[sdiy] Four bit to serial translator

[David Cornutt]

On Sunday, November 16, 2003, at 06:32  PM, René Schmitz wrote:
>
> Use an X(N)OR gate for each bit, and put an RC combination at one of 
> the inputs, and feed the bit directly to the other. Result is a short 
> spike when that bit changes. replicate n times. OR all XORs together.
>
I like it except for the RC circuits.  I had a very bad experience with
that kind of thing once.  The RC circuit reduces the slope of the
transitions.  If it takes too long to cross the logic threshold at the
input of the XOR gates, the output may oscillate.  You'd need to
use a Schmitt-trigger buffer on the RC circuit outputs.

My preference would be to latch and hold the previous value
for comparison.  In 4000-series logic, a pair of 4099 octal
latches would do it.  You'd need four 4030 quad 2-input XOR
gates to do the comparison, and a 4-input OR gate to sum them.
(4002, dual 4-input NOR, use other half as an inverter.)