7800 controller resistors?

[Garwor]

Hello there! Working on a home arcade controller that will have compatibility for 2600, 7800 and Colecovision. I've tried looking around but can't find a definitive answer -- how essential are those 620ohm resistors? I'm mostly wondering if they're likely to cause any interruption in function (or other problems) if they are left in the circuit for 2600 and Colecovision operation, or left out?

 

Just want to simplify the number of switches I'm using... the fewer the better.

 

Thanks.

[Shawn]

You MUST have the resistors in place for 2 button 7800 use.

[Andromeda Stardust]

You MUST have the resistors in place for 2 button 7800 use.

This. However the exact component value isn't too critical.

 

620 Ohms is fairly uncommon of a value. 560 or 680 Ohm resistors can easily be subbed in if 620 Ohm resistors are hard to source. I use 560 Ohm resistors in all my arcade controllers and they work great. I would definitely not recommend going lower than 470 Ohm however, or the transistor might sink excessive current in 7800 mode (21mA with both buttons pressed if using 470 Ohm resistors). I don't know what the maximum current draw on the transistor is. Normally in a game programmed for two button mode, if pin 6 ever gets pulled low, indicating that the input has been pulled to ground by a single button controller, the console will disable the transistor high logic output to avoid excessive current draw. It's a bit of a kludgy circuit design but it works.

 

Normally in 2600 or 7800 with single button compatibility mode, pin 6 (Atari 2600 FIRE) is held high by an internal 10k pullup resistor. With 5V VCC, the recommended voltage for low TTL/CMOS logic level needs to be below 0.8V. Usage of the stock 620 Ohm resistor would put this voltage level down to .29V on the input, about half that if both buttons are pressed. So the resistance value could go as high as 1.5k Ohm and still be within component tolerance (.65V = low) but this would be pushing it.

 

Basically, anything from about 470 Ohm to around 1k Ohm would probably work, but best not to stray too far off spec. I'm not sure what the total usable range is and it likely varies per console. Get as close to 620 as you can find, preferably 560 or 680 (both within 10% tolerance of 620) if 620 is not readily available. I use 560 Ohm resistors myself because Radio Shack stocks them. My two cents.