79 lines
3.4 KiB
Plaintext
79 lines
3.4 KiB
Plaintext
From: http://support.apple.com/kb/TA40730
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This note is about the cassette interface built into the Apple II and
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Apple II+, subroutines. An assumption made here is that the cassette
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recorder is in the proper mode, play or record, when the read and write
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routines are executed. Note also that the timing is approximate and may
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vary from one Apple to another.
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A record is a block of binary data. This data may be a BASIC or APPLESOFT
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program, a machine language program, or just binary data. Records representing
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BASIC or APPLESOFT programs are really two records, the length of the program
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and the actual program. A record consists of a header, synchronous bit, the
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actual data, and a checksum byte for error detection.
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Monitor record format
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+--------+-+-----------------------+-+
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| HEADER |S| DATA |C|
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+--------+-+-----------------------+-+
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BASIC program record format
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+--------+-+----+-+--------+-+----------------+-+
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| HEADER |S| LB |C| HEADER |S| PROGRAM |C|
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+--------+-+----+-+--------+-+----------------+-+
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Key: S = SYNC bit
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C = CHECKSUM byte
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LB = BASIC program length
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The header consists of 10 seconds of 770 Hz tone, (1 cycle equals 1300
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microseconds). This gives enough time for the cassette motor to attain speed
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and the plastic tape leader to go by. A subroutine called HEADR generates a
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shortened header between the BASIC length bytes and the BASIC program itself.
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The length of the header tone is controlled by the value of the accumulator on
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entry to the subroutine. This can vary from 0.2 seconds to 40 seconds. On
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entry the X register should be 0 and the carry flag should be set. HEADR also
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generates a synchronous bit at the end of the tone. HEADR resides at
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hexadecimal address $FCC9, or decimal address -882.
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The last cycle of header tone and SYNC bit
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---+ +-------------+ +-----+
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+-------------+ +----+ |
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| 1300 microseconds | 200 | 250 | header tone | synchronous bit |
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The synchronous bit, generated by HEADR, is one half cycle of 2500 Hz, (200
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microseconds) and one half cycle of 2000 Hz, (250 microseconds). It is used to
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signal the end of the header tone and the start of the data.
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The data is recorded on the tape with a low starting address and a high ending
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address. Each byte of data is shifted out most significant bit first, least
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significant bit last. A zero bit is made up of one cycle of 2 kHz, (250
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microseconds per half cycle) and a one bit is one cycle of 1 kHz, (500
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microseconds per half cycle). This works out to 2000 baud for zeros only and
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1000 baud for ones, or an average of 1500 baud.
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A zero bit and a one bit
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+-----+ +----------+ +
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+ +-----+ +----------+
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| 500 usec | 1000 usec |
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The checksum byte is written on the tape at the end of the data block. All
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during reading or writing each data byte is EXCLUSIVE OR-ed with the checksum
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byte. If the checksum computed during a read agrees with the checksum that was
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written out, then the data is probably good. This method will detect an odd
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number of errors for any of the eight bits of the byte.
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In writing data, the cassette output uses quite simple circuitry, a flip-flop
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connected through a voltage divider to the jack on the back panel of the
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Apple. Any time the address $C020 is accessed this flip-flop changes state.
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Accessing the flip-flop once every 500 microseconds generates a 1000 Hz
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tone.
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