Building the PiDP-11 Dec PDP-11 Replica Minicomputer

[dboreham]

I'm trying to build a real pdp-11/70. Mentioning the project to people I work recently with I realized that pretty much nobody knows the significance of this machine now. So perhaps worthwhile mentioning that it's the computer that the developers of the later versions of Unix used (by later I mean V6,7). It's also the target machine for Unix prior to the 1980s. Unix on an '11 is basically the ancestor of all the systems we use today. e.g. you could sit down at a terminal and you'd have familiar commands like cd, ls, grep, and you could to a first approximation get work done, write and compile C programs and so on.

[adrian_b]

I do not know if by "build" you mean building from scratch a real PDP-11, or just repairing an old real PDP-11, or just building an accurate emulator.

Most real PDP-11 from the later years of the seventies had processors made with bipolar bit-slice circuits from the AMD 2900 series and memories made with 16 kbit chips like Mostek 4116 and these had to be supported by a large number of MSI and SSI Schottky TTL integrated circuits.

Maybe a few such ancient integrated circuits in working state could still be found, but for a complete PDP-11 a very large number is needed (a 256 kB memory needs 128 DRAM chips, the maximum 4 MB memory needs 2048 DRAM chips, the CPU might need 40 to 100 bipolar logic chips). I do not believe that attempting to reproduce the large size and the great power consumption of a real PDP-11 can provide an improved self-teaching experience in comparison with the implementation of a clock-cycle-accurate emulator of PDP-11/70 on a modern FPGA board.

[zh3]

Oh, I think quite a few people do (plenty of PDP11 programmers about). It's easy enough to fire up an emulator and run a C compiler like its 1978 (or Fortran, come to that).

What's the definition of 'real' here?

[pinewurst]

Out of TTL or FPGA?

[musicale]

Original source of the PiDP kit that is being built:

https://obsolescence.wixsite.com/obsolescence/pidp-11

[watsocd]

The best I could find was a Dhrystone Benchmark of about 800MIPS for the PDP11/70.

The Raspberry PI3 is rated 2,411MIPS.

This emulator must be much faster than the original PDP11 even with the emulation software.

[zh3]

800MIPS? Not off an 11/70, for sure - much, much slower than that. 11/23's were as fast (or faster, depending on options and application) and they were maybe a single MIP or so (still got a couple of PDP11's kicking about, every so often I fire them up just to check my hearing).

[smackeyacky]

The RetroBytes guy on Youtube has a (slightly annoying) video featuring this kit. If I had the time it would be a fun project.

https://www.youtube.com/watch?v=bQUeMyAMQwo&ab_channel=Retro...

[mikewarot]

At Rose-Hulman they had a PDP-11/70 running RSTS that they had modified to have more than 128 terminals connected to. It's my understanding that it had 128K of RAM, and 144 terminals at one point circa 1982.

[aap_]

I made an 11/40 to hook up to the PiDP-11 panel: https://github.com/aap/fpga11 Unfortunately no disk or anything (that's always the hard part) and also it's not an 11/70. Need to do both of these eventually.

[musicale]

Kind of amazing the sort of functionality that old multiuser/timesharing systems were able to deliver. 128K (words, presumably) of RAM seems a bit low though for 128+ users.

[lalalandland]

This computer looks so cool. The colors, the switches, the lights :-)

[musicale]

Nice but I'd prefer an FPGA implementation. ;-)

[cptnapalm]

There are a couple: w11: https://github.com/wfjm/w11 PDP2011: https://pdp2011.sytse.net/wordpress/pdp-11/