Chapter 5
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5;
Sometimes there’s no substitute for viewing the waveforms. A digital oscillo-
scope is handy for viewing serial data. You can trigger on a Start bit or control
signal and the scope will display and preserve the waveform to examine at your
leisure. You can save a waveform and compare it to waveforms captured later.
When viewing RS-232 data, don’t forget that the data transmits least significant
bit first and that the logic levels are inverted. If the bits following the Start bit
are 11111110 from left to right on the screen, the value of the byte is 80h, not
FEh or 7Fh.
A logic analyzer is another tool for viewing serial data. Many logic analyzers
have eight or more channels so you can view multiple data and flow-control
lines at once. Also available are hardware and software data analyzers and testers
designed for debugging serial communications. These tools include capabilities
such as decoding data using various formats, triggering on specific characters,
and displaying statistics. Microsoft provides a free Portmon utility that displays
serial-port activity at the driver level. (Search microsoft.com for “portmon”.)
If you’re using a USB/serial converter and happen to have a USB protocol ana-
lyzer, the analyzer provides another way to view COM-port data.