9-track tape

Data is stored as 8-bit characters, spanning the full width of the tape (including the parity bit).

Various recording methods have been employed during its lifetime as tape speed and data density increased, including PE (phase encoding), GCR (group-coded recording), and NRZI (non-return-to-zero, inverted, sometimes pronounced "nur-zee").

The operator then initiates an automatic sequence, often by a single press of a button, that closes the protective window, starts the vacuum system, then moves the tape forward until the beginning-of-tape (BOT) foil strip is detected by an optical sensor in the tape path.

The outer two sensors, at the very top and bottom of the columns, serve to sense malfunctions in the feed mechanism during operation, prompting the control electronics to shut off all operation of the tape transport and vacuum system to prevent damaging the tape.

Because the supply vacuum column keeps a small, constant tension in the reverse direction, the capstan can feed backwards without the tape bunching up or jumping out of its path.

The sensing of BOT and EOT is achieved by shining a small lamp at the tape's surface at an oblique angle.

For example, some designs use a horizontal transport deck where the operator simply sets the tape reel in the supply reel bay, closes the door and presses the load button, then a vacuum system draws the tape along the path and onto a take-up hub within the mechanism.

The primary advantages of the 3400 system are the higher data density (6250 BPI) and support of the "autoloader" cartridge, first seen in the IBM 2420 model 7.

The 3420's autoloader cartridge enables the tape operator to mount the reel directly on the hub without having to remove the seal belt.

On the IBM AS/400 and iSeries there is the 9348-012 and it is a table top drive, flat mounting, but it autoloads the tape reel and auto threads it.

The maximum data capacity of a 2400 ft reel, with 32,767 byte blocks and recorded at 6250 BPI is 170 megabytes.

Typically, much smaller block sizes, such as 4K (4,096 bytes) are used, in which case the storage capacity of the tape is reduced to 113 megabytes.

Between blocks, there is an interblock gap, which varies depending on the density, but is typically 5/8 to 3/4 of an inch long.