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