180 BEGINNING OF THE MINICOMPUTER

Figure 7. The PDP-8.

In a fashion similar to the technical developments that marked the 18-bit family, the new 12-bit machine was physically smaller than its predecessor. This time, however, the change was more than simply a change from three cabinets to two or from two cabinets to one. It was a change from one cabinet to a half cabinet. The new small size meant that the PDP-8 was the first true minicomputer. It could be placed on top of a lab bench or built into equipment. It was this latter property that was the most important, as it laid the groundwork for the original equipment manufacturer (OEM) purchase of computers to be integrated into total systems sold by the OEM.

The improvements in logic density permitted by the new Flip Chip modules also influenced packaging and manufacturing methods. The PDP-8 logic modules were mounted in connector blocks, which were in turn mounted in frames. The two frames were each the maximum size that could be accommodated in the new Gardner-Denver automatic Wire-wrap machine. Automatic wire-wrapping was very important to the mass production success of the PDP-8 because it was both fast and accurate. The two wire-wrapped frames hung vertically and were hinged about a vertical axis at the rear of the computer cabinet. In some ways they resembled the pages of a book, with the wire- wrap pins on the surfaces that faced each other. The swinging gate backplane permitted access by maintenance personnel to both the connection pins and the modules.

Like its predecessor the PDP-5, the PDP-8 was a single-address 12-bit computer designed for task environments with minimum arithmetic computing and small primary memory requirements. Typical of these environments were process control applications and laboratory applications such as controlling pulse height analyzers and spectrum analyzers.

In addition to the originally envisioned applications, the PDP-8 was used for innumerable other applications. One of the most interesting was message switching. The PDP-8 message switching hardware assembled characters by bit sampling, checking the status of teleprinter lines at 5 times the anticipated bit rate to accurately recover data. Another interesting application was the TSS/8 small-scale general purpose timesharing system developed by Carnegie-Mellon University and DEC [van de Goor et a!., 1969]. While only a hundred or so systems were sold, TSS/8* was significant because it es-

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*TSS/8 was designed at Carnegie-Mellon University with graduate student Adrian van de Goor, in reaction to the cost, performance, reliability, and complexity of IBM's TSS/360 (for their Model 67). Although the TSS/360 was not marketed. it eventually worked and contributed some ideas and trained thousands for IBM. At Carnegie-Mellon (CMU), a TSS/8 operated until 1974 when the special swapping disk expired. The cost per user or per job tended to be about 1/20 of the TSS/360 system CMU ran.