How To Figure Out What Communication Protocol This Is? (Oscilloscope Readings)

Thread Starter

Mahonroy

Joined Oct 21, 2014
406
I am looking at a data line and trying to make sense of the data. I am curious if you know a way of identifying this? This data is on a RJ12 cable between different devices, and the cables can be quite long (e.g. 50 feet).

There are 6 wires total on the RJ12. 2 wires are ground. 2 wires are 24VDC power, and then 2 wires are the data... one wire idles high at 5VDC, then goes between 0V and 5V for the data. The 2nd line is the opposite... it idles low at 0VDC, then goes between 0V and 5V for the data (the lines are synced). This is a differential pair right?

At first I thought this was RS-485, but not sure if those voltage levels make sense for that. I was also thinking maybe its UART serial communication, but I can't make sense of the data and nothing translates over to ascii.

Here is a snapshot of some of the data:
1696623229148.png
The yellow line idles high at 5V, and goes to 0V and 5V for data.
The purple line idles low at 5V, and goes to 5V and 0V for data.


I figured the baud rate is 38,400:
1696623160564.png


Here is a chunk of data that I am trying to analyze, and I annotated a few things on it:
1696623360078.png

This is the raw binary data (ignoring the start bit and stop bit):
11110110100010000010000000001000011000100000000010011000001000011111101000000

I can't manage to correlate this to anything. I tried different settings with the built in serial decoder on the oscilliscope, and it tends to match up data in the middle with 8N1 serial (8 data bits, no parity bit, 1 stop bit). But the beginning data and the end data don't match up to anything. Also if I try to put the HEX into a HEX to ASCII converter, its just jumbled.

Any idea on how I can figure this out? Thanks and any help or advice is greatly appreciated!
 

nsaspook

Joined Aug 27, 2009
12,811
They are smart home devices. Things like a smart outlet, temperature sensor, etc. They plug into a single controller.
Is it too hard to say exactly what type and model the devices are? You might get much better answers with this information.
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
406
Is it too hard to say exactly what type and model the devices are? You might get much better answers with this information.
This is an example of one of them. They have zero information on the protocol so it doesn't really matter.
 
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Ian0

Joined Aug 7, 2020
9,510
Looks like RS485 to me. Certainly doesn't look like CAN. When the line is idle, there may be pullup resistors.
Your message says F6 88 20 08 62 00 98 21 F2
I wonder if it is in 8 byte messages, and Fx means the start of a message.
 
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