This is just sharing the GENESIS2PCE project from https://github.com/Infinest/GENESIS2PCE – all design credit goes to its author, Infinest. The adapter lets you plug a Sega Genesis / Mega Drive 3- or 6-button controller into a PC Engine console.

It recreates the circuit of an original PC Engine 6-button pad (74LS157 multiplexers and a 74LS163 counter) and adds an ATmega328P that reads the Genesis controller. Plug in a 6-button pad and it works as a real 6-button PC Engine pad for games like Street Fighter II; hold Mode at power-up to switch to 2-button mode for everything else, with an LED showing the active mode.

Seven slide switches give turbo on/off for buttons I–VI plus slow/quick turbo speed, and holding A+B+C+Start for 8 seconds swaps Run and Select (remembered after power-off). The board is fully through-hole and easy to hand-solder; the ATmega328P must be flashed with the provided hex file before fitting.