Neat idea.

I developed a manufacturing test tool several years ago, during the 
PIC16C???
era. This one sends out on ONE pin. It pumps out a manchester code (variable
time between bits, but the information is always a fixed percentage of 
the bit width).
I have it send out a  line of 16  bytes, usually (but not always) 
advanced one line of
16 with each pass plus 9 bits of RAM ADDRESS info, plus two start bits 
and two
stop bits. The total bit count is 16*8  + 9 + 2 + 2 = 141 bits. With a 
bit period of 5mS,
it takes 141*5 or 705mS per pass. I post a gap to allow the receiver to 
resynch.

The data looks like this:

_---_-__-          etc (A ONE then a ZERO)

The bit begins with a ONE, and ends when the NEXT bit starts. A ONE is 
defined
when the bit pulse with is MORE than 50% of its bit length, a ZERO is 
defined as
LESS than 50%. I run it flawlessly at 5mS per bit, but it operates well 
at 1mS per
bit as well, You can see that the LAST bit in the bit stream contains NO 
data.

The receiver for this is an old DOS 66Mhz laptop. The signal goes into a 
status
pin on a printer port. Together with a ground, that's all there is. A 
twisted test
cable can be as far away as 10 meters and recover the data reliably.

The way the data is recovered with two timers. The first one measures time
between bits, and the second measures the time the bit pulse is HIGH. On the
DOS machine, I used a bad serial port (logic worked, RS232 driver was
blown) to provide an internal timer, by sending data as fast as possible 
at 115kb
and counting each transmission time. That timer was used as a reliable fast
measurement. The standard tick clock is too slow (18 ticks/sec).

Jinx wrote:

>The little 2520 project I had trouble with at the weekend was this. I
>needed a way to look inside an F88 while it was running, so as to
>check what was happening to certain RAM registers. So I knocked
>up this RAM sniffer
>
>On a regular basis (every 2 seconds, as determined by a system tick)
>the F88 bitbangs its RAM contents out. The 2520 picks this data up,
>if set to, and shows it as 2 lines of 4 bytes, each line starting with the
>address. Two buttons allow you to move up and down
>
>Could be made into something flashy, but for now it does all I need
>it to. Maybe someone else can use it, I found it tremendously helpful
>
>Connections to the circuit are 4 clipped leads - 0V, 5V, clock, data.
>Some 2520 routines are there for another day and aren't used here
>
>===============================================
>If you aren't part of the solution, you're part of the precipitate
>
>
>
>
>
>  
>
>
> ------------------------------------------------------------------------
>
>
> ------------------------------------------------------------------------
>


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