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From: sb@pro-generic.cts.com (Stephen Brown)
Newsgroups: comp.sys.apple
Subject: Re: DISK II
Message-ID: <9275.infoapple.net@pro-generic>
Date: 13 Jan 90 08:01:00 GMT
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Organization: The Internet
Lines: 23

In-Reply-To: message from JONATHAN@NUACC.ACNS.NWU.EDU

Jonathan Gelfand asks about a Disk II that insists that all disks are write
protected.

Some Disk II clones use photoelectric sensors instead of mechanical (micro)
switches. I have a Quentin Disk II clone which had a bad infared LED which
caused it to think that all disks were protected.  A replacement LED came from
Radio Shack for about $4. If the drive has a mechanical switch, it can be
tested with an ohmeter. According to the schematic for the Disk II analogue
board (which appears on page 146 of the DOS manual), the Write Protect signal
comes from phase 1 from the disk controller (generated by the 9334/74LS259 on
the Disk Controller card -- probably okay) and ends up going through one part
of the 74LS125 on the analogue board -- probably the the problem.  Why not
take a logic probe to pin 2 of the 74LS125. If opening and closing the write
protect switch causes this to go up and down, then check pin 3 of the 74LS125.
If pin 3 is following pin 2, then your cable is bad. If pin 3 is not following
pin 2, then the 74LS125 is bad.  If pin 3 is not following the switch (and the
switch is okay), then I'd suspect the cable or the 9334/74LS259. Howzdat?

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