18 Jun
2008
18 Jun
'08
6:21 a.m.
The 279x (1,3,7) series chips actualy had a working pll data seperator.
Here is a link to the 279x controller chip data sheet:
http://retrotechnology.com/herbs_stuff/fdc_datasheet.pdf
As I recall, the 179x series chips could not reliably do sector reads from disks formated
on a 177x controller. The 179x could do a track read of the disk. Morrow provided a
program with the Disk Jocky controller which would do track reads, then write the track
back out creating a disk which worked well with either the 177x or 179x series
controllers. It was not a common issue, as very few people used single density once
double density became available, and new single density disks worked well with either
controller.
Les
18 Jun
18 Jun
8:32 p.m.
The 279x (1,3,7) series chips actualy had a working pll data seperator.
I thought there were more varients than that, but I don't haev the WD
data sheet in front of me. The PLL is quite easy to set up (there's a
test pin on the chip that you have to ground after reset IIRC, and then
you get 3 wavefroms on 3 pins of the chip that have to set to the right
frequency or pulse width using the 3 tweakers), but you need a 'scope.
FWIW this chip was used in some HP floppy drive units, permanently
strapped for MFM (double-density) use.
Here is a link to the 279x controller chip data
sheet:
http://retrotechnology.com/herbs_stuff/fdc_datasheet.pdf
As I recall, the 179x series chips could not reliably do sector reads
from disks formated on a 177x controller. The 179x could do a track
read of the disk. Morrow provided a program with the Disk Jocky
controller which would do track reads, then write
the track back out creating a disk which worked well with either the
177x or 179x series controllers. It was not a common issue, as very few
people used single density once double density became available, and new
single density disks worked well with either controller.
Eh? The 1770, 1772 (== 1770 with different step rates) and 1773 can all
do MFM operation, The 1771 is FM (single density) only, and has the
feature of beine able to write some odd data markers.
I have never had any prolems reading disks between a CoCo (using a 1773
controller) and a Model 4 (1793 controller). The only problem comes with
the Model 1 (1771), which did use one of those strange markers on the
directory track.
-tony
19 Jun
19 Jun
12:33 p.m.
Les wrote:
As I recall, the 179x series chips could not reliably
do sector reads from
disks formated on a 177x controller.
The 179x can read disks formatted on a 1771 just fine. What the 179x can't
do is distinguish nonstandard data marks. Most people using the 1771
were smart enough to use the standard data marks. Unfortunately, TRS-DOS
for the Radio Shack TRS-80 Model I did use nonstandard marks.
The IBM-standard formats have two different data marks, for normal
data and deleted data. The 1771 and 179x can both read and write
those data marks, and on read, distinguish between them.
However, the 1771 also supports two non-standard data mark patterns,
which it can write, and distinguish on read.
On the 179x, it is not possible to distinguish the non-standard data
marks from the standard ones. It's possible to write the non-standard
ones during formatting, but not when writing a sector.
This is why the "doubler" products for the TRS-80 Model 1 didn't
completely replace the 1771 with a 1791, but instead put them in
parallel and added switching logic.
Eric
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ard＠p850ug1.demon.co.uk -
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