Datasheet

Application Note
www.tektronix.com/oscilloscopes20
Working With RS-232
Serial triggering and analysis for the RS-232 bus is available
on most Tektronix oscilloscopes (see Appendix A). You
can view your RS-232, RS-422, RS-485, or UART data
conveniently on your oscilloscope, without needing to attach
to a PC or a specialized decoder.
Using the front-panel bus buttons we can define an RS-232
bus by entering basic parameters, such as the channels being
used, bit rate, and parity (see Figure 27).
In this example, we have chosen ASCII decoding; the
oscilloscope can also display RS-232 data as binary or hex.
Imagine you have a device that polls a sensor for data over an
RS-232 bus. The sensor isn’t responding to requests for data.
You want to find out if the sensor isn’t receiving the requests,
or if it is receiving the requests but ignoring them.
First, probe the Tx and Rx lines and set up a bus on the
oscilloscope. Then set the oscilloscope to trigger when the
request for data is sent across the Tx line. The triggered
acquisition is shown in Figure 28.
Here, we can see the Tx line on digital channel 1, and the
Rx line on digital channel 0. But we’re more interested in
the decoded data, shown above the raw waveforms. We’ve
zoomed in to look at the response from the sensor. The
overview shows the request on the Tx line and the response
on the Rx line. The cursors show us that the reply comes
around 37 ms after the end of the request. Increasing the
controller’s timeout fixes the problem by giving enough time for
the sensor to reply.
The oscilloscope’s RS-232 trigger includes these capabilities:
Tx Start Bit - triggers on the bit indicating the start of a byte.
Tx End of Packet - triggers on the last byte in a packet.
A packet can be ended by a specific byte: Null (00 hex),
linefeed (0A hex), carriage return (0D hex), space (20 hex),
or FF hex.
Tx Data - triggers on up to 10 bytes of user-specified data
values.
Rx Start Bit, Rx End of Packet, and Rx Data - these are like
the Tx triggers, but on the Rx line.
With Tektronix oscilloscopes, you can easily view RS-232
signals, analyze them, and correlate them to other activity in
your device.
Figure 28. Measuring time delay between messages on two RS-232 buses.
Figure 27. RS-232 bus set-up menu.