Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!ucbvax!hplabs!hpfcdc!hpfcdj!myers
From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: sync signal generator
Message-ID: <17660008@hpfcdj.HP.COM>
Date: 8 Aug 89 18:16:22 GMT
References: <89216.120136BHB3@PSUVM>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 40

>RS-170 is monochrome.  Isn't the 3-wire RGB RS-170-like video spec RS-343?
>I can't remember anymore.

Nope.  RS-170 IS monochrome, but the version of RS-170 which includes the
NTSC color encoding scheme (with accompanying slight changes in timing)
is RS-170A.  RS-343 (now superseded by at least RS-343A) was intended as
a high-resolution closed-circuit standard; RS-343A never really caught on
as far as its recommended timing (it is an interlaced standard), but the
signal level definitions are used by most high-resolution computer graphics
displays today.

The signal levels defined for these standards are (all into 75 ohms, and
assuming the BLANK (not the BLACK) level as the reference:

RS-170/170A:  White is 1V POSITIVE from reference
              Sync tips 0.4V NEGATIVE from reference. (Overall 1.4V p-p signal)

RS-343/343A:  White is 0.714V POSITIVE from reference.
              Sync tips 0.286V NEGATIVE from reference. (1.0V p-p overall.)

In RGB systems conforming to these standards, the sync is on the GREEN signal.
The red and blue signals conform to the signal level standards w/o the sync,
i.e., the red and blue in an RS-343-level system would be expected to have
a max. swing of 0.714Vp-p.  Sync may also be provided to the monitor via
a separate input or inputs, which are usually TTL-compatible.

Back to the original question - I don't quite understand the needs of your
system.  If the unit simply requires an external Vsync pulse to "sync up to"
before it will produce video, then simply feed it the output of a pulse
generator set to make 60 Hz with about a 2-3% duty cycle (sync pulse about
0.3 - 0.4 msec long, out of the 16.67 period).  This will result in video
which should be usable by any RS-170 monochrome display; no need to get the
59.94 Hz NTSC timing exactly, unless you're planning on syncing up to some
external video.  But in the latter case, you'd want to derive the Vsync signal
from that external source anyway - and that's a whole 'nother can of worms!


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers%hpfcla@hplabs.hp.com               |  sentient life-form on this planet.