The original IBM Personal Computer and IBM PCjr includes support for storing data and programs on compact cassette tape. It was common for home computers of the time, such as the

Apple II Apple II ("apple Roman numerals, two", stylized as Apple ][) is a series of microcomputers manufactured by Apple Computer, Inc. from 1977 to 1993. The Apple II (original), original Apple II model, which gave the series its name, was designed ...

, Commodore 64 and BBC 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 The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...

'' asked in 1982, "I'm still looking for someone who uses BM cassette tape Did IBM seriously think its system would compete with the VIC-20 and ZX81?" The IBM PC cassette format was not popular since very few were shipped without at least one

floppy disk drive A floppy disk or floppy diskette (casually referred to as a floppy, a diskette, or a disk) is a type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a ...

, and apart from one diagnostic tape available from

IBM International Business Machines Corporation (using the trademark IBM), nicknamed Big Blue, is an American Multinational corporation, multinational technology company headquartered in Armonk, New York, and present in over 175 countries. It is ...

, there seems never to have been any software sold on tape except IBM Typing Tutor created by

Microsoft Microsoft Corporation is an American multinational corporation and technology company, technology conglomerate headquartered in Redmond, Washington. Founded in 1975, the company became influential in the History of personal computers#The ear ...

, and the interface was not included on the follow-up PC 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 a

floppy disk A floppy disk or floppy diskette (casually referred to as a floppy, a diskette, or a disk) is a type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a ...

drive. The IBM PCjr was also seldom sold without a floppy disk drive, but it also had two

ROM cartridge A ROM cartridge, usually referred to in context simply as a cartridge, cart, cassette, or card, is a replaceable part designed to be connected to a consumer electronics device such as a home computer, video game console or, to a lesser extent, ...

slots for loading commercial software, which offered better convenience and reliability.

Use

An IBM PC with just an external cassette recorder for storage can only use the built-in ROM BASIC as its

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

, 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 15 h 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 The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...

blocks with a 2-byte CRC. Programmers can also operate the cassette relay by writing to its I/O address. The cassette, disk, advanced, and cartridge versions of IBM BASIC includes statements for cassette operations, but these features only work if the machine had a cassette port. The data transfer speed is from 1-2 kilobits per second, compared to the disk drive's 250 kilobits per second. In 2020, the cassette interfaces of the IBM PC and PCjr were utilized to boot FreeDOS off a custom vinyl disc.

Data format

The technical reference for the specifies that the WRITE-BLOCK routine turns on the cassette drive motor and transforms each byte into bits. A (1) bit corresponds to a timer period, (0) bit corresponds to , which results in a recording speed of . First of is written. One synchronization bit . A synchronization byte of . 256-byte blocks of data and a 2-byte CRC is written until all data is transferred.

Connector pinout

The IBM PC uses a female 5-pin DIN connector (the same as the keyboard connector) for the cassette port: Pinout: * 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 , bit : = on, = off.

References

{{Magnetic tape data formats Home computer peripherals IBM storage devices Computer storage tape media

Use

Data format

Connector pinout

See also

References