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From: johnhlee@sol.cs.cornell.edu (John H. Lee)
Newsgroups: comp.sys.amiga
Subject: Re: Blowing up your 1950
Message-ID: <49205@cornell.UUCP>
Date: 3 Dec 90 16:40:55 GMT
References: <964.2754A82B@weyr.FIDONET.ORG> <36456@cup.portal.com> <606@cbmger.UUCP>
Sender: nobody@cornell.UUCP
Reply-To: johnhlee@cs.cornell.edu (John H. Lee)
Organization: Cornell Univ. CS Dept, Ithaca NY
Lines: 35
Summary:
Followup-To:

In article <606@cbmger.UUCP> peterk@cbmger.UUCP (Peter Kittel GERMANY) writes:
>In article <36456@cup.portal.com> thad@cup.portal.com (Thad P Floryan) writes:
>>David.Plummer@f70.n140.z1.FIDONET.ORG (David Plummer)
>>On the early IBM-PCs (and maybe still for all I know about "those" systems),
>>it was possible to change video attributes by altering the values written to
>>the registers in the video controller chip.  An errant program (or a malicious
>>Trojan Horse) could also change the value in the register all the way down to
>>ZERO.  Just imagine what used to happen when, say, an expected 15KHz AC signal
>>on the CRT coil(s) is now pure DC.  Yes, it WOULD and DID destroy the CRT.
>
>Yes, and you know why? Because PCs use the same 6845/6545 video controller
>chip as in the SuperPET (or also 8032) mentioned in the original posting!
>This 6845 appears also today as subset in the registers of every PC video
>card. Was a quite useful part.

Was?  I've looked extensively into the 6845 because my old Heathkit H89's
terminal section (the H19 portion) used it.  So does a Perkin-Elmer terminal
I have.  It looks like a pretty nifty chip--relatively compact, yet generates
almost everything you need, plus is configurable in almost anyway you want
including 0Hz refresh rate.  In fact, I'm thinking about designing a simple
graphics board with it.

Anyways, the 6845 isn't the reason for monitors being destroyed.  It's
low-cost design.  My H89's analog video subsystem was designed to be an
independent monitor.  It had a simple local vertical oscillator with a
free-running frequency of ~60Hz that just reset itself to vertical-sync's.
It essentially acted like a simple phase-locked loop.  You couldn't destroy
the monitor by incorrect programming of the 6845 controller.  (You could
reprogram the terminal since it was basically a Z-80 computer with firmware.
In fact, mine has a 3rd-party ROM which added a couple of features.)

-------------------------------------------------------------------------------
The DiskDoctor threatens the crew!  Next time on AmigaDos: The Next Generation.
	John Lee		Internet: johnhlee@cs.cornell.edu
The above opinions of those of the user, and not of this machine.