a comment »

thanks! I've read it and i've checked successfully!
- Apr 6, 2008

very good document
- siva prasad nandyala, wipro. siva.nandyala@wipro.com - Mar 13, 2008

Very useful technical information. K.N. Lakshmi Pathi Advanced Electronics Company Riyadh
- Kadaba Lakshmi Pathi, Advanced Electronics Company. knlpathi@hotmail.com - Feb 3, 2008

Very useful technical information. K.N. Lakshmi Pathi Advanced Electronics Company Riyadh
- Kadaba Lakshmi Pathi, Advanced Electronics Company. knlpathi@hotmail.com - Feb 2, 2008

Hi, I'm trying a different type of environment of loopback. The loopback suggested by you is done locally. Wondering how the below can be done :- Step 1 : Tx characters from modem A to Modem B using hyper term. Step 2 : modem B upon receiving the characters auto tx the characters received back to modem A. A summary of the enviorment is that whatever Modem A transmits, Modem B will do a auto loopback to modem A. Are there any AT commands that support this operation? Any advice is most appreciated. Regards, Zhiwei
- Zhiwei. chiopigwinner@hotmail.com - Jan 10, 2008

thanks!
- pham hung, DH Bach Khoa Ha Noi. pkh_1985@yahoo.com - Jan 7, 2008

Thank you. This web site was a big help because I have been looking for a way to do this.
- larry.harrison@ucps.k12.nc.us - Nov 7, 2007

Simply Superb, But Need Further Details
This type of tesing I have already done and its working really great for me. But the problem that am facing is, I use MSCOMM32.OCX to connect to the COM Port. I get message saying PORT ALREADY OPEN, When actually Port is closed and not used by other process and Vis Versa. Can any one put in some idea about why this happens. Thanks in Advance.
- lakshmikanthkamath@gmail.com - Jun 27, 2007

Thanks!
thank you very much, it really helps. =)
- Jun 8, 2007

Great!
This helps out a lot, thanks!
- Apr 22, 2007

Perfect, thanks
Who would have thought I'd finally get some use out of that Quarterdeck loopback plug set all these years later? Thanks!
- Andrew. andrew.rich@gmail.com - Feb 12, 2007

Great Document
Exactly what I needed to sove a problem with a new Serial card.
- Jake. jatexas@camtel.net - Jan 11, 2007

Superb Article
This is exactly the article I was looking for. Thanks a lot.
- Susanta Mondal. cse_susanta@rediffmail.com - Sep 25, 2006

I am not clearly know that when I use modem connect mode, what type of the loopback connector I need, or modem connect mode can not be tested by loopback? Thanks!
- When I use modem connection mode?, china. golden_ctbu@yahoo.com.cn - Apr 25, 2006

Great document
I was really looking to do some serial loopback tests, and this document also had the way to test them using HyperTerminal. Great document!
- Dustin Martínez, Electronics School, University of Cauca (Colombia). tbb_mb@yahoo.com - Jan 24, 2006

great technical document
Thanks for this summary of testing serial ports!
- Oct 17, 2005

Looking to test Serial Port via DOS in Interrupt mode!
Looking to test Serial Port via DOS in Interrupt mode! Any quick examples I can "borrow"?
- Roger Owens, Hamilton Sundstrand. roger.owens@hs.utc.com - Apr 3, 2002