[sdiy] Roland's MB187123 / RD87123 Alpha Juno DCO chip

[Tom Wiltshire]

Hi All,

I recently bought a NOS Roland '87123 chip, the "DCO" (NCO really) from the Alpha Juno synths. My thought was to get it on the breadboard, and get a PIC talking to it so I can run some experiments and find out a bit more about it. Information about it is sketchy but not completely absent.
Richie Burnett and I have discussed this chip in the past and Richie has already worked out quite a lot about it, so it's not a complete unknown by any stretch. But (for example) there's no documentation of the register layout at all as far as I can find.

Roland provide both a pinout and a diagram of the chip in the Alpha Juno 2 Service Manual:

https://electricdruid.net/wp-content/uploads/2026/08/MB87123PinoutDescription.jpg <https://electricdruid.net/wp-content/uploads/2026/08/MB87123PinoutDescription.jpg>
https://electricdruid.net/wp-content/uploads/2026/08/MB87123Diagram-original.jpg <https://electricdruid.net/wp-content/uploads/2026/08/MB87123Diagram-original.jpg>

What I don't have is any clue about the bus timing, but given there's only 8 data lines and two control lines, how hard can it get?! 

That said, would anyone like to have a *more* educated guess at what the control lines do?

I suspect that the ALE (Address Latch Enable) line controls whether data goes into the Address latch, or to whichever data latch (Wave or Pitch) is selected by that address if not asserted. That would then leave the !CS Chip Select line to act as a Latch Enable and let data into the latch. There's other unknowns, like how many clock cycles it takes for the data to be latched in. Plus the chip seems to include amplitude control features which aren't shown in the diagram (controlled by the wave data latch, maybe?). I've been reading the datasheets of few other sound chips from a similar era (the famous 6581 SID and the SN76489, for example) figuring that bus-controlled sound chips from the 80's probably work similarly. I'm not sure that's really true, but it's thrown up a few things to watch out for at least.

Finally, I'm sort-of curious about those two "TST" pins. Looks like "Test" to me. but what does a test mode do on such a chip, and why didn't Roland use it in the Alpha Juno? (both lines are tied permanently low). Maybe that's not it.

Any thoughts or pointers appreciated.

If I get anywhere with this, I'll post a page with what I discover. When I'm done or give up, I'll be selling a Roland 6-oscillator DCO, one careful owner, only drove it to church and back on Sundays, etc etc, lol.

Thanks,
Tom

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