Keyboard PS/2 to Parallel converter

Holger Veit holger.veit at
Tue Jan 8 03:27:17 CST 2008

Jim Brain schrieb:
> If you didn't care about sending codes back to the keyboard, then 
> you'd just need an 11 bit shift register
> The previously mentioned 
> is almost there, 
> it just needs 1 more F/F
> 2 LS174s would work for the F/Fs. the CLK line needs to be inverted, 
> but you can probably get away without doing it.
> A divide by 11 counter (LS163 with and AND on bit 0,1 and 3 driving 
> CLR) should do the trick.
The price you then pay is additional effort in the target machine to 
parse and decode make/break codes of the keyboard which is usually 
significantly more expensive than a $2 PIC or ATMEL. Think of an 
Apple-II that wants certain ASCII codes at the parallel keyboard input. 
You don't want to modify the system (e)prom to convert
the raw codes returned by the keyboard - no space there for tables. You 
also don't want to add a simple conversion eprom at the shift register 
output to translate the codes into ASCII as this won't deal with the 
break (release key) stuff;  it's possible to "ground" unwanted code by 
translating them to e.g. 0xff and then use a 7430 to suppress the strobe 
signal but yet another  circuit then.

No, I am not yet at a point to use a microcontroller just to blink a 
LED, which have been done with two cross coupled transistors in the 
past, but a PIC is meanwhile the more compact, easier, more flexible 
solution to "convert-from-A-to-B" problems.


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