Decoding Unknown Serial Coms - Page 1
Forum section: electronics / beginners

author: mgreenhough
author_rank: Newbie
timestamp: February 16, 2025, 09:16:30 pm
content: Hi guys, I have a GME XRS radio and I am trying to decode the serial protocol used to initiate TX so that I can add a Push To Talk switch. I have used a cheap clone logic analyser to intercept pins 7 & 8 but when using the UART decoder in Pulseview I can't seem to get any standard frame settings to fit. Is it possible they are using the "Switched 13v" as a clock for i2c? My analyser won't handle that voltage. The main chip on board is a R5F2L368CNFP that seems to support both protocols but the pinout (if correct) obviously suggests UART? Is there other asynchronous standards I should try to apply? Modbus doesn't fit either...

author: Electro Fan
author_rank: Super Contributor
timestamp: February 16, 2025, 10:12:14 pm
content: Yes, it’s very possible that you are seeing +/-13V.See 1.1 in link below.https://www.ti.com/lit/an/slla544/slla544.pdf?ts=1739733420513Older logic analyzers are more likely to support such voltage, newer/less expensive LAs probably less likely.

author: mgreenhough
author_rank: Newbie
timestamp: February 17, 2025, 12:44:49 am
content: Thanks for the reply. So it's possible that even though the data "looks" good, it might be corrupted because of the high voltage?

author: ledtester
author_rank: Super Contributor
timestamp: February 17, 2025, 02:16:27 am
content: The standard way to convert RS232 to TTL is with a MAX232 or similar chip as found in "RS232 to TTL modules", e.g.:https://www.amazon.com/HiLetgo-MAX3232-Converter-Module-Serial/dp/B00LPK0Z9Abut you can also do it with just a few transistors:https://scienceprog.com/alternatives-of-max232-in-low-budget-projects/https://www.electro-tech-online.com/custompdfs/2006/12/RS232-Shifter-v2.pdf

author: mgreenhough
author_rank: Newbie
timestamp: February 18, 2025, 04:22:00 am
content: Thanks guys! So the data I got through a MAX232 module looked very similar. (screen shot) I used 8n1 in this instance to try and decode and all other standard settings produce similar results, including with parity bits.Any suggestions? Looks more like a packet based or streaming protocol?

author: hexreader
author_rank: Frequent Contributor
timestamp: February 18, 2025, 06:16:33 am
content: looks to me like 8 bit, even, 1 stopthe big problem with your decoding is that the decoder is expecting "idle high" signals, but your signals are "idle low"either change your decoder to expect inverted signals, or change to reading the TTL signals of your MAX232, rather than the approx +/- 12 Volt signals on the transmission cableMAX232 is an inverting devicenotice how your decoder detects a start bit a few bits after the first low to high transitionit should show start bit one bit-time after the first low to high transition

author: mgreenhough
author_rank: Newbie
timestamp: February 18, 2025, 06:32:26 am
content: Thanks hexreader! Yes I did notice that after posting. Unfortunately with inversion applied and even parity it still seems off?

author: hexreader
author_rank: Frequent Contributor
timestamp: February 18, 2025, 06:39:27 am
content: that is looking a whole lot betterclearly my "even parity" guess was wrongNext guess has to be nine bits, no parity, 1 stop bitEDIT - oops - just spotted the framing errorsbrain not working too well right now8,n,1 ??

author: mgreenhough
author_rank: Newbie
timestamp: February 18, 2025, 06:50:15 am
content: Unfortunately not 9n1 either. The screen shots don't show it (if it'd allow me to upload the .sr file i would) but there doesn't even appear to be a common header or start/stop indicators if it was a variable frame length protocolEDIT - Haha, mine either! No go on 8,n,1. or almost any other combo I've tried.

author: hexreader
author_rank: Frequent Contributor
timestamp: February 18, 2025, 07:17:20 am
content: Brain still scrambled, but since you have the data, the tools and the patience...9,n,19,e,19,o,1It is very bad news if it is nine bits and parity. Only ancient hardware tended to use 9 bits AND parity. Few modern devices can handle itmay look again later, if IQ improvesstill looks very much like normal async rs232 to me

author: mgreenhough
author_rank: Newbie
timestamp: February 18, 2025, 08:01:25 am
content: No bueno of any of the above unfortunately.

author: mgreenhough
author_rank: Newbie
timestamp: February 18, 2025, 08:33:22 am
content: Sooooooo, fitting that I posted in the beginners section because I think I've made an absolute rookie mistake!I misinterpreted some instructions for finding the baud rate... bare with me while I try to ascertain the Actual baud!

author: BennoG
author_rank: Frequent Contributor
timestamp: February 18, 2025, 08:59:35 am
content: It also looks like the sample rate is too low, choose a sample rate at least 5 times the baud rate (preferable 10 times)Benno

author: mgreenhough
author_rank: Newbie
timestamp: February 18, 2025, 09:56:17 am
content: Thanks for your help guys! Yes, the sampling was definitely too low! 57600 Baud, 8,n,1! Now I just have to figure out what it actually means Hahaha! Looks like AT Command?

author: hexreader
author_rank: Frequent Contributor
timestamp: February 18, 2025, 10:53:55 am
content: Quote from: BennoG on February 18, 2025, 08:59:35 amNice catch BennoKicking myself that I did not think of that