The C2N232 is a small RS-232 interface that can be plugged to the cassette port of an 8-bit Commodore computer:
In some cases, the C2N232 can be used without an RS-232 extension cable.
The C2N232 is the first universal data transfer solution for Commodore computers:
big computercan be anything that supports an RS-232 interface at 38400 bits per second and the C programming language.
Compared to other solutions, the C2N232 is:
Using RS-232 improves the portability on the
side, as RS-232 is available on virtually all serious computers,
ranging from IBM PC compatibles to Unix workstations and big
mainframes. All software is distributed as source code, and patches
to those systems that are not yet supported are welcome.
A cassette port is available on all 8-bit Commodore computers except the SX-64. On the 264 series, the connector is different, but the signals are compatible.
The hardware consists of only two integrated circuits: an Atmel AT90S2313 microcontroller clocked at 8 MHz, and a Maxim MAX232 RS-232 transceiver circuit.
The RS-232 connection is electrically far more stable and safe than connecting parallel interface chips together. Many an interface chip could have been saved by using an RS-232 connection. Even if there are severe grounding problems whose consequences exceed the absolute maximum ratings of RS-232, using RS-232 will limit the damage to easily obtainable driver chips, and not to proprietary or no-longer-available interface circuits.
The following functions have been implemented in the 128 bytes of RAM and 1024 sixteen-bit words of program memory on the AT90S2313 in the firmware:
Only four bytes of the 128-byte memory are reserved for the microcontroller execution stack. Everything is interrupt-driven, and the interrupts that drive the custom send mode may occur during RS-232 receive interrupts.
The idea of designing the C2N232 arised on the 30th of June 2001, when Marko Mäkelä visited a fellow Commodore hacker in Newcastle upon Tyne, Christopher Phillips. Christopher had no data connection between his Commodore 64 and his main computer, an Apple Macintosh. The only feasible interface of the Mac is RS-232, and existing solutions that require a parallel port are clearly out of question.
The first prototype was assembled on a Vero board in July or August, and the software development started at the same time.
First, some code was written for reading the pulse widths of the
cassette write signal, to reverse engineer the details of the
Commodore cassette format, as the information supplied in the title
Commodore 64 Whole Memory Guide by Tim Arnot turned out to be
c2n program that encodes and decodes
Commodore cassette pulses was written, and the
utility was enhanced with the high-level Commodore cassette
Second, the function for playing back cassette pulses was developed. There were some problems both in the hardware and in the software. The hardware was corrected by adding a pull-up resistor on the cassette read line. This was done on the 11th of September 2001 when Juha Kouri and Turo Heikkinen were visiting Marko's place in Espoo and measuring the circuit with the digital oscilloscope Juha borrowed from his work place.
Designing the custom transfer protocol took several months due to other activities. The protocol was designed in such a way that it works on all 8-bit Commodores. In the idle state of the bus, both the cassette read and write lines are on the logical high level. Both receiving and sending is initiated by the Commodore pulling the cassette write line low. This ensures proper operation of the protocol even if the computer clears the input register that records high-to-low transitions of the cassette read line. The send operation is extremely timing-sensitive: the processor clock speed must remain constant during it.
The digital oscilloscope was needed again in October or November for debugging the custom transfer routines on the Commodore 264 series, whose cassette read input is level-sensitive and not high-to-low sensitive like in all other Commodores. At that time, nothing remarkable was reported in the news.
A prototype run of twenty C2N232 units was produced before entering
mass production. At the time of writing (22nd of May 2002),
the first unit was assembled on a professionally manufactured circuit board.
The electronic components that were ordered weeks ago will probably arrive
in June. In July, Marko will be able to deliver the first devices to eager