[sdiy] Negative-going triggers in the Boss DR-110 drum machine - Why?

[Roman Sowa]

The microcontroller data ports don't have enough oomph to quickly charge 
(via 220R resistor) the envelope/decay capacitor of every voice so it 
had to be boosted by PNP transistor, hence the need for active-low 
pulses. If there was enough juice in the micro pins, they would have 
used positive pulses and a diode instead.

Roman

W dniu 2023-09-29 o 12:31, Tom Wiltshire pisze:
> Hi all,
> 
> I've been reading old drum machine schematics, and I've got a query about the Boss DR-110. It uses negative-going trigger pulses from the processor, and I wondered if there is any substantial reason why?
> 
> For some of the sound-generating circuits, this makes little odds, but some of them have a transistor to invert the pulse before they can use it. Given how stripped-down this thing is (one-transistor "VCAs" all over the place, and even some of those are re-used by multiple voices) this seems a bit odd, like they wouldn't have used negative-going pulses unless they had to.
> 
> I wondered if it might be to do with the processor itself, which is a 4-bit Hitachi HD44790A-44, but I don't know anything about this chip. As far as processors of that era go, I know the Z80 and the 6502 and that's about it!
> 
> Anyway, any thoughts or pointers appreciated.
> 
> Thanks,
> Tom
> 
> Full schematic:
> 
> https://electricdruid.net/wp-content/uploads/2023/09/DR-110Schematic.png
> 
> Service notes:
> 
> https://electricdruid.net/wp-content/uploads/2023/09/Boss-DR110-drum-machine.pdf
> 
> 
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