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9-track tape

From Alfawiki
9-track tape
Company typeData storage format
IndustryComputer data storage
PredecessorIBM 7-track tape
Founded1964
ProductsMagnetic tape data storage media

9-track tape refers to a magnetic tape data storage format that was launched alongside the IBM System/360 in 1964. This format utilizes wide tape and reels similar in size to the earlier IBM 7-track, but it incorporates eight data tracks plus an additional parity track, summing up to nine parallel tracks. Data is encoded as 8-bit characters spanning the entire tape width, including parity bits. Over time, recording techniques such as PE (phase encoding), GCR (group-coded recording), and NRZI (non-return-to-zero, inverted) were developed to enhance both speed and storage density. Tape lengths can extend up to .

The System/360 and its use of nine-track tape played a crucial role in standardizing the byte size at eight bits.

This format was the predominant choice for offline data storage and transfer for more than thirty years. Nevertheless, it became outdated by the end of the 20th century, with production of tapes ceasing in early 2002 and manufacturing of drives ending shortly thereafter.

Typical operation

A standard 9-track tape drive consists of a tape transport mechanism—responsible for moving the tape between reels past read/write and erase heads—and associated control electronics. The transport system includes motors for supply and take-up reels, reel hubs, a capstan motor that ensures precise tape movement, a tape head assembly, guiding rollers to maintain accurate tape alignment, and vacuum columns designed to protect the tape from damage. Maintaining consistent low tension is essential to prevent data loss caused by tape stretching or speed fluctuations.

Loading a tape involves removing a protective "tape seal belt," mounting the reel, threading the leader through guiding rollers to the take-up reel, and then initiating an automatic sequence that closes the protective window, activates vacuum systems, and advances the tape until it detects the beginning-of-tape (BOT). At this point, the drive signals its readiness to the host computer.

Tape motion control relies heavily on the capstan motor regulated by feedback from an optical tachometer to maintain smooth and steady velocity. Ramp generators facilitate controlled starting and stopping of the tape movement to preserve proper spacing between data blocks.

Vacuum columns serve as buffers between the precise capstan-driven movement and the reel motors by holding small loops of tape under low tension. Sensors—either optical or vacuum-based—monitor these loops to adjust reel motor speeds accordingly and detect any feed errors, which can trigger an emergency stop to avoid damage. Due to this vacuum tension system combined with careful tape path design, pinch rollers are frequently unnecessary.

Tape drives support bidirectional motion; they can read data moving either forward or backward as commanded. During reverse movement, the supply-side vacuum column maintains tension to prevent tape bunching. Unlike audio tape players, capstan and head assemblies remain engaged during fast forward or rewind operations, only disengaging during rapid rewind sequences. Some models provide a "fast search" function that enables rapid movement between data blocks before slowing down for normal reading.

Tapes are equipped with an end-of-tape (EOT) foil strip that alerts software when reached during writing operations so data finalization can occur before running out of media.

Detection of BOT and EOT is accomplished using angled lamps that shine light onto these foil strips; reflected light activates sensors which halt tape motion. This detection method can cause false triggers when exposed to camera flashes near the drives.

While this overview describes a typical transport design, various manufacturers produced alternative configurations including horizontal deck models with automated threading or microprocessor-controlled direct drive systems without vacuum columns.

Technical details

9-track tapes operating at 800 bits per inch using NRZI encoding and 1600 bits per inch with PE (phase encoding) incorporate an inter-record gap (IRG) to facilitate stopping and starting between data segments. In contrast, 6250 bits per inch GCR tapes utilize a narrower IRG.

On 9-track tapes, reflective markers are placed on the non-data side at specific distances from the start and end points, indicating beginning-of-tape (BOT) and end-of-tape (EOT) positions. The lengths of the leader and trailer sections provide sufficient slack to accommodate tape movement around the reels and permit final data blocks beyond the EOT marker in multi-volume sets. Operators may trim damaged leader tape; however, if the BOT marker becomes detached, it impairs data retrieval, requiring replacement of the BOT label and potentially causing data loss.

The recording densities available for nine-track tapes include 800, 1600, and 6250 8-bit bytes per inch, which translate to approximate storage capacities of 22.5MB, 45MB, and 175MB respectively on a standard reel.

IBM generations

IBM Model 2400 Series 3400 Series
Model numbers 2401, 2415, 2420 3410, 3420, 3422, 3440
Density (bits/in/track) 800, 1600 800, 1600, 6250
Tape speed (in/s) 18.75–200 75–200
Transfer rate (byte/s) 15,000–320,000 1,250,000
Interblock gap (in) 0.6 0.3-0.6
Rewind speed (full reel) 1–4 minutes 51–180 seconds
Length of reel (ft) 2400 max 2400 max

2400 Series

Debuting alongside the System/360, the IBM 2400 Series marked the introduction of the nine-track tape format. These tapes and reels maintained compatibility in size with earlier IBM 7-track devices such as the IBM 729, although legacy tapes necessitated specialized drives equipped with compatibility modes for reading and writing.

3400 Series

Released with IBM's System/370 platform, the 3400 Series provided enhanced data density capabilities reaching up to 6250 bits per inch. This series also introduced the "autoloader" cartridge feature—initially seen on the IBM 2420 model 7—which removed the requirement for a plastic "tape seal belt." This advancement enabled operators to mount reels directly onto hubs without removing seals or manually threading tape paths, thereby reducing setup durations and operator errors while boosting overall operational efficiency.

Other IBM units

Certain IBM tape drives like the model 8809 from the 1980s employ flat mounting systems without vacuum columns and demand manual tape loading and threading. Supporting densities of 800 and 1600 bits per inch, this drive is compatible with the IBM System/36. For IBM's AS/400 and iSeries systems, the tabletop-mounted model 9348-012 features flat mounting with autoloading and automatic threading capabilities for reels, supporting both 1600 and 6250 bits per inch densities.

Other information

A full reel measuring approximately 2400 feet recorded at a density of 6250 bits per inch using block sizes up to 32,767 bytes can store as much as 170 megabytes. Utilizing smaller blocks such as those around four kilobytes lowers capacity to roughly 113 megabytes. Tape formatting is commonly done in either EBCDIC for IBM environments or ASCII; tapes may be configured as "labeled" with headers, "unlabeled," or contain "non-standard labels," depending on system compatibility.

Data is generally written in blocks separated by interblock gaps ranging from about five-eighths to three-quarters of an inch. Reducing these gaps enhances storage efficiency and increases read/write throughput; however, it also raises the risk that corruption within one block could result in loss of multiple records simultaneously.

Standards

  • ANSI INCITS 40-1993 (R2003): Unrecorded Magnetic Tape for Data Exchange, featuring 9-track tape with 800 cpi NRZI, 1600 cpi PE, and 6250 cpi GCR encoding methods
  • ISO/IEC 1863:1990: 9-track magnetic tape, 12.7 mm (½ inch) wide, employing NRZ1 encoding at 800 cpi or flux transitions per inch
  • ISO/IEC 3788:1990: 9-track tape of 12.7 mm width utilizing phase encoding at rates up to 1600 cpi
  • ANSI INCITS 54-1986 (R2002): Recorded Magnetic Tape for Data Interchange with 6250 cpi GCR encoding
  • ANSI INCITS 27-1987 (R2003): Specifications for Magnetic Tape Labels and File Structure in Data Interchange
  • ECMA 62: Standards for data interchange on 12.7 mm, 9-track magnetic tape

Other drive manufacturers

Media manufacturers

  • 3M (later spun off as Imation) — original innovator and primary producer of 9-track magnetic tape.
  • Graham Magnetics — noted as the final manufacturer producing new 9-track tape as of 2001.

External links

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