S100 Floppy Controller Question - test-drb@ccmp.vtda.org

15 Nov 2007

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Subject: Re: S100 Floppy Controller Question
   From: M H Stein <dm561 at torfree.net>
   Date: Thu, 15 Nov 2007 04:21:34 -0500
     To: "'cctalk at classiccmp.org'" <cctalk at classiccmp.org>

---------Original Messages:
Date: Tue, 13 Nov 2007 07:48:21 -0500
From: Allison <ajp166 at bellatlantic.net>
Subject: Re: S100 Floppy Controller Question

Subject: Re: S100 Floppy Controller Question
   From: M H Stein <dm561 at torfree.net>
   Date: Tue, 13 Nov 2007 01:13:48 -0500
     To: "'cctalk at classiccmp.org'" <cctalk at classiccmp.org>

<snip>

I think you might have misunderstood, Allison; sounds like you thought I was
talking about replacing an FDC with one other than what the BIOS is configured
for; obviously that will require mods to the BIOS. 
Ok, then same fdc but sufficiently differnt drive requires new (modified) bios.

Changes that affect a bios:

 Motor on/motor runs continiously.

 Step rate, head load delay, motor on delay

 Different on disk format (likely with 8 to 5")

 What I was responding to was a previous post that
suggested replacing an 8"
drive with a 5 1/4" HD drive might require mods to the BIOS, and I just wanted
to mention that this is not the case with a Cromemco 16/64FDC, to which a
TM848 and a JU475 appear identical as long as the jumpers on the 5 1/4 drive
are set correctly and it supplies /READY, and that this may also apply to some
other controllers. Also, if the controller has both 8" and 5 1/4 connectors and 
they are effectively in parallel as they are on the Cromemco, you may not 
even need a 50<>34 pin adapter cable. 
if the drive supplies ready _MAYBE_.  If the Drive has the same step rate.

-------------Reply:

Well, I should know better than to argue (again ;-) with you of all people, but...

Since we're talking about 'modern' 5 1/4" HD drives, Motor on and Ready 
are jumper selectable on all the drives I've run across, step rates etc. are 
faster than the 8" equivalent (except perhaps the PerScis), and with the 
same speed and data rate the disk formats are identical, no? 
the drive used are a factor and many of the later designed systems 
had what could be described as an adaptive, they have a bios written 
to accomodate a wider variety of drives.

...
 In any case I'm only specifically talking about my
Cromemco system and it 
seems to work fine for me. It gives me 1.1 MB on a 5 1/4HD diskette instead 
of 360KB and lets me deal with both 5 1/4 and 8" images conveniently in a
single 5 1/4" FH bay box, without the complication of the 24V supply, 50 pin 
cable, unreliable old 8" drives etc. To initially read & convert my 8" disks
I just 
temporarily plugged an external drive w/PS into the unused 50 pin connector.
I haven't tried it yet, but it should also make recreating an 8" image on a PC
easier, and it puts some 5 1/4HD drives & disks to a good use, which otherwise 
would end up on the shelf or in the trash since I don't use them on PCs. 
Cromemco is one such example and much later than Altair/Tarbell.

...
 After installing the drive I happened to run across
some old posts in the archives 
dealing with this very topic (on a Cromemco); apparently it was easy and worked 
fine for some and not for others. I just wanted to add my name to the former
group in case it had relevance to someone else with a Cromemco FDC or
_perhaps_ even with a different make of controller if it's similar, and especially 
mention to one of the original posters that it might not be necessary to kludge up a
50<>34 pin adapter cable (which he was reluctant to make) if the two interfaces 
on his FDC are handled the same way as on the Cromemco FDC. 
Same was the case for AmproLB, SB180, and Compupro.  Generally most fo the 
floppies are close to "setup, plug and play".  However really old drives
for example SA800, SA400 it's fairly easy to get the signals right but things
like 40ms step rate (sa400) and 12ms step rate(sa800) have to be driven
by software. After about 1981/82 design had fewer problems and 5.25 floppies 
started to look more alike than different electronically.

The gotchas still lurk out there.

Allison

...
 mike 