[sdiy] Starting my ASM-1

[Ian Fritz]

Regarding keyboard interfaces:

Information on the EN Newsletter may be found at:
http://electronotes.netfirms.com/
The "Builder's Guide and Preferred Circuits Collection" includes the 
original EN analog and digital keyboard interfaces.

The analog unit was quite fussy to get working properly, and I gave some of 
my modifications in EN #67. The modification for adding a non-retriggerable 
trigger consisted of the following:

1.)  A digital inverter driven by the keyboard Gate signal.

2.)  A 555 one-shot triggered by this inverter's output. The period of the 
one-shot is slightly longer than the "normal" trigger.

3.)  An AND gate combining the new 555 pulse and the original trigger.  The 
output trigger pulse from this gate only occurs immediately following a 
Gate leading edge, i.e., when all keys had to be up before the current 
keypress. The resulting trigger signal is accurately synchronized with the 
"normal" trigger, because of the use of the AND gate.

The digital scanning design (originally in EN #68) is a superior design. 
But if I were to do this now, I probably would use a microcontroller to 
scan the keys and drive a good D/A converter.

   Ian