DEC VT-78

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The minicomputers of the PDP8 family were redesigned 13 times in the course of 25 years between 1965 and 1990. The instruction set itself remained almost unchanged, the model was only adapted to the current semiconductor technology, with the effect that the current models, while keeping the instruction set, became smaller, faster and cheaper:

Model Year Pieces Technique PDP8 65-69 1450 Transistor PDP8/S 66-70 1024 Transistor PDP8/I 68-71 3698 TTL PDP8/L 68-71 3902 TTL PDP8/E 70-78 >10000 TTL MSI Omnibus PDP8/F 72-78? >10000 MSI Omnibus PDP8/M 72-78? >10000 MSI Omnibus PDP8/A 75-84? >10000 TTL LSI Omnibus VT-78 78-80 Intersil 6100 DECmate I 80-84 Harris 6120 DECmate II 82-86 Harris 6120 DECmate III 84-90 Harris 6120 DECmate III+ 85-90 Harris 6120

The VT-78 is thus the PDP8 in the next to last semiconductor technology. DEC had started by the middle of the 1970s to develop its own single-ship PDP8 that should have been used to realise the PDP8/B. When, in 1975 Intersil put on the market the IM6100 microprocessor (CMOS-8), DEC stopped the development and instead built a PDP8 compatible single-board computer into its VT-52 terminal. That product was then called VT-78.

Since the IM6100 has no MMU, and the instruction set of the PDP8 heavily relies on the non-volatile property of magnetic core memory, the construction of a PDP8 compatible computer that can run the operating system OS/8 requires a great amount of additional logic circuits.