IBM cassette tape
| Industry | Computer hardware |
|---|---|
| Founded | June 16, 1911 in Endicott, New York, United States |
| Headquarters | Armonk, New York , United States |
The initial IBM Personal Computer and IBM PCjr models supported data storage through compact cassette tape. This storage medium was widely used among home computers such as the Apple II, Commodore 64, and BBC Micro, where cassette tapes served as a more affordable alternative to floppy disk drives. During the 1980s, numerous commercial software titles for home computers were distributed on tape.
Despite this, the IBM PC's cassette format did not gain popularity, mainly because most systems were equipped with at least one floppy disk drive. Beyond a single diagnostic tape provided by IBM, the only notable software available on cassette was IBM Typing Tutor developed by Microsoft. The subsequent PC XT discontinued the cassette interface entirely. Even with minimal demand, IBM continued to offer a Model 104 variant without a floppy drive until the original PC line was phased out in 1987.
Sales of the IBM PCjr rarely occurred without a floppy disk drive; however, it included two ROM cartridge slots for commercial software, enhancing user convenience and reliability.
Use
When operating solely with an external cassette recorder, an IBM PC relies on built-in ROM BASIC as its operating system, which natively supports cassette functions. In contrast, IBM PC DOS does not inherently support cassette operations, though third-party software solutions are available.
Control of the cassette motor and data read/write processes is facilitated by BIOS interrupt call 15h. Data is organized in 256-byte segments accompanied by a 2-byte CRC. Programmers can manipulate the cassette relay through its designated I/O address. Versions of IBM BASIC—including cassette, disk, advanced, and cartridge editions—feature commands for cassette use but require a physical cassette port.
Transfer speeds for cassette data range between 1 and 2 kilobits per second, which is significantly slower compared to disk drives that operate around 250 kilobits per second.
In 2020, the cassette interfaces of the IBM PC and PCjr were utilized to boot FreeDOS from a specially crafted vinyl disc.
Data format
The WRITE-BLOCK routine on the IBM PC 5150 activates the cassette motor and converts each byte into individual bits: a (1) bit corresponds to , while a (0) bit corresponds to , resulting in a specific recording speed of .
The routine begins by writing the first of , followed by a synchronization bit set to and a sync byte . Data is transmitted in blocks of 256 bytes with an appended 2-byte CRC until all data has been recorded.
Connector pinout
The cassette port on the IBM PC employs a female 5-pin DIN connector, identical in design to the keyboard connector:
Pin configuration:
- Pin 1: MOTOR CONTROL COMMON
- Pin 2: GND
- Pin 3: MOTOR CONTROL RELAY (6 V/1 A)
- Pin 4: DATA-IN (500 nA at 13 V with transmission rates between 1000–2000 Baud)
- Pin 5: DATA-OUT (250 μA, configurable jumper at either 0.68 V ("AUX") or 75 mV ("MIC"))
Motor control is executed through Intel 8255 port , bit : setting this bit to activates the motor, while clearing it turns it off.