Date   : Fri, 30 Oct 1992 07:47:40 PST
From   : Sprague.Wbst311@xerox.com
Subject: Re: Adding 5"1/4 to Xerox820II ????

> There excists a 5" 1/4 package without any extra offboard controller,
> is this correct?

Yes, though I don't like the fact that it draws it's power from the 820 itself,
rather than having it's own power supply.  That is the reason that the 16/8 was
not offered with 5.25" disks (unless you had the Disk Expansion Module or DEM).
The power supply was not spec'ed to supply power for the 820-II, the 16/8 board
_AND_ the 5.25" drives (though many could actually do it).  In any case, if you
make your own cable, _don't_ try to power the floppy drives from the 820, using
ribbon cable!!!  :-)

The floppy controller in the 820-II and 16/8 (and the built in controller in
the 820)  works for both 8" and 2.5" drives.  Pin 2 on the 37 pin interface
told the controler which type of drive was attached.  If open, then 8" drives
were connected, if closed, then 5.25" drives were connected.  So this pin must
be grounded, if you want to add 5.25" drives.


Xerox 820-II Disk Drive Connector (J1)

 1     n.c.
 2     8/5" Select        (open=8"/ground=5")
 3     Two Sided       (Optional signal)
 4     Index
 5     Drive Select 1
 6     Drive Select 2
 7     Side Select
 8     Head Load
 9     Direction Select
10     Step
11     Write Data
12     Write Gate
13     Track 0
14     Write Protect
15     Read Data
16     Low Current     (Optional 8" signal)
17     Drive Ready     (8" only)
18     +12 Vdc
19     +5 Vdc
--     --------------
20     Ground
21     Ground
22     Ground
23     Ground
24     Ground
25     Ground
26     Ground
27     Ground
28     Ground
29     Ground
30     Ground
31     Ground
32     Ground
33     Ground
34     Ground
35     Ground
36     +12 V Return
37     +5 V Return

Note that there is no MOTOR ON signal, as 8" drives did not use it.  You need
the type of 5.25" drives that have a jumper to internally tie Drive Select to
Motor On (the 360K Chinnon's I have do this).  If you don't have drives that do
this, you will need to create an interface, something like IBM's "twisted"
cable, to derive two seperate Motor On signals.

Assuming you connect modern day half height floppys, be sure to run the 820's
CONFIGUR program, and set the floppy step rate from the generic 30 mS (needed
for the old Shugart SA400 drives) to 6 mS, which is used by all modern drives.
This not only will make a noticable speed improvement in the computer, but the
drives will make less noise.

Feel free to contact me if you need more info.

                               ~ Mike  (sprague.wbst311@xerox.com)