RE: 1541IDE and 1541IDE-S -- and 8250IDE?
Date: 2007-12-13 07:22:11

Hallo Jim,

> You are correct, of course, and it's trivial to change it.  However,
> does make the memory map funky:
> $00-$1f=IDE registers low byte
> $20-$3f=IDE registers hi byte

Nope, A5 is just needed to read/write the High byte of the data ie.
D8..D15. Have a look at IC7A, the first half of a 139, you will see that
both A5 and R/W are responsible for selecting the 573's. In fact only
the addresses $20 and $30 are used, the rest are mirrors of those two
What does $30 you may ask? If you have a look at IC7B, the second half
of the 139, A5=1 makes it to output (H)'s only, thus disabling the
interfaces. Yes, interface_s_ as this 139 also enables you to connect a
second IDE interface to the, at this moment, not used outputs Y2 and Y3!
If you don't want two interfaces, connect A4 to the CS of this 139 and
input 3 of the other half. 

Regarding the 8250 discussion. IMHO it is possible to alter the decoding
scheme of a CBM in such a way that all the I/O still appears at the
original addresses but won't be mirrored anymore, thus freeing enough
space for our own purposes. IMHO a 74LS154 will do the trick.

But, as said before, Jack started the discussion mentioning the 8250:
see the subject. Nice idea but I forsee one problem: are there any well
documented sources available? I have them of the 8050 but I already
found out that the ROMs of the FDC's of the 805 and 8250 are quite
If it was up to me, I would start in the same way as I did with the
1541: soldering the interface and trying to make this work first only
using a BASIC program. My question: is it possible to force a program up
on the FDC of an IEEE drive ???? That I really would like to know!
The IEEE drives have a big advantage: a sepperate FDC. IMHO this means
we can adjust the ROM so the FDC is able to handle an harddisk without
having to alter the software for the other 6502. Remark: don't forget
that the ROM for this 6502 contains some routines meant for the FDC.

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