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I have a device with two chips without part numbers. It looks like their using RS232 for serial communication (proper setup, right voltage), but I do not know the bus settings (speed, parity, etc.). Is there any way to determine the bus settings without brute force (trying everything)?

I have a multimeter and an oscilloscope on my workbench.

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If you have an oscilloscope, it should be pretty easy to determine at least the baud rate. Then you just need to play with the parity and stop bits.

If you don't have it, there's no real way around bruteforce. However, usually you don't have to try all combinations.

  • probably at least 90% embedded devices I've seen use 115200-8-N-1 (115200 baud, 8 data bits, no parity, 1 stop bit), no flow control.
  • of the rest, majority seem to use 8-N-1 with a lower baudrate, such as 38400
  • in one case, I saw 38400-8-E-1.
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  • @CamilStaps Um, that's what I describe in the second part of my answer. Or did you mean something else?
    – Igor Skochinsky ♦
    Mar 26 '13 at 17:49
  • 9600-N-8-1 is very common as well. My approach is usually to start there and work up if that works since many devices support multiple speeds.
    – Remko
    Mar 26 '13 at 22:39
  • As said above try 9600-8-N-1 and 115200-8-N-1 and you've hit 98% of the cases in my experience.
    – ixje
    Mar 27 '13 at 7:59
  • 19200-8N1 is also quite common...
    – cb88
    Apr 16 '13 at 20:02
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A simple logic analyzer, such as the Saleae is invaluable for finding simple transmit serial pins. Receive serial pins are harder due to them being silent.

Are you sure that this is RS232 and not just serial? It's pretty rare to see RS232 on embedded systems unless they're industrial. RS232 goes way above TTL levels.

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  • I'm sure, the voltage level actually is why I thought it to be RS232. There are some modules, GPS e.g., that use the full RS232 protocol. But they are rare indeed.
    – user187
    Mar 26 '13 at 18:09

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