ISA ( PC ) 8" floppy disk CONTROLLER ???

[Chuck Guzis]

Well, Fred, not exactly--you're correct in that it's used for innter 
tracks, but RWC/TG43 is a signal INPUT to the drive.

The function is to reduce write current by about 20% to improve the 
readability of tracks 44-76.  It's not on all drives--the Shugardt
SA800, for example, doesn't have such a line.

The issue is one of "bit shifting".  When a transition is written close 
one already present, the tendency will be to "push" the already written 
transition away from the one being written.  This can lead to "bit 
crowding".  This effect can be seen if one has a controller, such as the 
Catweasel, which records a histogram of time between transitions.  Outer 
tracks tend to show nice clean frequency peaks with relatively empty 
"valleys" between them--that is, transition times fall in nice, neat 
"bins" with little overflow adjacent ones.  Go to the inner tracks and 
things are a lot more "fuzzy" statistically.

RWC/TG43 was an attempt on early controllers to mitigate this by 
lowering write current, particularly on FM-encoded disks.

Sometime along With MFM, the idea of "precompensation" came along.  This 
is done in the controller, not the drive.  The idea is that instead of 
spewing bits out of a shift register in the controller straight to the 
drive, one can run them through an extension of the shift register, so 
one can determine whether to advance or delay slightly the transition 
being recorded, based on what has been written and what will be written 
adjacent to the current bit.  This will cause the transitions upon 
reading to be closer to optimal.  You can still use the RWC/TG43 
facility on the drive, but the effect is far less pronounced when the 
controller-side precompensation takes place.

I hope this wasn't too confusing.  It can be a bit puzzling.

--Chuck

On 02/04/2015 12:45 PM, Fred Cisin wrote:
>>> I've seen the wiring info in the manual and I just looked through the PDF
>>> version of the manual I have. I could not find any mention of TG43 in my
>>> quick look through. Do you by any chance know where the info is? Thanks.
> On Wed, 4 Feb 2015, Chuck Guzis wrote:
>> TG43 is also sometimes called "RWC" for reduced write current.  On many
>> later 8" drives, this is a NC.
>
> Oversimplified:
> On inner tracks on a constant speed drive, the data is closer together.
> Therefore, the reliability is lower.
> (Some variable speed drives will change speed and write less data
> on inner tracks)
>
> One effort to help a little is "write precompensation".  On inner
> tracks, if adjacent flux transitions that would end up real close
> together are written slightly out of position, slightly further
> apart, it can help reduce the problem of flux transitions being
> too close together.  As part of doing that, there is a signal
> available called "TG43" ("track number greater than 43") to help
> decide when to enable precomp.
>
>
> It is NOT needed for reading, but is sometimes used to enable
> precomp for WRITING.