The C128 Simple VGA Adapter Page

Introduction

Welcome to the C128 Simple VGA Adapter Page! The C128's 80-column mode uses an RGBI (aka CGA) type output with a 15kHz scan rate. If you have an old Commodore monitor that supports it you are set. But, if you want a bigger newer display you will need to convert the RGBI to VGA or HDMI. This can be expensive. However, some modern LCD monitors or TVs can support 15kHz VGA signals. The remaining issue is that VGA is an analog system, but RGBI/CGA is a digital signal. This simple C128VGA adapter takes the 4 digital signals (R,G,B plus intensity) and uses a simple resitor and diode DAC circuit to convert those to the 3 R,G,B analog signals at the correct level. The adapter does not do any type of scan conversion and does not do have the "brown fix", but with the correct monitor it is a simple and inexpensive solution. As a bonus it does not need any external power. It also has an RCA composite output jack for 80-column mono.

Information

This is a passive design using minimal components for a small and simple solution. It does not need power as the DAC just combines the R,G,B, and I signals into RGB at the correct VGA voltage of 0.7V to 1.0V levels. It does not alter the Horizontal or Vertical sync as those are 5V levels. It does not support any type of VGA ID signals or two-way communications. The VGA monitor must handle all this automatically.

It does not support "Brown Fix" which means the colours may not exactly match what would be seen on an old CGA-type monitor.

It does not alter the scan frequency. It is not a scaler or scan doubler. Your VGA monitor must support 15kHz scan rates. Alternately, you can feed the output to an adapter or scaler box if your monitor does not support 15kHz or you want to convert the output to HDMI etc. Note: This adapter passes the H and V sync so any adapter requiring composite sync will not work.