87 lines
3.5 KiB
Markdown
87 lines
3.5 KiB
Markdown
# petwifi
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An ESP8266 expansion for the Commodore PET / CBM.
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## Description
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Unlike the C64, the Commodore PET does not have a serial port.
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For that reason, most serial port adapters for the C64 don't work, and an
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alternate interface must be found. Furthermore, the user port on the PET
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doesn't supply any power, so an external power supply would be necessary.
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There is hope, however: The cassette port supplies 5V power and has
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suitable logic lines that can be used to emulate a serial port.
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The drawback is that serial communication must be fully emulated in
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software.
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Due to the different voltage levels of the ESP8266 and the PET,
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some voltage conversion is necessary. Luckily, this can be achieved with
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minimal circuitry: Converting from 5V to 3.3V can be done with a
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resistor divider. For the opposite direction, no conversion is
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necessary, because the voltage swing from 3.3V CMOS output drivers
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is sufficient to drive NMOS input gates.
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Special care must be taken for the MOTOR control line (which is used as
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CTS control input on the ESP): It is driven by a darlington array on
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a 9V supply rail.
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The other pin mappings are as follows:
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| PET pin | ESP pin | Direction | Cassette #1 internal | Cassette #2 internal |
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|----------------|---------|-----------|----------------------|----------------------|
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| Cassette Write | RXD | PET->ESP | VIA PB3 | VIA PB3 (same as #1) |
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| Cassette Read | TXD | ESP->PET | PIA1 CA1 | VIA CB1 |
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| Cassette Motor | CTS | PET->ESP | PIA1 CB2 | VIA PB4 |
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| Cassette Sense | RTS | ESP->PET | PIA1 PA4 | PIA1 PA5 |
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A suitable serial signal (RS-232-like) must be generated/decoded in software.
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Make sure that you don't access the cassette port #1 while the adapter is
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connected.
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For programming info, refer to this excellent document:
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http://www.6502.org/users/andre/petindex/local/pet-io-2.txt
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## RS232 Waveform
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| Logic | L | H |
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|-------|-----|-----|
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| CMOS | 0V | 3V |
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| RS232 | +3V | -3V |
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| Start | +3V | |
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| End | | -3V |
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_ ___ ___ ____
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H | | | | | |
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L |_| |_| |___|
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S 1 1 0 1 1 0 0 1 E
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_ _ ___
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+3 | | | | | |
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-3 _| |___| |___| |____
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S 1 1 0 1 1 0 0 1 E
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## PET RS232 Driver
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On the software side, the RS-232 interface is realized as a loadable driver for
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the PET. It can be compiled for loading into RAM, or for flashing onto a
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ROM chip.
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The driver defines four entry points:
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| Function | Entry point (load at $7000) | Description |
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|-------------|-----------------------------|-------------|
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| `rs_init` | $7000 | Initialize the driver and the hardware |
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| `rs_uninit` | $7003 | Disable the hardware interface |
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| `rs_read` | $7006 | Read one byte from the input buffer |
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| `rs_write` | $7009 | Write one byte to the output buffer |
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Input parameters and return values for `rs_read` and `rs_write` are exchanged via the zero page:
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| Variable | Address | Description |
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|----------------|---------|-------------|
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| `rs_available` | $ed | Available bytes to read or write (size of the input buffer or remaining space in output buffer) |
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| `rs_data` | $ee | Data byte (return from read or parameter for write) |
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| `rs_status` | $ef | Status of the operation (0=ok, 1=read buffer empty, 2=write buffer full, 3=not initialized, 4=already initialized) |
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