Casio fx-7700GB Back Cover

The fx-7700GB back cover has a small circuit board for the power supply circuitry. The TC4049B is an inverting-type hex buffer IC manufactured by the Toshiba Corporation. These are usually used for interfacing between CMOS and TTL logic circuits, and can provide logic level conversion. As you can see, the back cover also has a jack socket for the Program-Link data connection, and wires (red, white, and blue) from this socket connect to the circuit board. Hence, they use the TC4049B to provide the interface and logic level conversion between CMOS and RS232C protocols.

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