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From: mathrich@mthvax.cs.miami.edu (Rich Winkel)
Newsgroups: comp.sys.ibm.pc,comp.sys.ibm.pc.programmer,misc.wanted
Subject: Re: Write protect for hard drive
Message-ID: <1990May25.213958.1141@mthvax.cs.miami.edu>
Date: 25 May 90 21:39:58 GMT
References: <2805@crash.cts.com>
Organization: U of Miami Dept. of Math. and Computer Science, Coral Gables, FL 33124
Lines: 39

There is a way to do it in hardware, although I've never tried it myself:
The signals in question are called 'write gate' and 'write fault'.  The
first is signalled by the disk controller to tell the drive to enable the
write circuitry, the second is signalled by the disk to tell the controller
that the write failed.  On an XT, write-gate is wire 6 on connector J1,
write-fault is wire 12 on J1.  The idea is to wire a switch between the
controller and drive so that, in the normal position, these lines run
straight through, and in the protected position, the WG line from the
controller is connected to the WF line from the controller, the WG line from
the drive is grounded, and the WF line from the drive is disconnected.  For
this you need a triple pole double throw toggle switch wired like so:

Drive side			Controller side

	_________________
	|		|
	x	x\	x
_________	  \
WG	|	   \		WG
	x	x   \	x----------
________________|    \
WF		      \		WF
	x	x      \x----------
	|
	|
     -------
      -----
       ---
	-
	
The x's are the solder lugs on the back of the switch.  The up position
would be normal, down would be protected.  (the inverted triangle under the
lower left 'x' means 'grounded'.  A convenient ground on the XT is line 1 on
connector J1)

Like I said, I've never done it, but I've been told it works, and it looks
right on paper :-).

Rich