RS-232 probe

This RS-232 probe is useful when troubleshooting a serial connection. It provides an optical indication of the line state and traffic on the line. 

Figure 1. Circuit diagram.

RS-232 has two states: +3 to +15 volt (logic 0, space) or -3 to -15 volt (logic 1, mark). The RS-232 probe displays the two states using LEDs, and you can see the states changing. The maximum baud rate on the line is 9600 baud. At faster speeds, you cannot see the LEDs flashing. 

The bit length in a 9600 baud signal is 104 microseconds, and with 10 bits being sent, the flashing will last about 1 milliseconds. The flashing can be detected by the naked eye. 

Please refer to figure 1. If the input of the probe is higher than 2.7 volt, the red LED is on (green LED is off). If the input is lower than -2.7 volt, the green LED is on (red LED is off).

The RS-232 probe is powered by two 9 volt batteries. The current consumption is low, and the batteries can last for a long time. Don't forget to disconnect the batteries when the probe is not used. 

Figure 2. Veroboard.

All components are fitted on a piece of Veroboard. This is my preferred way of building electronic circuits! The dent on LED1 faces away from T1. The dent on LED2 faces T2.

Best 73 from OZ1BXM Lars
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