In our first issue we discussed back up devices for stand-alone computer systems. We had contacted a number of libraries that had purchased an electronic back up system for the purpose of recording transactions while their primary system was not available. Usually the choice was one of three types: A tape cassette unit, a bubble memory device, or a portable terminal. We also described the somewhat newer CTI system which uses an Apple micro-computer and diskettes. A library has recently asked us why we did not discuss the relative merits of cassettes and diskettes for recording information. That hadn't occurred to us, but the following brief summary may be of value.
The concept of a tape cassette unit is simply one of recording each transaction at a terminal on a cassette tape for later entry into the computer, rather than transmitting the information to the computer immediately. Several cassettes may be filled during a protracted period of down time. Cassette capacities range from about 250,000 to 2 million characters for a standard Philips-type unit--comparable to a diskette. New cassettes with capacities up to 4 million characters will shortly become available. This increased storage capacity will be one advantage of cassettes over diskettes. Cassettes also usually cost 40 percent less than diskettes. Furthermore, the plastic package in which the cassette tape is protected means that dust, dirt and fingerprints are less of a problem than with diskettes.
On the other hand, diskettes have random access capability. One is, therefore, able to do more than sequential bogging of transactions; it is possible to search the diskette for specific information. This capacity is normally not used, however, because the number of back up terminals in most libraries is usually too small to do anything more than record transactions. If a library were to purchase enough back-up units to make utilization of this feature practical, the diskette might then be a superior choice.
The subsequent "batch loading" of the data that has been collected into the library's automated information system will require an appropriate input device. Libraries that seek to switch from one back-up system to another and wish to know how much this switch will cost, should determine the cost of the input device as well as the cost of the back-up units.