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An ESP8266 expansion for the Commodore PET / CBM.

Unlike the C64, the Commodore PET does not have a serial port. For that reason, most serial port adapters for the C64 don't work, and an alternate interface must be found. Furthermore, the user port on the PET doesn't supply any power, so an external power supply would be necessary.

There is hope, however: The cassette port supplies 5V power and has suitable logic lines that can be used to emulate a serial port.

The drawback is that serial communication must be fully emulated in software.

Due to the different voltage levels of the ESP8266 and the PET, some voltage conversion is necessary. Luckily, this can be achieved with minimal circuitry: Converting from 5V to 3.3V can be done with a resistor divider. For the opposite direction, no conversion is necessary, because the voltage swing from 3.3V CMOS output drivers is sufficient to drive NMOS input gates.

Special care must be taken for the MOTOR control line (which is used as CTS control input on the ESP): It is driven by a darlington array on a 9V supply rail.

The other pin mappings are as follows:

PET pin	ESP pin	Direction	Cassette #1 internal	Cassette #2 internal
Cassette Write	RXD	PET->ESP	VIA PB3	VIA PB3 (same as #1)
Cassette Read	TXD	ESP->PET	PIA1 CA1	VIA CB1
Cassette Motor	CTS	PET->ESP	PIA1 CB2	VIA PB4
Cassette Sense	RTS	ESP->PET	PIA1 PA4	PIA1 PA5

A suitable serial signal (RS-232-like) must be generated/decoded in software. Make sure that no cassette port #1 access is generated while the adapter is connected.

For programming info, refer to this excellent document: http://www.6502.org/users/andre/petindex/local/pet-io-2.txt