| TIC TAPE STORAGE | | | | that part of the chamber. An alternative system to |
| Beginning in the 1950s, magnetic tape has progressed | | | | vacuum chambers used swinging mechanical arms. |
| from the original half inch wide ferrite coated tape on | | | | Improvements in magnetic coatings allowed improved |
| open reels 10inch diameter and 2400ft long to the | | | | heads with smaller gaps to record more densely. PE |
| current cassettes mounted on drives in stacks. | | | | or Phase enabled was the next standard at 1600 bits |
| The first drives recorded 7 bits across the tape, | | | | or characters per inch. GCR (Group Controlled |
| being 6 data bits + parity. This was increased to 9 | | | | Recording) was a high density recording system, and |
| bits to accommodate the 8 bit byte, the backing | | | | the last to be extensively used on open reel tapes. |
| medium was changed to mylar for strength, and | | | | A later development, removing the requirement to |
| allowed 4800 ft to be placed on the reel with thinner | | | | have rapid start-stop times, was streaming tape, |
| tape. The main change was in the recording systems. | | | | which writes or reads continuously. A disadvantage |
| The first universally adopted 'standard' was NRZ (Non | | | | of this system is an overshoot on an error condition. |
| Return to Zero) referring to the crossing from | | | | Time is then lost relocating the data on the tape. |
| negative to positive of the magnetisation at speeds | | | | Current tape drives take a stack of cassettes and |
| up to 800 BPI. | | | | record at high density, using compression techniques. |
| The initial recording systems had the tape starting | | | | This makes the tape suitable for its main purpose, |
| and stopping at high repetition rates, so that it was | | | | backup of important, indeed all, data. However it |
| necessary to provide a buffer to prevent the tape | | | | emphasises the main disadvantage of tape, the |
| being dragged of the reel, or spilled onto the floor! | | | | relatively long access time. |
| This was normally accomplished by having two large | | | | Tape backups also take the form of an 'audit trail' |
| chambers for the tape to run in, one between each | | | | used on systems where 'real time' transactions are |
| reel (supply and take-up) and the read write head | | | | taking place, for instance airline reservation systems. |
| next to the capstan (drive) assembly. | | | | These require a continual backup of the data being |
| Each reel was driven by a reversible dc servo motor, | | | | updated on the system disks to be written to the |
| which was controlled from sensors in the chambers | | | | tape, so that an up to the second recovery can be |
| indicating the chamber was filling or emptying, due to | | | | made in the event of a system failure. |
| the action of the capstan feeding the tape, and the | | | | Another development in tape handling has been the |
| reel taking it up or feeding it out. The tape was kept | | | | use of libraries to handle tape cartridges. These store |
| under tension by vacuum pumps in the chamber, and | | | | up to 5,000 cartridges, and are accessed by robots, |
| the sensors could be vacuum switches or photocells, | | | | and controlled by a dedicated computer system. |
| indicating the presence or absence of the tape in | | | | |