DEC indicator panel

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RF11 indicator panel mounted, as usual, at the top of an H960 cabinet

Indicator panels, containing arrays of lights which provided information about the internal state of the machine, were provided by DEC for their earlier computers (such as the KA10), in common with the other computers of the day. Not only the CPU had such lights; other devices such as disk controllers also had their own indicator panels.

This tradition lasted through the early models of the PDP-8 and PDP-11, although the PDP-11 CPU light displays were modest compared to earlier CPUs. The earliest PDP-11 disk controllers were very much a match for the earlier machines, though, and included indicator panels which displayed a great deal of information.

Alas, the cost of such displays was high, and they were dropped from later controllers. (The only PDP-11 controllers with indicator panels were the RF11, the RP11, and the DX11; the RK11-C was wired for one, but DEC never even produced the 'inlay' - a clear plastic panel, with black paint with holes for the lights on the back, and white caption lettering on the front - for it. This was likely connected to the fact that these controllers were the only ones built out of a large number of small FLIP CHIPs in a large custom backplane.) This was a significant loss; in addition to helping with debugging (both hardware and software issues), they gave an extensive insight into how the machine was operating.


Indicator panel from a DX11

PDP-10 devices which used a 19 inch rack, along with the few types of PDP-10 main memory which came in such racks, used a standard indicator panel. The panel provided 4 rows of 36 lights each (although not all were used; unused lights were simply not wired up). Only the inlay was customized to the particular device.

The PDP-8 and PDP-11, although not 36-bit machines, also used the same indicator panel, but in general did not include any 36-bit fields. (The exception is the RP11, which was able to read packs written on a PDP-10, and thus included a 36-bit shift register, which was displayed on the RP11-C panel.)

The 18-bit PDP-15 also used this indicator panel.

The two groups differed in the white bezel; in the PDP-10's, the bezel is metal, and goes over the inlay; in the PDP-11's, the bezel is plastic, and the inlay is laid into the bezel.

Limited engineering drawings (they do not include the full mechanical drawings) for this indicator panel are included in the RF11 Engineering Drawings set; the indicator panel is shown on pp. 186-190. Of particular interest are the parts lists, which include all the inlays extant as of the date of the drawings (10/1972).

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