User:Electron9/IBM Cassette tape

IBM Cassette tape were included in the models 5150 (PC) and 4860 (PCjr) with a cassette port for connecting a cassette drive, and originally intended compact cassettes to become the 5150's most common storage medium. However, adoption of the floppy- and monitor-less configuration was low; few (if any) IBM PCs left the factory without a floppy disk drive installed. Also, DOS was not available on cassette tape, only on floppy disks (hence "Disk Operating System"). 5150s with just external cassette recorders for storage could only use the built-in ROM BASIC as their operating system. As DOS saw increasing adoption, the incompatibility of DOS programs with PCs that used only cassettes for storage made this configuration even less attractive. The ROM BIOS supported cassette operations.

IBM PC cassette recording signal

The technical reference for the IBM PC 5150 specify that the WRITE-BLOCK routine turns on the cassette drive motor and transforms each byte into bits where a (1) bit corresponds to a 1.0 ms timer period, (0) bit corresponds to 0.5 ms (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.