6 Nov
2020
6 Nov
'20
2:15 p.m.
On Fri, 6 Nov 2020, Enrico email.it via cctalk wrote:
First of all I would like to thank you all for every
suggestion that comes
from so much experience. I only know my Ferguson bigboard 1 pretty well
The z80 based machine was called GENERAL PROCESSOR Model T year 1981.
You can see it here: https://www.vintagesbc.it/?page_id=877 and the cards it
was equipped with here: https://www.vintagesbc.it/?page_id=879 even if it is
old information because I, together with a competent hardware technician and
another enthusiast who has already written an emulator HERE:
https://nippur72.github.io/gpmodelt-emu/ (you have to press CR to start then
BD and enter ...)
and the emulator video here:
https://www.youtube.com/watch?v=cRmmUYkaDxs&feature=youtu.be we are
proceeding to derive electrical schematics, save firmware, software, and try
to reconstruct and document everything.
You have to think that they used a WD1791 which has an inverted data bus and
in the firmware they use CPL instruction to invert it again so on the disk
the CP / M (CCP + BDOS + bios) is written all INVERTed.
Based on the firmware on eprom configuration it could manage:
- 2 drives 8 "(128x26x77) or 5" (128x18 (maybe 16) x40) all in FM and seen
as A, b, c, d
- 1 8 "DSDD single B unit and 1 HD 10MB Winchester drive (divided into 2
halves A and C)
At the beginning the engineers made ibm 3740 in side A. Only after they
passed to make a drive double face so we think they didn't change things
also because it seems to be possible to boot from both side A and side B and
we expect the side B of drive A it would in IBM 3740 format.
We noticed that the sectors always start from ZERO and not from 1 so it is
interesting what you say: ?you really should try unplugging the heads of the
drive and swapping them. That will put "side A / 0" on the second / B side
of the disk.! "
You did an excellent job of answering the questions!
The NAME probably doesn't matter, but it's always good to include, in case
anybody here might possibly have previously dealt with it.
With a WD179x, it is a little surprising that they didn't use MFM ("double
density").
On the other hand, 8" SSSD is "THE STANDARD", so must be supported, and/or
if earlier models had used the WD1771 FDC, this could be a legacy format.
The extreme standardization of CP/M 8" SSSD is another reason to justify
the replication onto second side, instead of changing the format. I once
pleaded with Gary Kildall to officially declare a 5.25" secondary disk
format standard. His reply was, "THE STANDARD is 8 inch Single Sided
Sigle Density."; it was understandable that he would not tolerate creation
of a "secondary standard". Although, we ended up with about 2500
different microcomputer disk formats, due to the lack of standards for 8"
DSSD, 8" SSDD, 8" DSDD, 5.25" SSSD, 5.25" DSSD, 5.25" SSDD,
5.25" DSDD.
(MOST of which could have been prevented with an official proclamation
from DRI.)
Swapping the head cables might be a simple way around it, IFF the second
side is formatted exactly the same as the first. One obvious possible
difference is the head number field in the sector headers. If EXACTLY the
same, then both would have 0 in that field. BUT, it could have 1 on the
second side (which will make it easier to read with an NEC chip). And,
the WD179x controllers didn't really care much about that field, so it
could be ANYTHING, including wildly wrong numbers. In most cases, the WD
chip wouldn't even notice if that field had the wrong value, but the NEC
chip needs to know what value to expect and look for.
If the imaging that included the second side was done successfully with
IMD, then that implies a 1 in the head number field on the second side.
Not insurmountable, but it means that just swapping the heads will still
require some special software.
If the imaging that included the second side was done successfully with
IMD, then consider the possibility of either doing the file transfers from
those images, or writing some minor software to split those images into
two separate single sided images.
The inverted data isn't a serious problem, although it is inconvenient
when looking at the data to see whether it is successful. But, it is easy
to work around.
Another possible complication, if you are trying to do the transfer on a
PC is that the WD179x controllers could start the beginning of a track
sooner after the index pulse than the PC NEC765 controllers can handle.
That is usually not too hard to work around, but is something to watch
for, particularly if you get failures to read the first sector of each
track.
Not all NEC floppy controllers can do FM and/or 128 byte sectors. Dave
Dunfield has created some test routines for checking that.
--
Grumpy Ol' Fred cisin at xenosoft.com