P112 [Don Maslin Archive]

Fred Cisin cisin at xenosoft.com
Sun Dec 1 16:40:05 CST 2019

On Sun, 1 Dec 2019, David Griffith via cctalk wrote:
> On the 8" drive question, I haven't done it myself, but I've received 
> reports from people who've successfully used them with the P112.  It 
> involves pretending they're HD 5.25" drives and using a hardware shim to 
> deliver more current when writing to tracks greater than X.  I don't 
> recall the specifics though.

As bill mentioned, though, 1.2M drives have 80 tracks; 8" have 77 tracks. 
Otherwise, the 1.2M is a 5.25" version of an 8" drive in most respects.
In fact, the first one that I ever got (at a swap) was a Mitsubishi, that 
had a 50 pin connector!

"Standard" (well, they sorta almost were) 8" drives, when they are on a 
track greater then cylinder #43 use a reduced current when writing.
Some drives expect a signal on pin 2 from the FDC to tell them; some have 
internal support for it, with the drive keeping track of the current 
track, and don't need that signal (such as Shugart 860, and the later 
versions of Tandon 848 (1E?, 2E?))

Disk controllers intended for 8" drives will usually have it.
PC controllers do not include it.

The DBIT FDADAP adapter (that I think Bill is using) handles it (in 
addition to the 34 pin to 50 pin cable pinouts)  Recent ones also have a 2 
digit display of track number.   Seek errors are not unknown, 
especially if the track to track step time is faster than the drive can 
keep up with (cf. Qume 142), so it is sometimes necessary to do a seek to 
track 0 and count steps back in again, to correct track number.

TG43 is absolutely NOT needed for reading.

Grumpy Ol' Fred     		cisin at xenosoft.com

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