# Notation for serial signals

Thread Starter

#### BladeSabre

Joined Aug 11, 2005
105
I'm analysing the signal used by a certain device, and I'm not quite sure what a good way to write it down would be. Here's an example (recorded through the sound card so the zero line is in the wrong place, but you get the idea):

http://img431.imageshack.us/img431/1110/wavestudiosaa1.png

The long (positive) pulses appear to be true / binary 1, and the short pulses false / binary 0. It uses 4-bit words. For numbers, the LSB is transmitted first. In the occasional case where an 8-bit number is needed, the low half is transmitted before the high half.

Time usually goes from left to right, for example in wave editors like that one, or a classic oscilloscope. Text is read from left to right too. So the easiest way to write the above section of signal is like this:

1000 1000 1001 0001
(0x8891)

That works because if I read from left to right, the bits go into my head in the same order they were transmitted. The tricky thing is, the numbers are all backwards. Suppose the first 4 bits mean Number 1. That looks like Number 8. The hex version is worse. (It would be unfortunate if "8" meant Number 1.)

If this was just a series of 4-bit numbers, I could reverse them.

0001 0001 1001 1000
(0x1198)

So now the first "1" really means Number 1. But this arrangement is a lot harder to write down when looking at an oscilloscope display. And suppose the second word represents an ordered sequence of "yes, no, no, no" - now that's backwards. (The bits really can represent yes/no events happening over a period of time, where the one occurring first is transmitted first.)

When two words are needed, it gets worse again. What if the middle two (19) meant "yes, no, no, no, yes, no, no, yes"?

(8-bit numbers are a little awkward too, though perhaps still better than they were the other way.)

Any suggestions? Is there some recognised way to solve this?

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

Is this a set of control signals from a remote? Four bit words (bytes) are a bit unusual.

Offhand, this is perfect for a computer to soak up, so the data may be jiggered in and old way without losing the original. This could also get clocked into a shift register. A number of them could accumulate several words for evaluation.

Anything has to be better than trying to copy bit patterns off a crt (unless you have a storage 'scope).

Thread Starter

#### BladeSabre

Joined Aug 11, 2005
105
It's not quite a remote, but near enough. (It's a toy, which talks to other toys of the same kind.) I've done quite a bit already, and I'm fairly certain about the 4-bit words. Like I said, there are some 8-bit values, but they sometimes appear across e.g. the 2nd and 3rd word (of 4).

I have a very nice storage 'scope. And I am trying to set up a computer link - but it would be nice if I had a good way of encoding it in my software, too

Similar threads