A long long time ago I did real-time programming for a high energy physics research laboratory, KMS Fusion, located in Ann Arbor, Michigan. I programmed a Digital Equipment Corporation (DEC) PDP-11/45, and I knew all the assembly instructions required to process an interrupt from the backplane to my code (eventually I wrote a multi-device driver for our CAMAC data collection hardware, but that might be another story).

We decided to replace the PDP-11/45 with a brand new PDP-11/84; when the computer arrived on our shipping floor, it was serial number 3 (as in three, yes) and had a preliminary installation and operations guide with a simple brown wrapping paper cover. And a few days before installation we learned that the floating point processor (FPP) was on engineering hold; luckily the firmware had support for long integer math and floating point operations, and we did very few floating point operations (it being a real-time system after all). That instilled lots of confidence in me.

The DEC field service tech came out and installed the system and it ran fine.The floating point unit followed a short time later and was installed.

Then one day that summer a thunderstorm rolled through and the power went off.  The PDP rebooted. My boss was livid – what about the battery backup? He had neglected to ask for that when we prepared the order… So I ordered one post-haste and it arrived in a few days along with another field technician to do the installation. He put the battery backup in the rack, wired it all up, and said we were ready to test it. I asked how we’d do that, and he pulled the plug on the computer.  When he plugged it back in the computer rebooted – whoops.

I started reading the preliminary manual, and it showed an option we needed to set in the bootup firmware (like the BIOS on PCs) to enable battery backup. We power cycled the computer while holding down the magic key sequence, got the options screen, but couldn’t find any of the options that controlled how the computer booted up. More digging  around that brown-covered bible led to a switch in back; that switch enabled “operator mode” so an operator couldn’t reset certain options when booting the computer. So we flipped that switch; alas, we still missed the magic setup options.

Now the field tech was getting worried – he called Colorado Springs, the support center for all things DEC, and he started trying things they suggested. After a couple hours and no progress, I went in the computer room and started playing with the console.

At one point I got the options screen up and then pressed Control-S followed by the QWERTY keys backwards while holding down the control key. I got a huge surprise: a prompt labeled with the chip set name (KJ11 something-or-other). I stared at that for a good minute and then hit return – all the setup options appeared! I tried it again (good thing I remembered what I’d done), and it worked!

So I turned off the computer, went to the door and asked the DEC field tech to put the Colorado Springs folks on hold, I had something to show him.

I turned on the computer and got to the options screen. I pressed the control key sequence and said, “Presto chango” and the chip set prompt appeared. “What’s that,” the field tech asked, and I told him, “Just wait, there’s more.” As I hit the return key I said, “Abracadabra!” When he saw the options we needed appear, his eyes got wide and he asked, “How did you do that?”

“Its magic.”

And the DEC field techs called me a wizard ever after.

Once the folks at Colorado Springs heard, they figured out what had happened. It turns out the tech that installed the floating point unit accidentally fried the chip that stores the option settings. One more visit from a field service tech and we had a fully functional PDP-11/84 with battery backup.