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

Question

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.

Answer

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

/

"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)