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.
On 02/04/2015 12:45 PM, Fred Cisin wrote:
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.
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.
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.