It took me longer than I meant to, but I dug up the manual for the AtariLab package that I'd mentioned in the thread on getting kids into older computers. In case you weren't reading that discussion, the AtariLab system was a science kit for the Atari 8-bit home computers that included a cartridge, an interface box that plugs into the joystick port, and a variety of measurement probes. Atari must have manufactured them far in excess of demand (no! not Atari!), because they're still fairly inexpensive NIB. At any rate, here's the scoop on the interface box: it has 8 phono jacks on it, arranged into 4 pairs: PADDLE, PTRIG, CONTROL, and POWER. The PADDLE inputs are the analog inputs; they're simply expecting to see a variable resistor on the other end, just as you'd except. The PTRIG inputs are digital inputs corresponding to the joystick fire buttons. The CONTROL jacks are indeed digital outputs (this was the question that came up in the original thread). They are wired to the UP and DOWN pins of the connector -- the Atari joystick interface can be programmed to use these pins for output rather than input, it seems. You must use controller jack 3 or 4 on the original 400/800 and 1 or 2 on the XL series. The PTRIG inputs can be reversed in the same way, giving you a total of four digital output lines per interface. The POWER outputs are just connected to the +5V supply of the joystick interface. The manual doesn't indicate how much current you can draw from the interface, but I imagine it's not much. So there you have it: 2 analog inputs, 2 digital input/outputs, 2 digital outputs, and 2 power connections.