The originalIBM Personal Computer andIBM PCjr includes support for storing data and programs oncompact cassette tape.[1]
It was common forhome computers of the time, such as theApple II,Commodore 64 andBBC Micro, to use cassette tapes for storage due to the lower cost of hardware and media compared to floppy disks. A wide range of commercial home computer software was available on tape throughout the 1980s.
BYTE asked in 1982, "I'm still looking for someone who uses [IBM cassette tape]. Did IBM seriously think its system would compete with theVIC-20 andZX81?"[2] The IBM PC cassette format was not popular since very few were shipped without at least onefloppy disk drive, and apart from one diagnostic tape available fromIBM,[3] there seems never to have been any software sold on tape except IBM Typing Tutor created byMicrosoft,[4] and the interface was not included on the follow-upPC XT. Despite this lack of popularity, up until the original PC's discontinuation in 1987, IBM continued to offer a Model 104 which shipped without afloppy disk drive.[5]
The IBM PCjr was also seldom sold without a floppy disk drive, but it also had twoROM cartridge slots for loading commercial software, which offered better convenience and reliability.
An IBM PC with just an external cassette recorder for storage can only use the built-inROM BASIC as itsoperating system, which supports cassette operations.IBM PC DOS has no native support for cassette tapes, though software can be written to provide support.
BIOS interrupt call 15h routines are documented in the technical reference manual that turns the cassette motor on and off, and read or write data. Data is written with a lead-in section, and formatted in 256-byte blocks with a 2-byteCRC.[6] Programmers can also operate the cassette relay by writing to itsI/O address. The cassette, disk, advanced, and cartridge versions ofIBM BASIC includes statements for cassette operations, but these features only work if the machine had a cassette port.
The data transfer speed is from 1-2kilobits per second, compared to the disk drive's 250 kilobits per second.[1]
In 2020, the cassette interfaces of the IBM PC and PCjr were utilized to bootFreeDOS off a customvinyl disc.[7][8]
The technical reference for theIBM PC 5150 specifies that the WRITE-BLOCK routine turns on the cassette drive motor and transforms each byte intobits. A (1) bit corresponds to a1.0ms timer period, (0) bit corresponds to0.5 ms, which results in a recording speed of1000 - 2000bit/s.
First256 bytes of11111111 is written. One synchronization bit0. A synchronization byte of0x16. 256-byte blocks of data and a 2-byteCRC is written until all data is transferred.[6]
The IBM PC uses a female 5-pinDIN connector (the same as the keyboard connector) for the cassette port:
Pinout:[9]
Motor control:8255A port0x61, bit3:0 = on,1 = off.
I have never encountered a PC program on tape for sale. In fact, about the only use of the cassette port that I am aware of is the homespun and jerry-rigged use of this port as a poor-man's serial port.