FAQs from Rohde & Schwarz

How can I set up the RTH for correct RS232 / UART decoding?


I want to decode a RS232 / UART signal with my RTH. Which settings must be done to perform a measurement? Currently my scope decodes but shows me the wrong character. I am sending the letter "c" in a loop but the decoder shows only "N" or "F" instead.


The following options must be available to perform CAN bus analysis on the RTH:

  • R&S®RTH-K2 UART/RS232 triggering and decoding (1325.9975.02)

The RTH user manual gives information about this option in chapter 9.4 (Version 1326.1578.02 ─ 15) beginning on page 198. It also describes the UART interface in general. Please perform the following steps on your RTH to measure the information of a UART / RS-232 signal:

The best way to set up the measurements is the following (same way for RS232 as well as UART TTL level):

  • Connect CH1 to the serial bus signal source and be sure to have data coming through the port
  • Be sure to be in the Scope Mode ([MODE]-"Scope")
  • Press <PRESET>
  • Press <AUTO SET>
  • Change the time Base to e.g. 100 ms/div to have enough data available for the test
  • All the following steps will need to scroll the available menu up or down to find the wanted menu point.
  • Open the "Bus" Menu by touching the R&S logo on the right lower screen and then choose "Bus"
  • Change "Bus Protocol" to UART and change "State" to "I"
  • Set the "Display Format" to e.g. ASCII
Set the "Display Format" to e.g. ASCII
  • "Config" opens a new menu
  • "Polarity" has to be set on "Idle Low" for RS232 but on "Idle High" for UART TTL

Explanation: While UART is TTL level and the Idle State is high.
RS232 is inverted after level conversion (+/-3 up to +/- 15 V), so here the Idle State is low.
This can be the reason for diplaying wrong / shifted characters.

  • "Find Level" automatically changes the Threshold and Technology settings
  • Set communication parameters (e.g. Bit Rate, Data Bits, Parity, Stop Bits, Bit Order to LSB First for RS232)
Set communication parameters
  • Press "Back"
  • Press the "R&S Logo" again to leave the menu now. If all settings are fine, you should see the following screen now:
Leave menu
  • It is also possible to display the content in list format:
  • Press <MODE> and choose Protocol
  • The display will now change to the list mode
Display list mode

This FAQ article was written using FW