Decoding Unknown Serial Coms

[mgreenhough]

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...

[Electro Fan]

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=1739733420513

Older logic analyzers are more likely to support such voltage, newer/less expensive LAs probably less likely.

[mgreenhough]

Thanks for the reply. So it's possible that even though the data "looks" good, it might be corrupted because of the high voltage?

[ledtester]

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/B00LPK0Z9A

but 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

[mgreenhough]

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?

[hexreader]

looks to me like 8 bit, even, 1 stop

the 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 cable

MAX232 is an inverting device

notice how your decoder detects a start bit a few bits after the first low to high transition
it should show start bit one bit-time after the first low to high transition

[mgreenhough]

Thanks hexreader! Yes I did notice that after posting. Unfortunately with inversion applied and even parity it still seems off?

[hexreader]

that is looking a whole lot better

clearly my "even parity" guess was wrong

Next guess has to be nine bits, no parity, 1 stop bit


EDIT - oops - just spotted the framing errors
brain not working too well right now

8,n,1    ??

[mgreenhough]

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 protocol

EDIT - Haha, mine either! No go on 8,n,1. or almost any other combo I've tried.

[hexreader]

Brain still scrambled, but since you have the data, the tools and the patience...

9,n,1
9,e,1
9,o,1

It 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 it

may look again later, if IQ improves
still looks very much like normal async rs232 to me

[mgreenhough]

No bueno of any of the above unfortunately.

[mgreenhough]

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!

[BennoG]

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

[mgreenhough]

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?

[hexreader]

It also looks like the sample rate is too low
Benno

Nice catch Benno
Kicking myself that I did not think of that