Computer Nerd Kev
Projects > Vectrex-2600
Vectrex Controller to Atari 2600 Adapter
A while ago I was asked to build an adapter to allow a Vectrex controller to be used with an Atari 2600 video game console. What follows is the design that I came up with. If you don't want to build one of these yourself, I can make one for $100 AUD, same terms as for my VecAdapts.
This adapter will also (probably, double check voltages etc. before building) work with other machines that support the Atari 2600 controller pinout, including:
And possibly including these systems with some modification or different pinouts:
Changes to 2600 pinout:
The fire button connections are easy, simply connect all the Vectrex buttons to "fire" (pin 6) on the Atari. The joystick requires a bit more work though, because the Vectrex uses an analogue one while the Arari is designed for digital. A bit of comparator work is in order...
Here's the schematic. I had a go at doing it on the computer for once, but as you might notice I fought with it all the way.
As you can see, the circuit is simply a quad Op-Amp (an LM324 in my case) comparing the X and Y voltages from the Vectrex controller to the voltages on their respective voltage divider. The trimpots are set to the the joystick's center position.
Note that the bottom resistor value is different between the X and Y sides. This is because the two axis have different voltage ranges.
Also, controllers that are worn such that their joysticks don't return properly to center may be difficult to use unless the center voltage range is extended to accomodate them by adjusting the resistor values. The trade off will of course be that the joysick will have to be moved further in order to register each direction.
The Vectrex V+ voltage (Pin 7 Vectrex) is wired to the Atari 5V line (Pin 7 Atari), and both GND and V- connections (Pin 8 & 9 Vectrex) are to Atari GND (Pin 8 Atari). These power lines also supply the Op-Amp comparator circuit.
The circuit itself was constructed on Veroboard (Stripboard) in a compact manner. This diagram (generated with the VeroDes design program), shows the component locations and connections:
Red X means cut track.
Green O---O links are made with small solid core wire (preferably insulated). The one under the IC goes on the bottom side of the board.
Blue squares are connections for wires to plug or socket.
the rest are components with types and values as noted in the schematic.
The design file for "VeroDes" (you might have to right-click and "Save As" in some browsers).
I find Veroboard a very convenient way to make compact one-off circuits, though it can be easier to make design and construction mistakes (make sure those tracks are completely cut!) than with other methods.
First thing to note is that I haven't done this (yet). The problem is that the Vectrex uses the potentiometers in its joystick in a voltage divider arrangement, while the Atari uses the potentiometer in its paddles as a variable resistor and reads its resistance. The Atari appears to read the resistance by measuring the time taken for a capacitor to charge to a specific voltage. Due to the very different potentiometer values (and travel distances) used in the two controllers, the most practical way to moderate the rate of capacitor charge relative to the voltage or resistance of the Vectrex controller potentiometer would likely be a PWM signal with an On Time determined by the Vectrex controller potentiometer.
While a 555 timer IC could be used to generate the PWM signal according to the voltage or resistance of the Vectrex controller joystick potentiometer, finding the exact component values to use may require some experimentation to achieve the correct "feel" for the movement. Especially contributing to this is that I have been unable to determine the frequency at which the paddles are sampled (and the charging capacitors dumped to ground thereafter) in documentation. Monitoring the voltage on one of the paddle capacitors with an oscilloscope would probably reveal this.
A programable resitor IC could also be used, though the maxiumum current able to be passed to the capacitor and discharging transistor (inside the TIA) should be checked to make sure it doesn't exceed the specifications of the IC. At zero potentiometer resistance, the current could be towards 3mA, which can exceed the ratings of some such ICs. A microcontroller or logic circuit would have to be provided to specify the resistance values to the programable resistor.
The Atari 2600 schematics available in the AtariAge Archives are most valuable. For the Atari 2600 PAL, NTSC, and (luckily) the TIA chip that, among many other things, processes the paddle inputs. Sheet Two of the TIA schematic contains the paddle section in the upper Right. Here's the relevant section (97KB) so you don't have to download the entire 4.9MB of the full resolution sheet.
With the power lines as described above, the Vectrex controller joystick voltages are as follows (well, mine are anyway):
Note how small the changes are, especially in the Y Axis, indicating a very short travel distance for the potentiometer