Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mandrill!neoucom!wtm
From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: comp.sys.ibm.pc
Subject: Re: How did this program burn out two monitors?
Summary: CGA monitor heritage
Keywords: better living through electricity
Message-ID: <1242@neoucom.UUCP>
Date: 27 May 88 17:34:04 GMT
References: <10244@agate.BERKELEY.EDU> <17460@glacier.STANFORD.EDU> <247@cucstud.UUCP>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 23


IBM did contract a japanese CRT manufacturer to make the CGA
monitor.  It is also quite true that the CGA monitor's function is
heavily based on NTSC-like tv technology.  IBM, being IBM though,
decided to make life nasty for would-be cloners of the CGA.  IBM
specified that the hoziontal sync should be positive going, rather
than negative going.  While it would be realtively easy for a
cloner to flip the sync polarity needed, it gave IBM several months
as opposed to several weeks lead time with an exlusive market, thus
assuring sales of their own monitor.  From a technical standpoint
there is no reason to have used positive sync, other than to be
nasty.

Since the MDA monitor was only  supposed to be designed to display
text, IBM probably felt that no precautions to prevent goofy scan
rates was necessary since nobody other than the ROM BIOS would be
programming the 6845 CRT controller chip.  We all know about that
assumption! :-).  Hercules brewed up a minor storm when they
released their board that added graphics as a possiblity for the
MDA monitor.

--Bill
  wtm@neoucom.UUCP