Bill Gunshannon bill.gunshannon at hotmail.com
Sun Apr 8 15:11:58 CDT 2018

On 04/08/2018 02:11 PM, Fred Cisin wrote:
>>>> I do have to admit that I find it hard to believe that the cable to 
>>>> the
>>>> floppy can actually make a difference.
>>> A minor point, . . .
>>> On 5150/5160/5170, the SECOND drive is a straight cable, FIRST drive
>>> is crossed.  Thus, drive A: is at the end of the cable, B: is in the
>>> middle of the cable.
> On Sun, 8 Apr 2018, Bill Gunshannon via cctalk wrote:
>> Yeah, knew that from other systems.  IBM really screwed that up.
>> Should have left it the way it was and have people set the Drive Select
>> themselves.
> Yep!
> IBM didn't think that users could be trusted to get that right.
> Or at least Computerland couldn't be trusted to get it right.
> Radio Shack used a different approach to drive select by cable instead 
> of making use of the well documented drive select on the drive.
> Radio shack jumpered all drive selects on on the drive, and pulled 
> pins in the cable.

Or, as in the case of some of the 1000 series (I had a TX my father an 
SL) punch holes in the cable to
break the connection.

> As opposed to IBM jumpering both drives as B: and twisting the cable 
> for A: (which also provided different control of motor)

Which also limited you to two drives.
>>> IF that is correct, then your first drie is straight through. That
>>> also means that an unkeyed cable can be reversed, as one more to try.
>> Reversed cable will result in the drive being active constantly.
>> Easily noted by the LED being on constant and the drive running.
> I meant reversing BOTH ends, end for end, giving same wiring.
> in case some lines of the cable are flaky.

Too many different cable for that to be likely.

>>> I did not see any mention of the disk format.
>>> If it is 512 bytes per sector MFM, with sequential sector numbering,
>>> then even USB drives should work for making disks.
>>> A different sector size, or even numbering sectors from 0, would be
>>> problematic for some USB drives.
>> I have no idea of the format.  I got the images and rawrite.exe and
>> told the computer to make them.  They were unusable when I used
>> a USB External floppy but worked fine when I used a real internal
>> floppy.
> Since they mimiced the 5150 cabling, I was hoping that maybe they had 
> made the format similar, or at least the same physical format.
> There are more efficient physical formats (using 1024 bytes per sector 
> easily gives you 400k/800K instead of 360K/720K)
>> I need to get the systems running before I start playing with reading
>> and writing weird formats.  But that is coming.
> Should be fun.
> Is the FDC a 765 variant?
> or a WD 179x variant?

SMC 37C651.  Supports  500 Kb/s, 300 Kb/s and 250 Kb/s Data Rates.
Something else we lost with the PC.

>> As a side note, I did get the system to boot and run from my floppy
>> emulator with a USB stick.  Have to boot twice.  First time you get
>> the unrecognized format error second time boots fine. Interesting.
>> Good to know for when I am testing on other systems as well.
>> Small steps, but advancing, just the same.
> Good luck!



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