The other cable end should have the RX/TX pins shorted using a paper clip or a piece of wire. Insert the cable Insert the unmodified end of the cable into the serial port you want to test. Performing a Loopback Test with Windows HyperTerminalĪfter you modify the null-modem cable as directed in the sections above, you can perform the loopback test. You also need one RS-232 or RS-485 null-modem cable per port, as shown above. * This is only a convertor cable/box to male DB9 connector. Below is a list of serial cables National Instruments offers: 1. The type of null-modem serial cable you need depends on the type of serial card you have. However, if you do short the pins, short pin# 7 to pin# 3 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). 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 a connector very similar to the network cable connector, but the wiring scheme is different. The RJ-45 connector is am 8-pin connector, as shown below. 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: The figure below shows the female DB-25 connector with pin# 2 and pin# 3 shorted. The DB-25 connector is a 25-pin connector, as shown below. The figure below shows the female DB-9 connector with pin# 4 and pin# 8 and pin #5 and pin# 9 shorted. The DB-9 connector is a 9-pin connector, as shown below. The figure below shows the female DB-9 connector with pin# 2 and pin# 3 shorted. RS-232 DB-9 Connector The DB-9 connector is a 9-pin connector, as shown below. RS-232 DB-9 Connector RS-485 DB-9 Connector RS-232 DB-25 Connector RS-485 RJ-45 Connector Serial Cables Performing a Loopback Test with Windows HyperTerminal Follow the steps below to modify the appropriate cable. ![]() ![]() 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. To perform this test, you need to modify your null-modem serial cable temporarily so that the transmit and receive pins are connected. ![]() How to Do a Serial Loopback Test Overview A loopback test allows you to send and receive data from the same serial port to verify that the port is operational. Login or Create a user profile.ĭocument Type: Tutorial NI Supported: Yes Publish Date: Feb 18, 2008 How to Do a Serial Loopback Test- Developer Zone - National Instruments
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