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.