What have I been up to?

Last summer I came across my somewhat tattered copy of the PDP-11 Processor Handbook. This dates to my first job as a software engineer in the early 1980s for EMC Controls. EMCC was a process control system manufacturer whose products ran everything from bakeries to steel mills to the Trans-Alaska Pipeline. A descendant runs the ventilation systems for some of Boston's road tunnels.

I got hired there because they used Intel 8080A microprocessors in their EMCON-D2 process loop controllers, and a friend of mine who worked there knew I knew a lot about the 8080A. My first day I was informed they didn't have an immediate need for work on the 8080A software, but would I like to do a sysgen on a PDP-11/44? I replied, "What's a 'sysgen'?" I never touched a single line of 8080A software in the two years I worked there, but became something of an of an expert on the RSX-11M-Plus operating system.

My first big project was the continuous annealing line for Bethlehem Steel in Burns Harbor, Indiana. It used a PDP-11/44 as a supervisory computer, which ran a mathematical model of the system to update process loop controllers. My second big project was porting the company's EMCON-D3 software to a VAX-11/750 running VAX/VMS. It was my knowledge of RSX and VMS gained at this job that guided my career through 1999. But that's a story for another time.

There are quite a few hobbyists who have build working PDP-11 systems from parts and systems acquired on eBay and other channels. This didn't appeal to me, as it would be a huge money sink and I already have central heating in my house. But, flipping through my old book, I wondered how hard it would be to recreate a PDP-11/44 in Verilog. The field engineering prints are now available online, and I started perusing them.

The PDP-11/44 was designed at a time when field-programmable logic devices (PLDs) like the Signetics 82S100 were becoming available for reasonable cost. The 11/44 uses 10 of these devices. While the programming of some of them could be inferred from the 11/44 schematics, some of them were more opaque. Fortunately DEC saw fit to install all of them in sockets, so if I could find a set of 11/44 CPU boards I could read their programming directly.

Buying a set of PDP-11/44 CPU boards solely to read the PLDs looked to be an expensive proposition, so I began hunting around for a set I could borrow. I found that the Rhode Island Computer Museum had two 11/44s, and I wrote to the museum curator asking if I could use theirs. They said they'd be happy to help, but suggest I contact the System Source Computer Museum in Hunt Valley, Maryland, as they were much closer to me.

System Source turned out to be located only a couple of miles from the former site of EMC Controls. Tours are welcome but by appointment only, so I booked a tour for the following week. My tour guide was Bob Roswell, one of the owners of the museum and a wealth of information.

After the tour, I asked Bob if I could speak with him privately. I explained my quest for access to a PDP-11/44 and he readily agreed to make theirs available. I then asked if they might be interested in volunteers to help with their restoration activities, as their goal is to have their museum artifacts running rather than static showpieces. He said they would love to have more volunteers.

On my next visit Bob took me to their warehouse to look for their operational PDP-11/44 system, which had not been on display for a while. We were unable to find it among new acquisitions (picture the end scene from "Raiders of the Lost Ark") but we did find two PDP-11/44 CPU chassis on a pallet wrapped tightly with pallet wrap. We relocated these chassis to a conference room, and I began disassembling them to inventory their contents. Disappointingly, neither contained any CPU boards. Instead, I found a variety of Unibus peripheral interface cards and, rather surprisingly, two FP-11 floating point processor boards which are a bit rare.

One of these cards laying loose amongst the others made me stop and stare in surprise:

It's a bit hard to make out here (and Blogger's image compression does us no favors), so here's a zoomed-in crop of the logo on the board:

That's the EMC Controls logo, the same company I worked for in the early 1980s. We affectionately referred to the two up-pointing arrows as the "Toilet Seat". Apparently this PDP-11/44 was sold to a local company by EMCC to run their manufacturing process, and had somehow made its way to the System Source Computer Museum after being removed from service. 

This isn't a Unibus card, or any other sort of DEC-related bus card, though. It's a Multibus card, which goes into an Intel-based machine. The EMCON-D3 software ran on Intel 8086 systems even when I worked there, so it may have been from one of them. I'm guessing it was tossed into the 11/44 chassis as it kinda fell into the same category of surplus process control electronics.

I contacted the friend of mine who got me the job at EMCC decades ago. She still works with the descendants of the EMCC systems, and I offered her the board as nostalgia. She replied that she had several of these in her testing and support lab and didn't need another. 😲

Later we located the operational PDP-11/44 system. I was provided with a table and a hand-operated pallet jack with which to move the PDP-11/44 system to where I could open it up. I have several photos of the boards as I worked with them, but the only one I'll bother sharing is this one of my homebuilt PLD reader copying a PLD.

I sent copies of the PLD images to 9track.net, but they don't appear to have been posted in the 7 months since.

As with so many hobby projects, my PDP-11/44 project has ground to a halt. It's not that I ran into an impassible impediment. I've just been too busy.

While talking with Bob Roswell, I asked him what sort of project I might do as a museum volunteer. Unsurprisingly, there are dozens of projects for a motivated volunteer with technical skills. Here's a list of a few of the ones I've tackled:

• Restoring the electrical wiring in a century-old IBM master clock. (The clock mechanism itself was restored by a professional horologist.) I believe this clock was designed to operate bells in an industrial facility to indicate the beginning and end of shifts and breaks. Some of the museum staff have come to call this "Reece's Clock".
• Restoring a Tempest-shielded Model 28 Teletype and adapting its electronics so it can communicate with ASCII devices over RS232. It works, but I need to replace a gear with a broken tooth.
• Testing a Mark-8 computer (a 1974 design based on an Intel 8008 µP and assembled in 1984), and designing an acrylic display case for it. If you were at this year's Vintage Computer Federation - East event you may have seen this on display.

In the nine months I've been volunteering at the museum, I've gone from asking for projects to getting specific requests from staff. About a month ago I got a text from the museum management asking when I'd be on site next. I've kept a pretty regular schedule of being on site two days a week, so this struck me as rather odd with a hint of urgency.

One of the museum's recent acquisitions is a CDC 160A computer system dating from 1961. It was restored by Paul Allen's Living Computer Museum, was acquired by the System Source museum this fall, and is now slated for display at the VCF West event August 1st & 2nd. After some bulb replacements and repairs to a relay in the console display, the CDC 160A CPU runs well. However, the two peripherals we have for it, a CDC 167 punched card reader and a CDC 161 typewriter, were in unknown condition. After removing some mouse nests, other volunteers had gotten the card reader to feed cards but it didn't appear to transfer data. I'd been asked to look at this but I'd just started to read about the system. The typewriter was being looked at by one of the museum staff.

It seems the typewriter is mechanically worn so badly it's unlikely to be repairable. Without the typewriter all we'd have would be a static display of a big desk with some switches, lights, and very loud fans. The reason I was asked when I'd be in next was that they want me to build something that would plug into the CDC 161's vintage electronics in place of the defective electro-mechanical typewriter. And it needs to be designed, built, tested and packaged by the beginning of July so the system can be trucked 2,900 miles (4,600 km) from Maryland to California.

More on this in future posts.