IBM cassette tape
It was common for users of home computers of the time, such as the Apple II, Commodore 64 and BBC Micro, to use cassette tapes for storage if they could not afford a floppy disk drive, and in many cases there was a wide range of commercial software available on tape. This, however, was not the case with the IBM PC – very few were shipped without at least one floppy disk drive, and apart from one diagnostic tape available from IBM, there seems never to have been software sold on tape. An IBM PC with just an external cassette recorder for storage could only use the built-in ROM BASIC as their operating system, which supported cassette operations. IBM PC DOS had no support for cassette tape.
The IBM PCjr was also seldomly sold without a floppy disk drive, but it also had two ROM cartridge slots on the front of the unit – a much more convenient and reliable option for loading software.
No market developed for commercial product distribution on cassette tape and the cassette interface was dropped on the subsequent IBM XT.
Using from software
BIOS interrupt call 15h routines were documented in the technical reference manual that would turn the cassette motor on and off, and read or write data. Data was written with a lead-in section, and formatted in 256-byte blocks with a 2-byte CRC. Programmers could also operate the cassette relay by writing to its I/O address. The cassette, disk, and advanced versions of IBM BASIC included statements for cassette operations, but these features only worked if the machine had a cassette port.
The data transfer speed was from 1000 to 2000 bits per second, compared to the disk drive's 250 kilobits per second. The technical reference for the IBM PC 5150 specifies that the WRITE-BLOCK routine turns on the cassette drive motor and transforms each byte into bits. A (1) bit corresponds to a 1.0 ms timer period, (0) bit corresponds to 0.5 ms which results in a recording speed of 1000 - 2000 bit/s. First 256 bytes of "11111111" is written. One synchronization bit "0". A synchronization byte of 0x16. 256-byte blocks of data and a 2-byte CRC is written until all data is transferred.
The IBM PC used a female 5-pin DIN connector (the same as the keyboard connector) for the cassette port:
- Pin 1: MOTOR CONTROL COMMON
- Pin 2: GND
- Pin 3: MOTOR CONTROL (6 V/1 A) RELAY
- Pin 4: DATA-IN (500 nA with 13 V at 1000-2000 Baud)
- Pin 5: DATA-OUT (250 µA jumperable either at 0.68 V ("AUX") or 75 mV ("MIC"))
Motor control: 8255A port 0x61, bit 3: 0 = on, 1 = off.
- Williams, Gregg (January 1982). "A Closer Look at the IBM Personal Computer". BYTE. p. 36. Retrieved 19 October 2013.
- "Diagnostics for IBM Personal Computer 1.02 (Cassette)"
- "IBM PC and PCjr Cassette Waveforms". 091207
- IBM (1983). IBM PC Technical Reference Handbook. Comment: Also includes a complete listing of the ROM BIOS source code and schematics of the original IBM PC.