IBM PC data cassette

Revision as of 05:47, 20 January 2013

Yes, the IBM PC did have a cassette port at one point. Actually, only the original IBM Personal Computer model 5150, and the later PCjr (intended as a low-end home computer), had this; other PCs did away with this feature which was rarely used on this platform given that just about everybody got their PC with at least one disk drive. Apparently, though, IBM felt that when they made their entry into the PC field (after years of regarding small computers as "toys" unworthy of their attention) they needed to be sure their machine duplicated all the features present in other brands of personal computers around at the time, and most of them had cassette interfaces then because disk drives had been so expensive in recent history (and those computers were aimed at home/hobby markets with limited budgets). By the time of the IBM PC, disk drives had come down in price, while IBM's computers were aimed at a more upscale business/professional market, so cassettes saw little use there.

The format consisted of 1-millisecond-long pulses for each 1 bit, and 0.5-millisecond pulses for each 0 bit. The tape starts with a leader of 256 bytes of all 1 bits (hex FF), followed by a single synchronization bit (0), and then a synchronization byte (hex 16, the ASCII character from the C0 controls designated as SYN). The number of bytes in the data follows, followed by the data itself, grouped into 256-byte blocks (each followed by a 2-byte CRC).

References

• Reading IBM PC and PCjr Cassette Waveforms
• Discussion of IBM PC cassette interface
• Wikipedia article