BIT-C-128 Video DAC for Commodore 128/128D, IBM CGA, and Tandy CGA Plus

Uses for the Video DAC

The most common application for the BIT-C-128 Video DAC is driving a VGA display through a low-cost video scaler such as a Gonbes GBS-8200 or GBS-8220 video converter. (Sample output using this configuration is pictured below at right.)

Whichever way the video DAC is used, it may be connected to the computer through an optional DE-9 extension cable. The length of such a cable should not exceed about 6 feet or 2 meters. Optional jackscrews are recommended to secure the cable to the video DAC.

Driving a VGA Display

The most common application for the BIT-C-128 Video DAC is driving a VGA display through a low-cost video scaler such as a Gonbes GBS-8200 or GBS-8220 video converter.

What you'll need:

This combination will convert digital output from Commodore 128 or 128D computers in 80-column mode, IBM CGA, or Tandy CGA Plus and display it on a VGA monitor.

Using 40-Column Modes

Commodore 128 and 128D computers use a separate circuit to generate their 40-column video output, so the BIT-C-128 Video DAC is not needed to display 40-column video.

Output in 40-column mode may be viewed in grayscale simply by adding a video cable to feed the luma (Y) signal (or—less ideally—the composite video signal) into the Y channel of the component video (YPbPr/YCbCr) input of the Gonbes unit, as shown below left.

Color from 40-column modes may be displayed by adding a component color separator such as the Ambery SDV1 (which sells for $80) and 3 RCA male-male cables (or a triple RCA cable), as shown below center and below right. The Ambery SDV1 may be connected to the Commodore 128 using a circular DIN cable with either S-Video (for sharpest display) or composite video (yellow RCA).

Note that the Commodore 128 may operate in either 40-column mode or 80-column mode, based on whether the "40/80 DISPLAY" key is set up (for 40 columns) or down (for 80 columns) when the computer is turned on or reset; Commodore 64 mode (40-column) may be entered from Commodore 128 modes (40-column or 80-column) by using the command "GO 64". For more information, see C128 System Guide - Section 2.

Switching the Gonbes unit's input (cycling through RGBS, RGBHV, and component) may be done by pressing its "SW" button, and switching the Ambry SDV1's inputs (between CVBS and S-Video) may be done by pressing its button.

40-column Commodore 64 mode using composite video into Y channel of component input on GBS-8220, using:

  • Gonbes GBS-8220
  • AC adapter for GBS-8220
  • Commodore circular DIN to composite video (RCA) cable

40-column Commodore 64 mode using composite video via Ambery SDV1 component output to GBS-8220, using:

  • Gonbes GBS-8220
  • AC adapter for GBS-8220
  • Commodore circular DIN to S-Video cable
  • Ambery SDV1 (with included AC adapter)
  • 3 RCA male-male cables (for composite video)

40-column Commodore 128 mode using composite video via Ambery SDV1 component output to GBS-8220, using:

  • Gonbes GBS-8220
  • AC adapter for GBS-8220
  • Commodore circular DIN to S-Video cable
  • Ambery SDV1 (with included AC adapter)
  • 3 RCA male-male cables (for composite video)

Driving an Analog RGB Display

The video DAC may also be used to drive slow-scan analog display directly. For example, the video DAC has been tested driving the analog RGB and composite sync inputs of a Sony PVM-1342Q, as pictured at right. (Note that the PVM-1342Q also has a digital RGBI input that a C-128 can drive directly using only an extension cable with DE-9 connectors, although the colors displayed will differ slightly without the video DAC.)

Use of the analog CRT demonstrates the video DAC's high signal quality, including the accuracy of its colors and the sharpness of the characters. (Please forgive the photograph's poor focus at the edges, especially the bottom right.)

Driving a display like this, the video DAC will need a source of 5-volt power, such as from a USB port or power adapter.

For the easiest and nicest solution, here's what you'll want:

To Make Your Own Cables

If you would like to solder your own wire connections, here's how:

KitWire
Color
Connector
Pin
Function
power red center+5V DC power supply
 blackoutsidepower supply return ("ground")
signal black 6,7,8 *signal return ("ground")
 yellow 14vertical sync
 gray ** 13horizontal sync (jumper removed) or composite sync (jumper present)
 red 1red channel analog output
 green 2green channel analog output
 blue 3blue channel analog output

* VGA signal return ("ground") pins are 5 (digital horizontal sync return), 6 (analog red return), 7 (analog green return), 8 (analog blue return), and 10 (digital return for vertical sync/blank and VESA DDC). For best performance, connection to only analog signal returns (pins 6, 7, and 8) is recommended.

** Gray wire used on Gonbes cables. Color may vary on 6-wire kit.

I highly recommend sealing each connection with heat-shrink tubing and adding strain relief to the wires coming from the D connector, as I do.


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