PDP-11/20

           <<<    >>>    ^^^

The oldest PDP-11 - it is even older than the LAB8/E in the museum.

In 1970, Digital Equipment launched a new computer series on the market: The PDP-11. Unlike all other machines from DEC, this series has a word length of 16 bits. Although, for the sake of consistency, the octal numbering system is still deployed. The first model was the PDP-11/20. It had 4 kwords of core memory and a frontpanel similar to the panel of the PDP8. The maximum amount of memory was 32 kwords without MMU and 128 kwords with the optional MMU (less the I/O area at the end of the addressing range).

The fate of this exemplar was much like that of Sam Hawkens' old leather jacket: Over the years it had been patched so much that almost nothing was left from the original. First, the 4k core memory was replaced with a 24k semiconductor memory. This is the Wire-Warp frame at the top of the rack. This improvement is also historical: the ICs are from 1973, the boards from 1976.

Then, the guys at the RWTH Aachen were fed up with the light bulbs and a capable mechanic replaced them by putting LEDs into the front panel. Unfortunately, the frontpanel cracked when drilling the holes.

When we recieved this machine, some switch handles were missing from the front panel. Due to the lack of matching spare parts, we had to take some brown handles from a spare PDP-8 front panel.

Because we wanted to attach the DECtape drive to the PDP-11/34, and the poor 11/20 was now without core memory (non-volatile) and without mass storage, we wrapped two boards with EPROMs (shame, shame!) in which we burned the original Paper-Tape-BASIC. If the machine is powered up, the BASIC can be loaded from the EPROM.

Closeup of the frontpanel

The crack is quite visible unfortunately.