Building a PDP-11 from Parts?

Who knows a lot about PDP-11 hardware? What are the minimum list of parts needed to build a functioning PDP-11? And can you boot off of a SCSI disk?

This depends somewhat on what you mean by “functioning”. If you just want something that runs the PDP-11 instruction set, you can get an LSI-11 or Soviet clone, wire it to some latches and SRAM, and have a SBC in a dozen or two chips. If your want to be able to run existing PDP-11 software, it’s going to take a lot more than that.

PDP-11 devices tend to be relatively smart, using relatively high level commands written to registers mapped into memory, and thus are somewhat complex. Some, like the console device, would map in a straightforward fashion to a typical UART chip, but disk controllers and the like would require more work.

You can easily emulate a PDP-11 using software such as SIMH, and there are hardware emulators which emulate down to the actual bus interface, such as the UNIBONE or QBONE.

I think the answer to your question depends quite a not on what you’re ultimately trying to accomplish.

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Hi and thanks for responding. I have booted BSD 2.11 on SIMH but want to try PDP-11 hardware now. I would like to start with a QBUS and add cards. I could start with assembly bootstrap but would eventually like to boot RT11 from a floppy, and later something from a disk.

That is going to be difficult to build from parts, depending on what you mean by “from parts”. Booting RT-11 from a floppy is a very achievable goal with a random PDP-11 that you might find, if you keep your eyes open; RT-11 will run on almost any configuration. Small machines w/out lights and switches front panels are not always particularly expensive (like the 11/03 or 11/23), and RX01/RX02 drives and controllers are pretty easy to find.

If you mean finding QBUS backplane, a QBUS CPU card, a QBUS disk controller, and a QBUS memory card, and building a machine that way, you can absolutely do it, but it will probably cost you more than just keeping your eyes open and buying a machine. In particular, small PDP-11s often sell based more on where the buyer is willing to collect them from than anything else. (Larger machines have buyers who are willing to travel long distances.)

Another option is to find a QBUS backplane and put a QBONE in it; that will allow you to drive “real” PDP-11 hardware with an emulated CPU. I don’t think it’s particularly cheaper than a small QBUS machine, but it will emulate a much larger QBUS machine (including something capable of running 2.11 BSD) if you wanted to. It would represent a middle ground between a “real” PDP-11 and a pure emulation.

There are plenty of QBUS cards on eBay but the complete PDP-11 systems are listed for several thousand $. For QBUS you need the QBUS itself, the power supply, and a terminator, correct? Do you also need a rack? For cards, you need a QBUS CPU, QBUS disk/floppy controller, QBUS RAM, and QBUS I/O? I/O does not come as part of the other cards, correct? Thanks!

A minimal system would be:

  • A power supply
  • A backplane (there are several configurations
  • A CPU
  • Memory

For a small backplane, you can get by without a terminator card. Many CPU cards have a console port built in. I think there may be some with a small amount of memory, as well.

You may also need or want:

  • A floppy disk controller
  • A terminal interface
  • Other disk controllers

It is true that you’ll see systems listed for several thousand dollars on eBay, but a small (64 to 256 kB, or 32 to 128 kW, depending on who is counting) QBUS machine can be had for much less than that if you watch carefully. Check more local exchange and marketplace sites and join some PDP-11 groups, and you’re likely to turn one up for a few hundred dollars, not a few thousand. UNIBUS systems, systems with blinkenlights consoles, and any system with > 18 bit addressing is going to be more expensive. Non-floppy drives get expensive fast.

Shipping is a huge problem for these machines, so don’t underestimate the “I’m local and can pick it up” potential. :wink:

Also often big IRON requires big power. Ebay now and then often has something PDP 11 ish
from the USSR. Rare USSR Soviet Ceramic Electronica Mk1red3 PDP-11 Chipset CPU - The Tetris CPU not working or for parts only Ben.

None of the QBUS PDP-11s even need a 20 amp outlet. The PDP-11/70 is often wired for three phase, but it just uses it as three independent single phase 120 V power circuits, in the US versions (as far as I know, at least). An 11/70 is expensive and hard to find, though, and not QBUS.