Tarbell Cassette Interface
A fast, reliable S-100 bus cassette interface from 1976.
The Tarbell Cassette Interface, first advertised in 1976, is an expansion card designed for the Altair 8800 and other computers using its S-100 bus. Created by Don Tarbell and sold through Tarbell Electronics, it gained a reputation for speed, reliability, and popularity, eventually becoming a de facto standard for storing data on compact cassettes.
Its recording format uses Manchester encoding: a "1" bit becomes "01" on tape, and a "0" becomes "10." This is done by loading a byte into a shift register triggered by a 555 timer. While the system can operate at any audible frequency, it is normally set to 3,000 Hz. Because each data bit requires two output bits, the effective data rate is 1,500 bits per second (187 bytes per second). When reading, clock recovery adjusts for tape stretch and other variations, allowing data to be recorded at any hardware-supported rate; the manual suggests speeds up to 560 bytes per second.
When the interface launched in 1976, the 300 bits per second Kansas City standard was also in use. Tarbell later modified the system to read those tapes. This modification raises the main oscillator from 3,000 Hz to 4,800 Hz. It writes "00000000" (on tape, "1010" repeated four times) for a "1," or "01010101" ("0110" repeated four times) for a "0." Since the system halves the basic rate and uses eight output bits per data bit, this yields the standard 300 bits per second of the Kansas City standard.
Tarbell Electronics also sold other products, including TARBELL CASSETTE BASIC in 1978 and a dual disk drive subsystem compatible with Shugart Associates drives. That subsystem includes a Tarbell floppy disk interface, introduced in 1979, which plugs into any S-100 bus computer.
- Designer
- Don Tarbell
- Manufacturer
- Tarbell Electronics
- Introduced
- 1976
- Encoding
- Manchester encoding (1 bit output as '01', 0 bit as '10')
- Default frequency
- 3,000 Hz
- Effective data rate
- 1,500 bits per second (187 bytes per second)
- Maximum suggested rate
- 560 bytes per second
Lore & Background
The Tarbell Cassette Interface was advertised in 1976 as an expansion card for the Altair 8800 and other S-100 bus systems. Its encoding scheme is essentially Manchester encoding, where a '1' bit in the original data is output as '01', and a '0' bit is output as '10'. This is accomplished by reading a byte into a shift register periodically triggered by a 555 timer. Although the system can be run at any audible frequency, it is normally set to 3,000 Hz. As each data bit requires two output bits, the effective data rate is 1,500 bits per second, or 187 bytes per second. On reading, the system uses clock recovery to calculate the original data rate to account for tape stretch and other effects, meaning data can be recorded at any hardware-supported rate, with the manual suggesting up to 560 bytes per second.
At the time of its release in 1976, the 300 bits per second Kansas City standard was also in use. Tarbell later modified the system to support these tapes. This modification increases the main oscillator from its normal 3,000 Hz to 4,800 Hz, and then writes either 00000000 (on tape, '1010' ×4) to produce a single '1', or 01010101 ('0110' ×4) to produce a '0'. Because the system divides the basic rate by two, and each bit requires eight outputs, this produces the standard 300 bits per second output of the Kansas City standard.
Reader's Guide
The Tarbell Cassette Interface's significance lies in its role as a de facto standard for compact cassette data storage on S-100 bus systems, particularly the Altair 8800. Its design by Don Tarbell and sale through Tarbell Electronics starting in 1976 provided a fast and reliable storage solution at a time when the 300 bits per second Kansas City standard was also in use. The interface's ability to run at any audible frequency and its use of clock recovery for reading allowed flexibility in data rates, with the manual suggesting up to 560 bytes per second. Its popularity helped establish Tarbell as a household word in the S-100 bus community. The interface's legacy includes its later modification to support Kansas City standard tapes, demonstrating adaptability. Tarbell Electronics continued to build on this foundation, later releasing TARBELL CASSETTE BASIC in 1978 and a Shugart Associates-compatible dual disk drive subsystem with a Tarbell floppy disk interface introduced in 1979, which plugged into any S-100 bus computer.
Did You Know?
- It uses Manchester encoding where a '1' bit is output as '01' and a '0' bit as '10'.
- Tarbell later modified the interface to support the 300 bits per second Kansas City standard.
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