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

[Tom Wiltshire]

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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