| A loopback test is a simple way to verify that RS-232 communications hardware
is working. You may test a radio interface and your computer's serial port
using this method. A loopback test simply sends data from a
port, through a jumper, and back to the serial port. If everything is working,
you will be able to type something on the keyboard and see it appear on the
terminal window. The signal travels from your keyboard, out the serial port,
thru the jumper, back into the serial port, and is displayed on the
screen. This tests the
port, the level converter device, and any connecting cable. If the
loopback jumper is in place, and you do not see your typing, something is
malfunctoning or the jumper is not connected properly.
We recommend using the built-in terminal in LOGic. It is easier to configure
than HyperTerminal. If you have not purchased LOGic, you my use the LOGIC demo at
http://hosenose.com/logic
However, any terminal program will work,
including Windows HyperTerminal or the terminal in TRX-Manager.
In LOGic, go to tools/setup/misc ham setup. Select the Data Terminal
page. Note that while the terminal is intended to communicate with a TNC
or multimode controller, we will be connecting the radio interface to it.
The baud rate isn't critical, so long as it is slow enough for your hardware to
support it. 9600 is a good choice. The setting for stop bits doesn't
matter. Set Parity to None, word length to 8 bits.
If using a terminal other than LOGic, you may have additional settings to
deal with. Turn DTR on. This provides power to our level converter
interfaces. Select Full Duplex or turn off Local Echo.
It is recommended that you first do a loopback test on a known good serial
port. This will verify that you have everything set up correctly and are
performing the test properly.
Testing the serial port
Type something in the terminal. (In LOGic, make sure the cursor is in
the lower input field. This provides immediate input and output to
your port. The top input field is for packet radio and only sends data
when you press Enter). You SHOULD NOT see your typing displayed on the screen.
This chart shows the location of pins 2 and 3 on a 9-pin male
connector (the type found on the back of your computer)
1 2 3 4 5
6 7 8 9
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Now,
short pins 2 and 3 (the transmit and receive pins) of the serial port together.
The pins are usually numbered. Get a flashlight and look closely, or use
the chart provided on this page. A breakout box is the ideal device for shorting
these pins, but they are not easy to find and are fairly expensive. Use
micro jumper clips, a larger alligator clip with a nose wide enough to bite both
pins, or have a partner hold a clean miniature screwdriver over the pins.
An RS-232 extension cable that is known to be good can simplify things.
Just use the extension cable to extend the port on the back of your computer so
you don't have to work behind your computer.
With the transmit and receive pins shorted together, type something. What you type should be displayed on the screen. If this test
fails, your port is bad, the jumpers on the board are set incorrectly, or it is
not the port number you think it is.
The ports on the back of your computer are often not labeled. We get a
lot of tech support calls simply because the customer does not know which port
is which. A loopback test is a good way to determine which port is whuch.
On an ATX tower case, COM1 is usually the uppermost 9-pin port.
Testing the level converter interface
Level-converter interface pin diagrams
Short the pins indicated with red together.
This view is from the outside of the connector, looking at the end of the
pins.
Kenwood and Yaesu Big Din
•
• •
•
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Yaesu Mini-DIN
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• •
• •
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Follow this procedure when testing radio interfaces other than ICOM: Plug the
interface into the computer port without connecting the radio. Type something.
You should NOT see your typing. If you do, there is a short somewhere. Now,
short the transmit and receive pins on the radio side of the interface together
(consult the diagram on this page). Type something. Your typing should
appear on the screen. If it does not, the interface or cable is bad.
For the Icom line, which uses CT-17, LCU-3-Icom, and other similar circuits, the testing procedure
is different because there is no separate transmit and receive lines to jumper
together. The transmit and receive lines are already shorted
internally. Just plug the interface into your serial port. Do not
plug the interface into the radio. Type something,
and you should see your typing. Unplug the interface, and type something.
You should not see your typing.
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