121 lines
4.3 KiB
Markdown
121 lines
4.3 KiB
Markdown
# petwifi
|
|
|
|
An ESP8266 expansion for the Commodore PET / CBM.
|
|
|
|
## Description
|
|
|
|
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 external | 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 you don't access the cassette port 1 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
|
|
|
|
## RS232 Waveform
|
|
|
|
| Logic | L | H |
|
|
|-------|-----|-----|
|
|
| CMOS | 0V | 3V |
|
|
| RS232 | +3V | -3V |
|
|
| Start | +3V | |
|
|
| End | | -3V |
|
|
|
|
```
|
|
_ ___ ___ ____
|
|
H | | | | | |
|
|
| | | | | |
|
|
L |_| |_| |___|
|
|
S 1 1 0 1 1 0 0 1 E
|
|
|
|
_ _ ___
|
|
+3 | | | | | |
|
|
| | | | | |
|
|
-3 _| |___| |___| |____
|
|
S 1 1 0 1 1 0 0 1 E
|
|
```
|
|
|
|
## PET RS232 Driver
|
|
|
|
On the software side, the RS-232 interface is realized as a loadable driver for
|
|
the PET. It can be compiled for loading into RAM, or for flashing onto a
|
|
ROM chip.
|
|
|
|
The driver defines four entry points:
|
|
|
|
| Function | Entry point (load at $7000) | Description |
|
|
|-------------|-----------------------------|-------------|
|
|
| `rs_init` | $7000 | Initialize the driver and the hardware |
|
|
| `rs_uninit` | $7003 | Disable the hardware interface |
|
|
| `rs_read` | $7006 | Read one byte from the input buffer |
|
|
| `rs_write` | $7009 | Write one byte to the output buffer |
|
|
|
|
Input parameters and return values for `rs_read` and `rs_write` are exchanged via the zero page:
|
|
|
|
| Variable | Address | Description |
|
|
|----------------|---------|-------------|
|
|
| `rs_available` | $ed | Available bytes to read or write (size of the input buffer or remaining space in output buffer) |
|
|
| `rs_data` | $ee | Data byte (return from read or parameter for write) |
|
|
| `rs_status` | $ef | Status of the operation (0=ok, 1=read buffer empty, 2=write buffer full, 3=not initialized, 4=already initialized) |
|
|
|
|
## Example BASIC Program
|
|
|
|
This program assumes that the driver is already loaded at start address $7000.
|
|
|
|
```basic
|
|
10 PRINT "Initializing driver"
|
|
20 SYS (28672)
|
|
30 PRINT "Waiting for input"
|
|
40 SYS (28678)
|
|
50 status = PEEK(239)
|
|
60 IF status <> 0 THEN GOTO 90
|
|
70 char = PEEK(238)
|
|
80 PRINT CHR$(char);
|
|
90 GOTO 40
|
|
100 PRINT
|
|
110 PRINT "Disabling driver"
|
|
120 SYS (28675)
|
|
130 END
|
|
```
|
|
|
|
## VICE Emulation
|
|
|
|
To help test the RS-232 driver, there is a patch for the Commodore computer
|
|
emulator VICE in the file vice-tape-rs232.patch.
|
|
|
|
It is intended for VICE 3.6.1 and prints out decoded RS-232 transmissions
|
|
from the driver to the console. Reading data into the emulator is not
|
|
supported yet.
|
|
|
|
To enable the emulated device, you need to open the "Tape port" settings
|
|
and select "RS-232 interface" for tape port 1.
|