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

[rburnett at richieburnett.co.uk]

An interesting little project!  I'd start by looking at the bus timing 
of the 8032 or whatever microcontroller runs the show in the Alpha Juno. 
As you say it has a shared address/data bus so some or all of the 
address bits will get output on the shared bus and latched by ALE 
activity, followed by the data itself. I'd try to get to the bottom of 
that and then maybe write some PIC code to spam the full address space 
with random commands to see if you can produce a response in the 
analogue output waveforms.  Then work out what writes have what effect 
and start building a memory map of registers in the DCO!

I think the DCO chip probably does all of the address decoding inside, 
because I don't remember much in the way of glue logic around it on the 
schematic. So it may only respond to a small handful of addresses out of 
the full 16-bit address space.

I bought a "non-working" Alpha Juno 2 back in the early 2000's with a 
view to desoldering the DCO chip and doing exactly what you're doing 
Tom.  In the end it turned out that it wasn't too hard to fix and I 
managed to get the synth up and running again. So, unfortunately I never 
did get round to figuring out the command structure to talk to the DCO 
chip.

If you get stuck I might be able to hang a logic analyser off the DCO 
chip in my Alpha Juno, but it would be quite an undertaking, and would 
be easier if we already had some idea of the address range that the DCO 
responds to.

I also might be interested in taking the DCO chip off your hands if you 
want to sell it on down the line :-)

Good luck,

-Richie,




On 2026-08-24 23:37, Tom Wiltshire wrote:
> 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/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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