In this instalment we'll go Atari!
We have the power!
Coleco was nice enough to build an expansion module that's basically a full Atari 2600 system without power supply, crystal and modulator. Those components are provided by the hosting CV system. To prevent interference, the expansion module halts the CPU, puts the VDP in reset so it doesn't generate video and cuts the clocks to the sound chip so it doesn't generate random sounds.
What do we need to add to make our little clone work with the Atari expansion module? Actually, very little. First, the Atari module requires power, mainly 5V, and our little power supply is more than capable of supplying the required current. Nothing to do there. Next, the Atari needs 12V, basically just for its video output buffer. To generate 12V from our internal 5V supply, install:
DCDC2 ROL-0512S 5V to 12V DCDC converter
This will supply the Atari module with 12V. Also, the expansion module needs -5V for its joystick ports (just like the Coleco BTW!) to offset the diodes present in the ColecoVision controllers. The charge pump on board is capable of supplying this voltage, just:
Set SJ20
If you want to support other expansion modules that possibly require more current, or you want exactly -5V (because the on-board circuit is more like -4.5) you can install its own -5V DCDC converter (DCDC1 ROL-0505S). Important: in this case remove SJ20!
Also, the expansion module needs clock, and should be able to send audio to the main board:
R60 470
C78 100n
Buffering...
Since you probably installed a TMS9928A or TMS9929A to use the RGB output, there has been no need for a composite video buffer (unless you're using the 9918A, in which case it's incompatible with expansion module #1!). With the Atari expansion module #1, you need the video buffer, but be sure to remove SJ12!.
Adding the video buffer is cheap, install these components:
U25 LM318D (SO08)
R35 75 Ohms
R36, R37 10k
C47 27p
C48 VF47/16K-C (6mm)
C49 10u/16V (5mm)
C79, C80 100n
FB1 0805 ferrite bead
Remove SJ12!
Unfortunately there are two resistors missing in revisions up to and including Rev.B. Add a 10k resistor (1206) Between C80 and ground as shown in the picture below. Add an additional wired 10k resistor between TP18 and LM318 supply as shown as well.
Again, that‘s really all!
Power up, and measure 5V, -5V and 12V at the appropriate expansion port pins, and you're set!
You don't know jack...
To output the (Atari) A/V signals, you need to install the RCA jacks J4, J6 & J7, but that means drilling holes in your precious ColecoVision.
You can also route the composite video through the Mini-DIN jack by installing a toggle switch to SJ5 to switch the Mini-DIN Composite Video pin back and forth between CVideo (for the expansion module) and CSync (for Coleco RGB Video).
Now all signals are available on the Mini-DIN connector, but whether or not you see Atari video depends on which wires are routed inside your Genesis RGB-Cable and how your TV deals with them! After all it's a RGB cable, and Atari doesn't output RGB, but most cables tell the TV it'll be getting RGB through the signal voltage. You can force some SCART-TVs to use composite by removing jumper SJ13 („A/V Voltage“), but it needs it again for RGB. In this case make the switch a DPDT to control SJ13 too.
So, if your cable/TV combo doesn't show a picture, you may have to solder a simple Mini-DIN to Composite Video cable. But at this point you're more than capable of doing that, and it's a small price to pay...
Happy Atari-ing!