https://groups.google.com/g/comp.sys.tandy/c/c8brrPNjZEs/m/XlCAr8P3BjAJ
Bill Vermillion
Mar 13, 2003, 11:25:35 PM
In article <WtPba.68256$
[email protected]>,
Larry Fosdick <
[email protected]> wrote:
I am contemplating replacing the 5 MB disk in my TRS-80 HD
with a 31 MB disk. I looked at the information on Ira's site
concerning this, and came up with a question.
Are the ST506 and ST412 specs different and incompatible?
Compatible to a point.
I've seen these two specs used together, separately, and in a
few documents, interchangeably. The bubble repair document on
Ira's site uses ST506, but the Seagate ST-4038 is specified
as ST412 MFM, not RLL). The drive seems to meet all the other
requirements for the old (large) controller, in that it has 5
heads and 733 cylinders.
Can anyone shed light on this?
Sure can.
All the early drives were pretty dumb and the formatting and setup
utilities did all the work. In modern drives the drives take care
of this.
In the ST506 you are limited to 70MB and heads 0 thru 7.
You also set the value for the track on which write-precompensation
occurs.
Write-pre comp is when you modify the signal to take care of
bit-shifting that occurs on the inner tracks were one bit can
affect an adjacent bit. Since identical magnetic forces repel
the bits want to shift away - and the pre-comp change the
timing/spacing so this doesn't occur. That's also something you
see in coated media that is relatively soft - which was the norm
for the old drives.
In the ST506 there is a line that is toggled when it is time
to use write-precompensation.
In the ST412 drives they handle write-precompensation internally.
The line that was used for write-precomp now becomes a head-select
line.
Normally you select heads 0 thru 7, but on the ST412 when you
toggle the old write-precomp - now head select, when you select 0
it is head 8, and 1 is head 9 and so on until you get 15 heads.
So you can used ST412 drives on an ST506 interface >>IF<< you only
format the first 8 tracks - heads 0 thru 7.
We used to put the ST4096 Seagate in the Model 16's to get 70MB.
The ST0496 is a 77MB drive with 9 heads. But if you tried to
format 9 heads using the standard controller - which used
the WD1010 controller chip that only knew about 8 heads - once
you went to head 8 [the ninth head] and since that was
write-precomp, you actually wrote over track 0. That is not what
you want to do.
To sum up:
ST506/ST412 is seen quite often. Pure ST605 interfaces were
used only in the early days. They are virtually identical except
for the write-precomp line becoming head select. You can use
ST412 drives in ST506 but DO NOT format for more than 8 heads.
You can also set write-precomp to the maxium number of cylinders
when using an ST412 on an ST506 controller - because the ST412
handles write-precomp intenally.
You used to have to know all this stuff if you used other than the
drives speced by the factory. The old 16's had a Tandem drive that
would fail in strange ways and would lose on head or the access to
one. I'd take the standard ST225 - the 1/2 height 5.25" and put
those in the 16 - or if I just needed to load the data from the
Bermoulli backups so I could xfer the data from the 16, I'd just
route the cable out the back and have the 225 on the table for the
time it took to transfer.
Bill
--
Bill Vermillion - bv @ wjv . com
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