Loopback Test

You can easily test your PC serial port and cable with a simple loopback test. On a 9 pin connector, the PC transmits data on pin 3, and receives on pin 2. With the cable disconnected from the board, you can short pins 2 and 3 together, as shown in figure 8. Most small metal objects will work, such as a paper-clip, though the test will be much less frustrating with two people, one to hold the connection on pins 2 and 3 while the other uses the computer.

Figure 1: Serial Port Loopback Test, Short Pins 2 and 3

While pins 2 and 3 are shorted, you should type into the terminal emulation window. As you type, the characters should appear on the screen, because they are sent on pin 3 and immediately received on pin 2. When the short is removed, no characters should appear on the screen as you type.

If this test fails, usually as no characters appearing when the pins are shorted, then something is wrong with the PC serial port, or the cable. Often times the ports on the back of your PC are simply mis-labeled and the cable is connected to the wrong one. In some cases, a serial port may not work because some other program is attempting to use it. It is rare for the actual serial port hardware to have a problem, except in the case of older PC motherboards where the serial port is connected by a ribbon cable to the board. Sometimes the ribbon cable comes loose, or is plugged in backwards or is not seated properly.

If the loopback test does not show characters appearing in the terminal emulation window as you type, you must resolve this problem before attempting to use the 8051 development board.