3
\$\begingroup\$

Note the different offsets for CH1 and CH2

This is a mystery signaling standard that I'm trying to sort out. It looks more or less like RS-485 to me, but I don't recognize the long looking start bit, and the "idle" state of the line looks a little strange to me. Using an RS-485 to RS-232 converter, I can't seem to get it to translate to anything intelligible. Note the different offsets for CH1 and CH2. When in the "idle" state (at the beginning) the two lines are at essentially the same potential, and when transmitting, they differ by the amounts show.

I guess in particular, I'm more curious about the framing than I am about the physical layer itself, since my rs-485 converter seems to interpret it OK (even if the data appears to be nonesense to me)

Edit:

Here's the character of the data over a longer time span, note the timebase (same vertical settings)

Longer time scale

Here's the close-up trace, but with infinite persistence turned on. I'm triggering on the long start pulse, which seems to be present for all messages:

Infinite persistence, triggered on the long start pulse

Improve this question
\$\endgroup\$
9
  • 1
    \$\begingroup\$ I think you may be jumping the gun a bit by assuming that whatever system you're using is bothering to follow a standard at all. It's probably something homegrown. \$\endgroup\$ – Connor Wolf Dec 31 '12 at 2:40
  • \$\begingroup\$ Can you set the trace persistence to infinite, and push a bunch of data over the bus? That should give a good idea of the aspects of the data structure that are fixed, and the aspects that change. Just make sure you're triggering properly first. \$\endgroup\$ – Connor Wolf Dec 31 '12 at 2:41
  • \$\begingroup\$ I may indeed be jumping the gun, but since the baud rate appears to be 9600 baud, I'm hazarding that this is interpretable by some conventional technique. \$\endgroup\$ – Ryan Dec 31 '12 at 3:12
  • 1
    \$\begingroup\$ ... sounds a bit like GPS messages (NMEA) from the baud rate and burst rate.... \$\endgroup\$ – vicatcu Dec 31 '12 at 4:07
  • 1
    \$\begingroup\$ Can you give us some info on what the device is, where it is used, and maybe some chip part numbers? This will help us greatly in guessing what this is. For example, the chip part numbers can give us insight on the electrical spec being used. The type of device can give us insight on an obscure protocol that you might not be familiar with. \$\endgroup\$ – user3624 Dec 31 '12 at 4:24
1
\$\begingroup\$

The physical layer does indeed look like RS485, but that's just describing the differential voltage drive and idle-state of the lines. A peek at whatever's driving the bus in the source kit would tell you - it'll most likely be some standard 485/422 chip.

What the data format is is a different question, for that it's easier to look at the output from a 485 chip and trace the 0's and 1's. If you are sending some known data back & forth then you should be able to determine that relationship pretty easily.

There may be some sort of messaging protocol above that, depending on the kit.

Improve this answer
\$\endgroup\$

Not the answer you're looking for? Browse other questions tagged or ask your own question.