ColecoVision controller circuit. Why pull down to -5V? - Page 1
Forum section: electronics / projects

author: mayor
author_rank: Regular Contributor
timestamp: December 28, 2022, 01:38:26 pm
content: Hello, I'm puzzled by the use of negative voltage in following circuit:U24A and B inputs are pulled down to -5V through a 1.5K resistor. The output is used to drive controller inputs (the controller essentially shorts pins 1-4/6-7, each pulled high, to pin 5 or 8 ) into a 74LS541, which should work withgroundand >=+2V to toggle outputs.It seems using ground instead of -5V would have worked just fine? I've actually tested this by bypassing the logic gates entirely with ground, no issues. Why would the designer go through the trouble?

author: jonovid
author_rank: Super Contributor
timestamp: December 28, 2022, 02:04:16 pm
content: Note: Schematic traced from actual board, accuracy can not be gauranteed.

author: mayor
author_rank: Regular Contributor
timestamp: December 28, 2022, 03:14:38 pm
content: Quote from: jonovid on December 28, 2022, 02:04:16 pmYep, but probing confirms this is how the actual console is.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 28, 2022, 03:59:53 pm
content: Would be nice to give your source, to give an better explanation ?From here :https://wiki.console5.com/wiki/Colecovision

author: Bud
author_rank: Super Contributor
timestamp: December 28, 2022, 04:09:17 pm
content: Could be done to increase control lines noise margin. Controller wires are long and may pick up noise which may trigger the 541 buffer. Biasing them with negative voltage for logical zero would increase voltage swing from 0...2.5v to -2.5...2.5v on the 541's inputs.

author: MasterT
author_rank: Frequent Contributor
timestamp: December 28, 2022, 04:32:13 pm
content: I see on this picture that composite video "reference blank" is 0.285V, that hard to get using common logic IC specified at around 0.4Vhttps://en.wikipedia.org/wiki/File:Composite_Video.svg

author: floobydust
author_rank: Super Contributor
timestamp: December 28, 2022, 07:39:05 pm
content: I think the schematic has too many errors to make sense of any use of -5V. Unless it is used for data bus termination, I see the 2.5V/270Ω resistors which are more likely for that.From my wasted youth, there are no analog inputs at the Coleco joystick ports. The car racing steering wheel plugged into the expansion module port.ColecoBIOS source codefor each of the two players, there are 5 bytes to read - joystick, keyboard, fire button, spinner, arm button.A -5V might be for an extra mux channel like for the Super Action Controller (8 pos joystick, 5x2 keypad, spinner, 4 fire buttons). I have one I can take apart and draw the schematic for because it might have more switches than the port has?The 'LS541 has the input Schottky clamp (same as '241), PNP and inputs still float high.The flip-flop outputs a strobe to select either the joystick or keypad to be read from the game controller.edit: it's only the two strobe outputs that appear to be pulled -ve if it was a tri-state buffer, not a 'LS00

author: mayor
author_rank: Regular Contributor
timestamp: December 28, 2022, 08:07:02 pm
content: Quote from: coromonadalix on December 28, 2022, 03:59:53 pmYes indeed, forgot to paste that in!

author: floobydust
author_rank: Super Contributor
timestamp: December 29, 2022, 01:54:15 am
content: I think I figured it out.The -ve voltage at the strobe flip-flop outputs is to overcome the voltage drop of the anti-ghosting diodes in the controller.You marginally meet 'LS541 VILmax. of 0.8V with a diode in series with every switch. They measure 0.6V plus cable loss. So I think they pulled the 'LS00 output low below ground for coverage as its VOLis too high.

author: mayor
author_rank: Regular Contributor
timestamp: December 29, 2022, 10:29:35 pm
content: Quote from: floobydust on December 29, 2022, 01:54:15 amHah, thanks, yeah, I can see that!