TG43 signal and the Nec 765

[Allison]

>
>Subject: Re: TG43 signal and the Nec 765
>   From: Dave Dunfield <dave04a at dunfield.com>
>   Date: Thu, 28 Jul 2005 08:40:38 -0400
>     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
>
>>TG43 is mostly a relic signal of older 8" drives.  Not all
>>of them used it.  Also it's supposed to be meaningless on read.
>>
>>I may add that more than a few designers also used demuxed TG43
>>to switch write precompensation in DD modes on 8" drives for 
>>better data reliability. 
>
>Can you elaborate.?

One of the things done when writing to a Floppy [it's done with all 
FDC chips if it's done at all!] is to deliberately twook the bit 
timing based on the bit pattern.  The FDC provides a set of control 
signals (in hardware) and drives a bit of logic to intentionally
smear the bits in time to allow for the media effects of close 
flux transistions.  In most cses this is all hardware but usually
the amount of precompensation is adjustable by both bit rate and
a non FDC control register (back in the 1791/765 days).  Some designs
further modified the precomp value for the inner tracks (TG43) as
drives didn't use TG43 or based on the deigners evaluation of bit 
crowding on the inner tracks might also alter the precomp timing to
optimize flux transistion spacing.  This is important as bits written 
to magnetic media tend to spread themselves out or crowd based on 
pattern when written closely.

Hope this helps.

>>Is this for a Cpro Disk-1(A) or something from the ground up?
>
>No, it for my ImageDisk utility, which is a replacement for
>TeleDisk with a documented image file format - I want to provide
>support for 8" drives that need TG43.

Ok, then I take it that this is in a PC FDC that is based on the 
765 core?

I ask as a way to point out that features and functions that the 
base 765 chip have as implemented on an something like a Disk1A
are very differnt in a PC.  Largely the PC design truncates the 
interface from the 765 to something simpler abd often far less
flexible.  Some of the really late chips (post 37C65) do not
even fully implement the full complment of interface signals
and may only have limited access to things like precomp timing
or worse those functions are "wired" by the chips designer to fit
their view of what floppies are (IE: antique = 8" floppy, 
unsupported).


Allison