The StorageTek 4400 (known familiarly as the STK 4400) which was delivered to the Atlas Centre in November 1989 advanced the automated handling of data in two ways. First, it had an order of magnitude greater storage capacity than the Masstor M860 that was in use at the time and second, its storage medium was the IBM 3480 18-track tape cartridge which was by then becoming an industry standard. An input/output port enabled the insertion and extraction of users' own 3480s which might be brought in from elsewhere.
The heart of the device was an annulus-shaped box 8 feet high and 11 feet in diameter, known as a silo. The inner and outer walls of the silo held racking which could accommodate up to 5,500 cartridges, each of which could hold 200MB of data, providing a total capacity of over 1 TB. At the centre of the silo was an arm mounted on a pivot, and the end of the arm could move between the inner and outer walls. At the end of the arm were two cameras, one pointing at at each wall, for reading the barcoded information that identified the individual tape cartridges. The arm also had two hands for picking and replacing cartridges. Cartridges picked by the arm were moved to a set of eight tape drives; access time to mount and thread a cartridge was about 20 seconds and up to a further 30 seconds might be taken if the data to be read happened to towards the end of the tape. In practice the system was capable of mounting and dismounting well over 100 cartridges per hour.
The movements of the robot were controlled by software running under VM/XA on the IBM 3090. The drives themselves were interfaced to IBM's MVS, CMS and the Cray operating systems COS and Unicos through software developed in house, so that requests for cartridge mounts from any of the operating systems could be handled automatically.
A second silo was added in July 1992, by which time longer and higher density cartridge tapes had also become available, and the total capacity, if such tapes were used, had risen to about 4 TB. By this time the StorageTek facility played a key role in the Atlas Data Store which had been gradually coming into service during the past few years.
