A shared connector for many devices
SIO is a 13-pin connector and a set of communication signals. Typical devices have two sockets for daisy-chaining. To communicate with a drive, the computer sends a command with a device identifier; the addressed drive acknowledges it and transfers data or the result. Several devices can thus share the connection. SIO is also the system routine handling this communication. In BASIC, C:, D: and P: provide access to cassette recorders, disk drives and printers respectively.
Standard cassettes: data recorded as sound
Standard tape recording represents data with two frequencies at a nominal 600 baud. Recordings are divided into records with gaps, so loading time depends on more than program size. The recorder converts the tape signal and works with POKEY's serial input. MOTOR lets the computer control tape movement. After PLAY is pressed, the tape starts only when the computer begins reading.
BASIC programs and boot tapes
CSAVE stores a BASIC program in the standard cassette format read by CLOAD. A boot tape instead contains a program loaded during computer startup, without first starting BASIC. On XL/XE, OPTION can disable built-in BASIC when a program needs its address space.
Turbo systems and recording copies
Turbo speeds up reading through different encoding, often with extra recorder circuitry and a separate loader. Reading requires hardware and software compatible with that turbo system. An emulator must reproduce the system; simply increasing emulation speed is insufficient.
A complete WAV or FLAC recording is useful for preserving an old tape. It allows decoding to be retried if CAS conversion omits a section or misreads data. CAS makes loading easier, but the audio recording is also worth keeping for preservation.