Early 3.5" Floppy Drives
ajp166 at bellatlantic.net
Thu Dec 15 14:21:05 CST 2005
>Subject: RE: Early 3.5" Floppy Drives
> From: "Chuck Guzis" <cclist at sydex.com>
> Date: Thu, 15 Dec 2005 11:30:29 -0800
> To: cctalk at classiccmp.org
>On 12/15/2005 at 1:18 PM Allison wrote:
>>;) your assumption is double density. 8" SSSD is not that fast.
>>I never said formats were the same or even dive interface only that
>>the data rates fly.
>Nope. I'm just going by the 765 data sheet:
>"Pin 19 - CLK - Single-phase 8 MHz (or 4 MHz for mini-floppies) squarewave
>IOW, if you supported 3.5 DD (or SD) floppies, you weren't going to be
>able to do an A1 8" floppy without changing the clock.
;) You know not the part you speak of. Question, what it that clock used
for? Hint data rates are NOT tied to it.
The internal timers (HLT, HST, SR) are. So if you need real fast or real
slow step rates the chip clock is important.
You have to read the apnotes and there was a users manaual at one time.
there are many things that if you apply WD177x or 179x rules to will not
make sense. Such as the use of TC.
>Same thing obtains for the WD 179x - "Pin 24 - CLOCK - This input requires
>a free-running 50% duty cycle square wave clock for internal timing
>reference. 2 MHz +/- 1% for 8" drives, 1 MHz +/- 1% for mini-floppies."
Not even close 765 is a wholly differnt animal. The Read operation needs
RDW and the write must have WC. Both are independent of the chipclock.
(note: the 37C65 and later parts integrate a lot of logic that was
external to 765 but logically still are. so their behavour is rule driven.)
>The 279x has a clock divider that's programmed by pin 17 (5/8).
Again differnt animal and based on 179x.
>Look at the grandaddy of the 8" XT drive controllers, the CompatiCard I.
>It uses port 7F2H to change the 765 clock from 4 to 8 MHz for 8" support.
>MFM/FM doesn't enter into the equation--that's programmed into the read and
>write commands and the data separator.
>But an old XT-era 720K 3.5"/5.25" controller couldn't do this
>clock-switching trick unless it could also support 1.2/1.44 media.
Nope not needed.
The serial logic of the 765 is decoupled from the control and status
logic. If you used 4mhz for chipclock and did 8" your fastest step rate
would be half as fast as if you used 8mhz. Same you be true for Head load
time and head unload time. Also when he chip is idle (not seen on PC hardware)
it scans all four drives for "Ready and Disk Change", the scan rate for that
would also be off by /2.
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