To measure single RS-232 signals, a measuring instrument with at least one channel is required. To measure full-duplex signals, an instrument with at least two channels is required. The maximum frequency on the RS-232 bus depends on the bit rate that is used. The instrument must sample at at least three times the bit rate on the bus, but preferable ten times higher, on each channel. For a 110 bit/s bus that would mean a minimum sampling frequency of at least 1 kHz on both channels. RS-232 uses voltages between -12 and +12 Volt, so the instrument should be able to measure voltages between -12 and +12 Volt. Since data transfers can be long, a long record length is preferred to capture the communication.

The Handyscope HS3, Handyscope HS4 and the Handyscope HS4 DIFF are suitable instruments to measure RS-232 signals.

To examine the transferred data in the measured signals, the various pulses have to be examined, the start and stop bits have to be taken out and the parity bit too, when used. The remaining bits have to be converted into readable data.

The TiePie engineering Multi Channel software is capable of analyzing RS232 signals, using the Serial analyzer.

In this example, a Handyscope HS4 is used to measure serial communication from a computer running a terminal program, in which text is typed.

Open your favorite terminal program (e.g. Hyperterminal) and set it up to communicate through a serial (COM) port using the following settings:

setting	value
bit rate	110
data bits	8
parity	None
stop bits	1
data transport control	None

Text typed in the terminal program should now be transmitted directly through the serial port.

Since only one channel is used to measure the RS-232 signal, Ch2, Ch3 and Ch4 of the Handyscope HS4 are removed from the screen.

We are using Ch1 to measure the Transmitted Data (TxD). To simplify recognition of the signal, it can be given a descriptive name (alias). To change the alias of a channel, right-click the channel in the object tree and select Set alias... and enter the required alias. Give Ch1 the alias "TxD".

RS-232 signals lie between -12 Volt and +12 Volt, idle state = -12 Volt. Therefore, set the channel input coupling of the channel to "DC" and set the input sensitivity of the channel to "20 Volt" full scale. That way both signal levels can be measured properly.

In our example, a 110 bit/s serial communication is used. Therefore, set the time base to a sampling frequency that is ten times higher, 1 kHz. Since we will be typing text in a terminal program, we need sufficient measuring time, e.g. 2 seconds. With a sampling frequency of 1 kHz, that requires 2000 samples record length.

In idle state, the TxD line has a logic "1", which means that the voltage on the line = -12 Volt. When communication starts, first a start bit, a logic "0" (+12 Volt) is transmitted. That means that the start of a communication sequence is a rising edge. Therefore, set the trigger type to rising edge. The trigger level and trigger hysteresis are not really important, as long as they are within the -12 .. +12 Volt range. Set the trigger level to e.g. 50% and the hysteresis to e.g. 2.5%. To make sure that the measurement is only started when the communication begins, the trigger timeout has to be set to infinite.

To analyze the RS-232 signals, the serial analyzer sink is used. Create one by right-clicking Sinks in the object tree and selecting Serial analyzer. This also opens an additional window, which will display the analyzed RS-232 communication.

The serial analyzer can handle up to eight serial communication signals, but in our example, only one signal will be analyzed. Connect Ch1, TxD, to the serial analyzer by dragging it on the serial analyzer sink in the object tree.

Port settings