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

[Gordonjcp]

On Mon, Aug 24, 2026 at 11:37:55PM +0100, Tom Wiltshire wrote:
> 
> That said, would anyone like to have a *more* educated guess at what the control lines do?
> 

I would think that your guess that ALE latches in the register address or data in the same way that the 8032 does it is correct. Unfortunately the service manual doesn't mention anything about the address decoding and it is clear that this is buried in the glue logic gate array!

I think I would probably look for clues by disassembling the the 8032 firmware and see if I could work out where the pitch values are written. In the Juno 106 this was pretty obvious because the address decoding to the dividers is laid bare but in the Alpha Juno I think you'd be looking for a lookup into some table of values that is then written to an "illegal" address.

If you had a "sacrificial" Alpha Juno that you were going to pull the DCO ASIC from you could probably inject addresses into the glue logic and see what lit up.

-- 
Gordonjcp