24 Aug
2005
24 Aug
'05
9:23 a.m.
Subject: Re: Floppy controller questions
From: Dave Dunfield <dave04a at dunfield.com>
Date: Wed, 24 Aug 2005 10:09:02 -0400
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
Now that I know the 765 better - makes perfect sense. This is a good example
of the problems I had with the NEC documention when I originally wrote the 6809
driver (mid 80's - my first experience with the 765). There are very few
references to polled transfers. Under "READ DATA", the datasheet says:
"After completion of the read operation from the current sector, the Sector Number
is incremented by one and the data from the next sector is read and output on the
data bus. This continued read function is called a Multi-Sector Read Operation. The
Read Data command may be terminated by the receipt of a Terminal Count signal. ..."
Well I have a copy of the original jinglish manuals from my days as product
engineer. After youve worked with a potload of the japanese/english translations
your own English becomes polluted. Your then speak missing small bits in sentence.
Sorry to those japanese speakers but, I was there and suffered.
Anywho, there were other cultural things that made etting answers to ambigious
or thinly worded explanations difficult. Often I'd end up in the lab testing
"just to see what was the case.".
As it turns out many engineers managed by inference to sort it out or by
simply hacking (try and see) methodology and the Compupro DISK1 (and 1A)
while DMA didn't use TC. Instead the driver programmed N and EOT to the
sector desired.
FYI: Another trick. Externall it's possible to tell DMA or other logic
if the transfer will be read or write without using a IO port. The
Write_enable (pin25) line to the drive can also be used to condition the
IO direction. Compupro used this for their DMA hardware for set read from
or write to memory. I'd figured this out when talking to RCA about 1802
hich has a DMA read and DMA write line. Using the WE line to conditionally
gate one or the other made doing 1802 floppy DMA less complex.
I may add that working with 765 you find thses and other things out. If
you work with 179X or 177X there are a similar set of "things you learn"
that allow you to exploit the device.
Allison