TTL Video For The Classic ATARI

Which Monitor?
     One of the issues confronting users of classic Atari computers today is the selection of a suitable monitor for your system. Unfortunately, no one display will provide optimum performance under all conditions. A color monitor, for example, builds a pixel out of triads of red, blue and green dots. This produces a pleasing display when viewed at a distance, but quickly becomes grainy and indistinct as you get within a few feet of the screen. A monochrome monitor doesn't have the color screen's resolution problem, but graphics lose much of their personality when displayed as shades of a single color. The newer VGA monitors have much smaller color triads which give them both unlimited color and high resolution, but they run at much too high a frequency to be used on our classic 8-bits. So, we have to compromise: text clarity or color graphics?

If most of your time (like mine) is spent in text modes, a mono chrome monitor is really your best alternative. A good television with direct video inputs will fill in for those occasional games and demos, while the bulk of your computing (programming, word pro cessing, telecommunicating, etc.) can be done on a nice sharp mono chrome screen. Sadly, the supply of high quality analog displays is very limited since most computers (reed: IBM) are now built with TTL (digital) interfaces. (See 'Super Video 2.1XL elsewhere in this issue for a more general discussion of video interfaces.)

Unmodified, the Atari requires an analog NTSC or PAL com posite monochrome monitor which are practically impossible to find any more. All you see for sale are these days are TTL units (and even those aren't quite as popular as they were a few years ago). What we need is a way to interface our Atari to a TTL monochrome monitor that we can use for text displays while keeping our com posite output for color graphics. That's the subject of this upgrade.

TTL Caveats

By adding my circuit to your 8-bit, you'll be able to run two separate monitors on your Atari: the normal composite sources and a 9-pin TIL device. I've used the TTL interface to drive both color IBM CGA (Color Graphics Adapter) monitors as well as mono chrome IBM MDA (Monochrome Display Adapter) displays, al though the wiring needs to be modified for each type.

Why use a CGA display? Well, the MDA interface only has the capability to display four levels of gray, where our Atari will normal ly output eight. This isn't much of a problem in most text modes, but does leave a few holes in graphics. PacMan, for example, loses two of its ghosts in the background when run on an MDA display. On the CGA monitor, we can diaplay all eight intensity levels, each as a different color rather than shade. Text still suffers from the pixel size, but it does make a very clear and colorful alternative to MDA. Keep that in mind when you plan your configuration. Other wise, the text on a TTL monitor is razor sharp and much easier on the eye. than any color monitor.

Another concern is the frequency difference between the 8-bit and an IBM. Horizontal sync frequency on a standard MDA monitor is 18kHz, while the Atari runs at 15.75kHz. To check for problems in this area, I tested a number of MDA displays on the upgrade. Most ran just fine, with some needing slight horizontal or vertical frequency adjustments. Making these adjustments can be signifi cant if you're using a monitor like the IBM 5151. This model has no external controls! I had to remove the cover (no simple task) and adjust the pots inside the cover. Another negative for the IBM 5151 is the slow P39 phosphors used on the screen. This long persistence surface causes a smeared image as an object is moved or scrolled. I can't recommend the IBM, although it does work with the interface. I suggest a good amber screen with a high contrast filter like the Amdek 310A or Samsung SM-125FA7. These monitors provide an excellent text display and are widely available.

Doing The Mod
The construction method I used on this project allows you to build the board and simply plug it into your computer: no wires need to be soldered to your motherboard. I prefer this technique since it makes it easy to remove the modification in case of a problem, and it allows a friend to build you a board without your system being out of service.

I used half of a small Radio Shack perftoard (#276-148) and 30 gauge wirewrap wire to mount the components. Sockets are optional but recommended. You'll have to remove the 4050 video buffer from its socket on your motherboard. If it's not socketed, cut it off the board and add a socket.

I used .018 diameter standoff header pin connectors to plug into the vacant 4050 socket. [These type headers are hard to find but are usedm the Wizztronics 256K RAM upgrade and are available from Best Electronics. -BP] This provides us with all the necessary signals and power. See Fig. 1.

The output cable in a short length of nine conductor ribbon cable with an IDC female DB-9 connector on one end and a ten pin dual row header-pin connector on the end that connects to the board. I mounted the DB-9 connector on the case of my 1200XL, but you can just run it out through the case seam if you like and let it hang free. The installed TTL board and cable are shown in Fig. 2.

Refer to Fig. 3 and the table below for a pinout description of a typical TTL monitor:

   Pin # Signal

    1     GND

    2     GND

    3     Red   (1)

    4     Green (1)

    5     Blue  (1)

    6     Intensity

    7     Video

    8     H-sync

    9     V-Sync

   Note (1): these pins are not used

              in MDA (monochrome) monitor..

		

Take note of the two designations for pin 8 of the DB-9. One is for a CGA monitor, and the other is for an MDA monitor. These monitors use different sync polarities and may or may not work with the wrong polarity. [A few brands of monitors have a switch in the back for positive or negative sync. The enterprising hacker might elect to install a switch here if your monitor lacks one. -BP] Choose CGA or MDA when you wire up the board (or install a switch).