Single Port Interface for Commodore 16 and Plus4 keyboardReplacement of MOS 6529B IC based on the logic 74273JUMPERS:Jumper settings to allow the best compatibility among motherboard revisions/models:VCC /RST jumper selects if 74HCT273 will reset according to the computer's push-button position (system reset, an additional wire to be connected) or tied high always (no IC clear, just power-on-reset) default: VCC (no wire)CLOCK jumpers select which active-low signal (_CS, R/_W, _CS&R/_W) does trig 74HCT273 default: &BOM LIST:1x High-Speed CMOS Logic Octal D-Type Flip-Flop with Reset 74HCT273 (DIP, THT)1x IC socket DIL20 0.100"/2.54mm pitch for the 74HCT273 (THT)2x Male Connector Header Through Hole 10 position 0.100"/2.54mm pitch circular pins for MOS 6529B (THT)10x Small Signal Schottky Diodes BAT43 or BAT42 (THT)1x Plastic or Ceramic Capacitor 0.1uF >16V (THT)1x Resistor 4.7K 1% 1/4W or 1/8W (THT)3x Male Connector Header Through Hole Right Angled 3 position 0.100"/2.54mm pitch (THT) or 0R resistors for the jumpers1x wire to connect /RESET pad to the reset push-button of the computer (optional, it depends on VCC /RST jumper)Designed together with SukkoPera and tested with C16 boards RevA and Bhttps://github.com/SukkoPera/LittleSixteenNOTES:The MOS 6529 is an I/O expander while 74xx245 and 74xx639 are bus transceivers.74xx245 and 74xx639 do not work at all.74xx273 suits the role better (do not use HC version, HCT is a must).It does implement the register to retain the written value and combined with open collector buffers, it can replace a faulty MOS 6529 as the keyboard controller.Schottky diodes are used to get the weak pull-up behaviour of the MOS 6529 (measured 7~8KΩ).Please mind that MOS 6529 is a bi-directional device, hence 74xx273 is unusable for the Plus/4 USER PORT because 74xx273 would completely ignore the ability to read from the bus.
