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

Frequently-Asked Questions (FAQs)

Q: Why would I need a BIT-C-128 Video DAC?

A: The Gonbes GBS-8200 and GBS-8220 accept only analog red, green, and blue (RGB) color signals with digital sync. (On these units, inputs with composite sync or separate horizontal and vertical sync are abbreviated RGBs and RGBHV, respectively.) The RGB color signals output by a Commodore 128 in 80-column mode (or IBM CGA or similar) is digital RGB plus intensity (RGBI) and vertical and horizontal sync. The BIT-C-128 Video DAC accurately converts the 16 digital RGBI codes to the intended analog RGB color levels plus composite sync, and enables convenient easy connection to a Gonbes GBS-8200 or GBS-8220.

Q: What will I need to get started?

A: Depending on your display and your computer, and what you want to do with them, follow the simple sequence below to determine what you will need. Note that a BIT-C-128 Video DAC with a Gonbes GBS-8200 or GBS-8220 will allow you to add more functions later. Every user will follow the first two steps, and most users only need the first 3 to 4 steps.

  1. Convert color signals from digital to analog. To use a Commodore 128’s 80-column output (or IBM CGA or similar), use a BIT-C-128 Video DAC to convert the digital color and intensity signals (RGBI) and separate sync (HV) to analog color plus composite digital sync (RGBs). Although the BIT-C-128 Video DAC is small enough to connect directly into most computers, many users find it convenient to extent the digital RGBI signals with a DE-9 extension cable and a to fasten the cable to the DAC with a pair of jack screws. (For your convenience, you can buy these with the DAC.) If your display accepts slower-than-VGA analog RGB (nominally 17kHz), all you’ll need is the right cable and cable adapter. (I sell those and also show how you can build your own.)

  2. Supply 5-volt power. Use a 5-volt power source of good quality. I sell 5-volt AC power adapters that I know work well, including a power adapter with a North American plug and a slim international power adapter with a folding North American plug and its clip-on plug adapters for most of the world. If you do not need a Gonbes unit (described in the next step), you may power your BIT-C-128 Video DAC from an AC power adapter via a 2-wire power kit, or from another 5-volt DC source that you might have handy.

  3. Increase scan rates from CGA to VGA. Most users want to connect to a display that accepts VGA input. To convert the slower CGA-rate (15.75 kHz) video signals to the faster VGA rate (31.46875 kHz), I recommend using an inexpensive Gonbes GBS-8220. (The Gonbes GBS-8200 is usually a little cheaper and also works, though arguably not as well.) Each Gonbes units is supplied with two cables that have a connector at one end and tinned wires at the other. One is a two-wire power cable, and the other is a six-wire signal cable. The tinned ends simply insert into the video DAC's terminal block, which has convenient spring-loaded contacts that clamp down on the wires.

  4. Connect your 40-column mode output. Those lucky enough to have a Commodore 128 will have a 40-column output from an entirely different video circuit. Some displays (such as the Dell U2410) will accept an analog composite IF (intermediate frequency) signal; if you have one that does, you can plug the signal right in, and use the switch on the display to select the input. Most displays don’t have one of these inputs, so I recommend getting a Commodore video cable, an Ambery SDV1 (with included AC adapter for about $80) , and 3 RCA male-male cables to plug the SDV1’s component video output into the Gonbes unit; you can then switch between inputs on the the Gonbes unit.

  5. Convert VGA to HDMI or DVI. Some users may want to plug this signal into a display that has only an input for HDMI or DVI. To do this, I recommend buying a Gana Link VGA to HDMI video converter for about $15. (Audio may also be fed into HDMI through this device by adding the right cable.) Converting from HDMI to DVI can be done by adding an inexpensive plug adapter.

Q: How does the BIT-C-128 Video DAC connect to a Gonbes GBS-8200 or GBS-8220?

A: Each Gonbes units is supplied with two cables that have a connector at one end and tinned wires at the other. One is a two-wire power cable, and the other is a six-wire signal cable. The tinned ends of each wire insert into the video DAC's terminal block, which has convenient spring-loaded contacts that clamp down on the wires.

Q: Why do you recommend using the GBS-8220 instead of the GBS-8200?

A: The GBS-8220 is a newer model and offers some improvements over the GBS-8200. The cables supplied with it also have slightly longer tinned ends, making them easier to connect into the terminal block.

Q: How does the BIT-C-128 Video DAC get power?

A: In most cases, the BIT-C-128 Video DAC gets its power from a 5-volt AC power adapter connected to a Gonbes GBS-8200 or GBS-8220; after inserting the two power wires (and six signal wires) into the terminal block, the 5-volt AC power adapter’s barrel connector plugs into the Gonbes unit, and the DAC gets its power directly from the power adapter through a connection on the Gonbes unit.

Those not using a Gonbes unit typically supply power from a 5-volt AC power adapter via a 2-wire power cable kit, though it can be supplied from any well-regulated 5-volt DC source.

Q: Why are the colors dark when I first connect my BIT-C-128 Video DAC and Gonbes unit?

A: For adjusting their analog RGB input levels, Gonbes GBS-8200 and GBS-8220 units provide three single-turn variable resistors (sometimes also called potentiometers or “pots”) near the input connector. These are small round knobs (usually orange) with a slot for a small screwdriver, which should be turned gently. (Note that they should turn freely, but less than 360 degrees.)

Although new units usually ship with these turned all the way up (counterclockwise), I sometimes hear that these arrive turned down.

To get the benefit of the unit’s full input range, you’ll generally want to keep at least one turned all the way up. You can tune the color balance by turning the input knobs down a bit; I recommend starting with turning down the red, which these units tends to display slightly high. (The BIT-C-128 Video DAC was desiged to provide perfect color output, which can be seen when feeding an analog monitor directly.)

Q: Any relation to Ray Carlsen?

A: No. It’s just a neat coincidence. I like Ray’s work and use (and love) a Commodore 128D(CR) that he restored, which was signed by its designer Bil Herd.

(When my grandfather immigrated from Norway, he adopted the name following an old Scandinavian tradition, as he was a son of my great-grandfather Karl.)

Q: Can you help?

Of course! The BIT-C-128 Video DAC is a specialty item that I sell to serve our community of like-minded Commodore 128 enthusiasts, and that I'm here to help in any way I can.

If you want help using a BIT-C-128 Video DAC (and especially if you didn't find the answer above) please send me a note via the contact page.

I also appreciate feedback, which helps me to provide a better experience to BIT-C-128 Video DAC users worldwide.


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