Eventually we had four of them. It's about 5 feet high, so you can get an idea of its size, and that doesn't even include the disks, tapes, and other peripherals (still, it was tiny compared to the IBM 360/91 in the next room). It requires three-phase power and isolated true-earth ground, raised floor, massive amounts of air conditioning, and weekly or biweekly "preventive maintenance." It has two million 36-bit words of main memory and accommodates 30 simultaneous users comfortably, and up to about 100 with increasing discomfort.
As you can see, there is not much in the way of lights and switches. But inside the main cabinet, not visible in the top picture, is the PDP-11/40 front end (used mainly to interface terminals and printers), which has a more entertaining front panel done in shades of blue:
The switches labled 15 through 0 could be used to load 0's or 1's into the 16-bit switch register, which could then be deposited into PDP-11 (not PDP-10) memory by using the the other switches (not that this was done commonly on the DEC-20; earlier computers, however, were booted by "toggling in" the bootstrap program in a like manner).
This shows the old VT52 terminal we kept on top of CU20A to display the current load average (the higher the number, the slower the computer; sometimes it got up past 100, where it would take several minutes for each keystroke to echo). "Load average" means the average ratio of working set to balance set over some period of time, like 1, 5, or 10 minutes.
Here's a full view of a small DEC-20 system from the vendor literature:
From left to right: An LP20F line printer, two TU45 tape drives, then the DEC-20 itself (front-end cabinet with serial line interfaces; disk/tape interface and memory cabinet; KL10 CPU cabinet), a CD20 card reader (ours didn't have any of these), and two RP04 disk drives. The terminals are a VT52 video terminal (DEC's first video terminal that had both upper AND lowercase letters) and an LA36 DECwriter hardcopy terminal. Between them is an RP04 3330 disk pack (92MB).
