Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!att!dptg!pegasus!dmt
From: dmt@pegasus.ATT.COM (Dave Tutelman)
Newsgroups: comp.sys.ibm.pc
Subject: Re: TTL Mono Monitor and Composite Mono Monitors
Summary: Highly non-trivial
Message-ID: <4423@pegasus.ATT.COM>
Date: 24 Jan 90 18:21:40 GMT
References: <13120.25bb055b@max.u.washington.edu>
Reply-To: dmt@pegasus.ATT.COM (Dave Tutelman)
Organization: AT&T Bell Labs - Lincroft, NJ
Lines: 64

In article <13120.25bb055b@max.u.washington.edu> scott@max.u.washington.edu writes:
>Could any who is knowlegable about monitors tell me what is the
>difference between a TTL monochrome monitor and a Composite
>monochrome monitor? Is it merely a different in the connector type.
>...how difficult is it to adopt a TTL monochrome monitor so that it will  
>accept the signals that a Composite monochrome monitor would accept.

I did the opposite conversion (TTL Hercules adapter to monochrome composite
monitor) about six years ago.  It was non-trivial.  What you suggest is
even harder.  Here's what you'll have to do:

1. The sweep frequencies are different.  You'll have to tweak the
horizontal sweep circuit of the TTL monitor to run about 20% slower.

2. The Sweep pulses (hor & vert) are combined in with the composite
signal.  You'll need a sync separator circuit (look in your favorite
TV circuit design handbook) to get them onto the separate leads
required by TTL.

3. Similarly, the intensity information is combined in with the
composite signal.  Separating this out to the intensity line is NOT
a standard TV circuit.  However, if you understood the sync separator
circuit really well, you could adapt the techniques to design an
intensity separator circuit.

4. Finally, you'd need a little level-shifting/impedance-matching
circuitry.  Simple, and probably passive.

When I did the opposite conversion, I had to solve #1 and #4,
just as you will.  #2 and #3 were much easier conversions
(like a gate and a couple of resistors and diodes) than
going from composite to TTL.

You don't give a reason for wanting to do this.  Here are a couple
of possible reasons, and their "solutions":

   -	You already have a TTL monitor and a CGA board, and you're
	cheap.  (That's why I did my modifications six years ago.)
	There may be a display board that looks to the bus and program
	like a CGA, but puts out a TTL mono signal; I don't know.
	However, that's likely to be much more expensive than just
	buying a cheap mono-composite monitor or Hercules-clone board.

   -	You want to run both CGA and MDA software on the same computer,
	into the same monitor (say, run games occasionally on a
	machine that's otherwise text-only).  In this case, don't
	spend money on a second CGA board.  Get a CGA-on-Hercules
	emulator (software) for your Herc board.  There are a
	couple of shareware products out there that work on about
	70% of the CGA software: SIMCGA and my HERCBIOS.

   -	You MUST run both CGA and MDA software on the same monitor,
	and cost is not an object.  Look into a (perhaps pricey)
	board/monitor combination that does this.  I know they exist,
	though I couldn't give you a brand name off the top of
	my head.

Dave
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|    Dave Tutelman						|
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|    Logical -  ...att!pegasus!dmt				|
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