8" disk drive project - maybe 3.5" project too??? - test-drb@ccmp.vtda.org

6 Nov 2007

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Subject: Re: 8" disk drive project - maybe 3.5" project too???
   From: Grant Stockly <grant at stockly.com>
   Date: Mon, 05 Nov 2007 22:50:45 -0900
     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>

I'm going to add some to my own topic.  The questions below still 
stand, but I've got more info!  : )

 I want to use a QUME 842 or SA800 in an altair
disk subsystem.

Both seem to have the same interface.

The FD400-5x0-5x1 manual seems to show the drives being quite similar.

The first thing I see is that there are extra signals.

"Trim Erase", "Write Enable", and "Write Data" where the
Shugart 
interface only has "Write Data" and "Write Gate".  I assume that 
gate and enable are the same thing.  The FD Pertec drives also have 
"IN", "OUT", and "STEP" instead of "STEP" and
"DIRECTION", but that 
is not a big deal.  That can be generated in software.

What I don't see is any input to perform the function of head 
current.  How does the shugart interface handle this?

Can I ignore the trim erase feature?  How do I generate a write busy 
signal?  I'm willing to make some circuits to fake these signals if required. 
More info:

The pinout of an Altair disk drive...of the wires connected.  Omitted 
pins are either not connected, power, or ground.  All lettered 
connections are ground.

6 - Output - Door Open (IDOP
7 - Head Current Switch (IHCS)
8 - External Trim Erase (Option) (IEEN)
9 - Input - Write Enable (IWEN)
10 - Input - Write Data Input (IWDA)
11 - Input - Step In (ISTI)
15 - Input - Step Out (ISTO)
16 - Input - Head Load (IHLD)
17 - Output - Index (IINXP)
18 - Input - Drive Motor On (IDEN)
19 - Track 0 (ITRK0)
20 - Output - Read Data Output (IRDA)

One thing that interests me slightly more is the possibility of a 
regular 3.5" disk connected to the Altair.  I don't care about the 
encoding or format being compatible with a standard controller.

It is very similar, but is also a soft sector disk/drive.  How close 
is close?  Can I just use a microcontroller with a timer to time from 
one index to the next and divide it up into 32 sectors?  Will this be 
close enough for the Altair?  Anyone know what kinds of errors I 
would get?  A microcontroller running at 16MHz would be able to count 
between pulses with 16MHz/1 accuracy.  With the SA800 I would have a 
sector hole sensor giving me the correct timing.

There still is the issue of the write trim erase feature, whatever that is... 
Write trim erase is a signal to the head assembly (driver) that turns on
the erase segment of the head for erasing the area either side of the head.
It's also called tunnel erase. This gives some tolerence to off track reading
as the intertrack gaps are "clean".  Tunnel erase is still part of all 
floppy drives but is activated by Write-enable and you never "hear" of it.

In 1976 all this was new and magical until Sugart came out with the much 
simplified 5.25 SA400.

...
 Any comments or ideas on the idea?  Is it worth trying?
 I've got a 
very weird tarbell card that formats and uses 3.5" disks as a 70k 
mini floppy.  I guess anything is possible.  : ) 
Sounds like a wither the DD or SD tarbel card that could interface
to most any drive using soft sector. 

Myself I'd persue something using a 5.25 or 3.5" drive with a current 
softsector interface to the drive and enough CPU smarts to fake looking
like the Altair interface which was dumb as a rock and depended on the 
8080 to do most everything.

Allison

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2 - Density Select (/REDWC)
8 - Index (/INDEX)
10 - Motor Enable Drive 0 (/MOTEA)
12 - Drive Select 1 (/DRVSA)
14 - Drive Select 0 (/DRVSB)
16 - Motor Enable Drive 1 (/MOTEB)
18 - Direction Select (/DIR)
20 - Head Step (/STEP)
22 - Write Data (/WDATE)
24 - Floppy Write Enable (/WGATE)
26 - Track 00 (/TRK0)
28 - Write Protect (/WPT)
30 - Read Data (/RDATA)
32 - Head Select (/SIDE1)
34 - Disk Change/Ready (/DISKCHG)

I'm not going to sleep a wink tonight.  I hate having ideas at 10PM...  : (

Grant  