Connect two monitors to the Commodore 128: we use the RGBI output

In today's video, let's see how to connect two color monitors to the Commodore 128, using the exit RGBI, in addition to the video output. The Commodore 128 is in fact equipped with a second graphics chip, the VDC, whose signals come out on the rear RGBI port.

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The RGBi port can be connected to an RGBI or CGA color monitor, plus it has a pin with a black and white composite video signal.

RGBI (CGA) to VGA 15kHz adapter, mounting

The first step in connecting two color monitors to the Commodore 128 is to build an adapter to connect the RGBI port to a VGA color monitor (which supports 15KHz signals). Ivan Sergio created the printed circuit on the basis of Travis Durf's scheme, available on the net and shown below.

Connect two monitors to Commodore 128, RGBI CGA to VGA 15kHz DIY assembly kit interface converter adapter diagram, Travis Durf

The adapter is not compatible with all VGA monitors. It is necessary that the monitor supports the horizontal sync frequency of 15KHz, instead of the standard ones, which start at 31KHz.

Connect two monitors to Commodore 128, RGBI CGA to VGA assembly diy kit interface converter adapter

We begin to mount the resistors, which are the lowest components. A little at a time, we insert them in their correct position, mounting them adhering to the printed circuit. Finally we cut off the surplus of the leads.

I had made a tin soldering tutorial, whoever is interested can find it by following this link.

We continue the assembly of the printed circuit with the sockets of the integrated circuits. We begin to weld 2 pawns for each plinth, at opposite corners, so that we can straighten the plinth by heating the two feet just soldered. Be careful to insert them in the correct direction, identified by a notch. When the sockets are straight, we can solder the other feet.

Let's move on to the 100nF filter capacitors. We solder them straight and adherent to the printed circuit board. Then we cut off the surplus of the leads.

Connect two monitors to Commodore 128, RGBI CGA to VGA DIY kit interface converter adapter

It is the turn of the pin strips. One at a time, to mount it straight and adherent to the printed matter, we begin to weld only one pin. Then we straighten it and finally we weld the other feet.

Connect two monitors to Commodore 128, RGBI CGA to VGA interface converter adapter diy kit

We mount the two connectors, the 9-pin and the 15-pin one. In addition to the feet, we also weld the side supports, which serve to strengthen the installation.

Connect two monitors to Commodore 128, RGBI CGA to VGA interface converter adapter diy kit, db9 dsub 9-pin connector

We also solder the power connector. The adapter needs an external 5V power source.

To complete, I wanted to add an electrolytic capacitor on the 5V power line, as an additional filter.

C128, RGBI CGA to VGA port interface converter adapter diy kit, dsub 15 poles high density

After having welded all the components, we can clean the printed with a toothbrush, a cloth that does not leave lint and alcohol.

Connect two monitors to Commodore 128, interface converter adapter kit diy PCB cleaning RGBI CGA to VGA, alcohol toothbrush rag

We then mount the three integrated circuits on their respective sockets, paying attention to the correct direction.

Since, with the connector that I bought, the power supply voltage did not pass on the predisposed pin, I had to mount a jumper. Details of the jumper can be found in the video.

Connect two monitors to Commodore 128, RGBI CGA to VGA 15kHz interface converter adapter diy kit assembled

At this point we can mount the jumpers on the CINVERTSV1 and HSYNCSC2 connectors. Although I have not found any differences by reversing the polarity of the pulses, I suggest starting to mount the jumpers in the position indicated in the photo.

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Connect two monitors to Commodore 128, RGBI interface converter adapter kit CGA to VGA 15kHz jumpers, CINVERTSV1 HSYNCSV2, reverse sync

Before powering up, I suggest testing any short circuits on the power supply line with the tester. With regard to the polarity of the 5V power connector, the positive is located outside the connector, while the central pin is the negative. The converter to connect two monitors to the Commodore 128 consumes about 50mA.

C128 double color monitor, RGBI CGA to VGA converter, 5V power supply connector, stabilized 50mA absorption

Connect two monitors to the Commodore 128, cables

Let's start connecting the SVideo cable for the number one monitor signals. The SVideo cable starts from the 8-pole DIN Video connector behind the Commodore 128 and arrives behind the TV. A composite video cable with a 5-pin DIN connector can also be used instead of the S-Video cable.

Connect two monitors to the Commodore 128, monitor one SVideo Video cable, DIN 8 poles

We connect the cable with two DB9 male-male connectors between the Commodore 128 and the RGBI adapter (CGA) – VGA. Pay attention to the pinout! It is not possible to mount a connector on the adapter to insert it directly into the RGBI port. The pin arrangement is not mirror-like, but pin to pin.

We also connect the VGA cable between the adapter and the second monitor.

Connect two monitors to the Commodore 128, RGBI CGA to VGA 15kHz interface converter adapter cables

Finally, we connect the external 5 Volt power supply, which must deliver a current greater than that absorbed by the converter, which is about 50mA.

Connect two monitors to the Commodore 128, the Basic test program

After turning everything on, let's load the example program, which you can find at the bottom of this page. On each monitor are displayed different writings, in color.

Connect two monitors to the Commodore 128, Graphic Color Vice emulator Basic test program, dual monitor, dual display

After typing your name, the first monitor displays “HELLO”, followed by the name, in different colors. The second monitor displays “GOOD MORNING”, followed by the name.

Connect two monitors to Commodore 128, C128 dual color monitor, RGBI CGA to VGA 15kHz Basic program, dual monitor port

As I wrote earlier, not all VGA monitors support 15KHz horizontal sync signals. There are lists of compatible monitors on the net. By connecting an incompatible monitor, the image will not be formed completely or will be incomprehensible. It is also possible that the monitor gives an error message such as "OUT OF RANGE", "OUT OF RANGE” or similar.

Basic commands to use two monitors with the Commodore 128

To learn more, let's see the commands in Basic with the VICE emulator for using two monitors with the Commodore 128. As soon as it is turned on, if the 40/80 column button is not pressed (40/80 display), the Commodore 128 is positioned on monitor 1, the standard 40 column one. We can also change the color of the writing using the command:

COLOR 5,numero_del_colore

To switch to the second monitor, connected to the RGBI port, use the command:

GRAPHIC 5,0
Connect two monitors to the Commodore 128, C128 dual color monitor, Vice emulator Basic program

Also on the second monitor you can change the color of the writing, using the same command COLOR just seen. To return to monitor 1, use the command:

GRAPHIC 0,0
Connect two monitors to Commodore 128, C128 dual color monitor, Basic program emulator Vice Graphic 5.0 Graphic 0.0 Color WinVice

The example program found at the bottom of the page is based on this principle.

Basic example program to manage the double monitor on the C128

Commodore-128-dual-monitor.zip, to be expanded, inside there is the Basic program in PRG and D64 format.

Credits: In the video tutorial, SID music is ACE II by Rob Hubbard.

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