Coin Meter (Sega Mega-Tech)

2min

When the Sega Mega-Tech boots up, the SMS system first disables the controller inputs on the Mega Drive by sending the IO Expander Port D2 (IC24 pin 22) high.
This is done by disabling the output enable for the 74LS257 Multiplexers (IC67 / IC45) and the !1G output (for UP/DOWN Buttons) of the 74LS244 Buffer (IC62) connect to the OR Gate 74LS32 (IC51 pin 8).
The two inputs of IC51 (that if high disable the output) go to:
Pin 9 (3A) connects to IO Expander (IC24 pin 23 Port D3).
NO CREDITS - Pin 10 (3B) connects to the IO Expander (IC24 pin 22 Port D2).
If either of these are high, it disables the buffer / multiplexers and the Mega Drive cannot read the controller buttons.
Inserting Credits
To insert a credit, send the A2 button to ground (CN2 pin 10), this will send IC24 pin 22 D2 to ground, and so long as pin 23 is also ground the controller buttons will be enabled.

Updated 28 Apr 2024

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