🚀 go-pugleaf

In article <DEVEN.90Apr12101123@netserv2.rpi.edu> deven@rpi.edu (Deven T. Corzine) writes:
>
>In article <DEVEN.90Apr12055052@netserv2.rpi.edu> I write:
>
>Deven> Now what confuses me is this: WHY is stepping a track a
>Deven> requirement to update the signal??
>
>On 12 Apr 90 12:17:58 GMT, new@udel.EDU (Darren New) said:
>
>Darren> I think the signal gets CLEARED when the drive is stepped.
>Darren> That is, once set, it stays set until the drive is stepped.
>Darren> Therefore, read the signal and then step the drive.  The next
>Darren> insert or remove will set the signal again.  -- Darren
>
>Meaning it's a latch bit set by a pulse when the disk is inserted and
>cleared when the drive is stepped?  No, that can't work either; it
>still seems in any evet that the drive must "know" at some level
>whether or not there is a disk there...  and it seems it must know
>whether or not you step the heads.  So stepping the heads makes it
>tell the computer.  Is this right?  If not, what?  If so, why?

>Deven T. Corzine        Internet:  deven@rpi.edu, shadow@pawl.rpi.edu

From looking at the hardware and a discussion here long ago, I believe
its like this:
The bare drive has a signal that says whether or not a disk is in the drive.
This signal goes to the little interface board between the drive and the
DB-23 connector (The interface is on the motherboard for internal drives,
but a seperate board on the 1010.)  Whenever this signal goes to 
"disk removed," even for an instant, it sets a latch on the board.

Every so often, the system comes along and polls this latch.  Then, it
steps the drive, which clears the latch.  If there's no disk in the drive,
the latch immediately gets set again.

The reason for the hardware latch is because otherwise it might be
possible to change disks really fast while the software wasn't looking,
or was tied up on some poorly-behaved system-stealing program.  If there
wasn't a hardware latch, the system wouldn't know that a disk had changed,
and would procede to trash the filesystem on the disk.  For example,
re-writing a cached directory entry from one disk onto another.

It probably could have been done some other way instead of stepping the drive.
I don't think there is an extra pin in the cable; some of the alternative
schemes could have been worse.  It looks like the idea was to change from the
standard drive interface as little as possible with that little interface
board; the other things the board does like latching the motor on signal and
reporting drive-id at powerup also fit nicely into unused bandwidth in the
original (34-pin) drive connector.
--------------------------------------------------------------------
Steve Tell					tell@wsmail.cs.unc.edu
CS Grad Student, UNC Chapel Hill.