RS-232/UART Triggering and Hardware-Based Decode (N5457A) for InfiniiVision Oscilloscopes

Introduction

Debugging systems that use an RS‑232 serial bus can be difficult, unless your scope is capable of triggering on and decoding the RS-232 protocol. Traditional methods of debugging serial buses such as RS-232 include manual bit counting. But this visual technique of counting “1s” and “0s” can be tedious and it is prone to errors.

Keysight Technologies, Inc. serial bus options for InfiniiVision oscilloscopes offer powerful triggering and provide unique hardware-accelerated decoding to help you debug embedded designs with RS-232 and UART serial buses faster. With the industry’s fastest serial decode update rates, you can more easily find and debug random and intermittent errors and signal integrity problems that you could easily miss using other serial bus decode tools.

Other oscilloscope solutions with serial bus triggering and protocol decode typically use software post-processing techniques to decode serial packets/frames. Using these software techniques, waveform-and decode-update rates tend to be slow (sometimes seconds per update), especially when you use deep memory, which is often required to capture multiple packetized serial signals.

RS-232/UART protocol decoding formats

You can serially decode transmit and receive signals in either binary, hex, or ASCII format. The protocol decode data is always displayed in a time-correlated trace below the captured waveforms. In addition, the decoded data can also be viewed in a tabular format using the "lister" display mode. Figure 2 shows a transmit and receive message decoded in hex format.

Figure 3 shows the same message decoded in a color-coded

ASCII format. Note the red “1” character at the end of this important message. Red is an indication that a parity error was detected for that particular byte. And with Keysight's 7000B Series oscilloscope, you can also easily search and navigate within the protocol lister display to find and mark particular events of interest with direct time- correlation to the waveform display.

Triggering

Figure 4 shows the available selections for triggering on RS‑232/UART transmit and receive signals, including triggering on either receive or transmit parity errors.

In addition to being able to trigger on specific transmit or receive bytes, this scope's "burst trigger" mode allows you to specify to trigger on the "Nth" byte within a burst of data bytes that satisfies the data entry.

Real-time frame/byte totalizer

In addition to flagging parity errors, InfiniiVision’s RS-232/UART option also provides a real-time totalizer/counter of transmitted and received frames/bytes, as shown in Figure 5. In addition, the option allows you to count parity error bytes and see a percent readout that gives an indication of the quality of your serial bus. This totalize function, which is not available in other oscilloscopes currently on the market, is independent from the scope’s acquisition or triggering. In addition, this RS-232/UART byte counter is not affected by either the oscilloscope’s acquisition window or scope dead-time. Totalize counts run continuously, even when the scope’s acquisition is stopped.