2797 query re AT&T 3b1

[John B. Milton]

In article <24223 at eagle.wesleyan.edu> flinton at eagle.wesleyan.edu writes:
>On the 2797 FD controller in the UNIX-pc, pins 17 and 37, though tied to
>ground (according to sheet 13 of the Tech Ref Manual's schematic), serve
>        _         ____
>as the  5/8  and  DDEN  pins, respectively.  That is, held high, pin 17
>should adjust the VCO frequency for 8" disks, while pin 37 should select 
>single-density (rather than default double-density) R/W/format operation.
>
>If one wished to operate with more floppy drives (of a variety of formats/
>sizes), one certainly needs to add FD selection switching (external to the
>2797) and appropriately hold pins 17 and 37 (which need first to be lifted
>from their hard-wired grounded condition) HI or LOW.
>
>Q.1: are there enough unused gates on the motherboard to do any of this
>under software control?  I'd be glad to hear about their successes from any
>having such a design.
>
>Q.2: even if FD selection must be done via external mechanical switching,
>is it possible to drive pins 17 and 37 HI and LOW via software and unused
>gates?

This one has been around on the guitar before. We have the WD2797 in our
machines, not the 2793. From the data sheet:

                      2791 2793 2795 2797
Single Density (FM)    X    X    X    X
Double Density (MFM)   X    X    X    X
True Data Bus               X         X
Inverted Data Bus      X         X
Side Select Out                  X    X
Internal CLK Divide    X    X

One could switch to the 2793, use internal clock divide and do side-select
externally (the way it should have been done), or one could switch the clock
externally from 1Mhz to 2Mhz. Other analog circuitry must also be changed.
I have an Advanced Digital S-100 Z-80 board that uses the 2793 to get both
5 1/4" 250k and 8" 500k data rates. It changes the !5/8 pin and biases the
analog stuff. Any way it's done, the driver still needs to be changed.

As far as unused gates, yes there are some, not the sort you need, and since
there are so many different mother boards, nothing you can depend on.

John
-- 
John Bly Milton IV, jbm at uncle.UUCP, n8emr!uncle!jbm at osu-cis.cis.ohio-state.edu
(614) h:252-8544, w:469-1990; N8KSN, AMPR: 44.70.0.52; Don't FLAME, inform!