PDP-11/53 – Overview



I picked up this PDP-11/53 in Feb 2014. You can read a little about it here.


Somewhat unorthodox to say the least but strangely practical. It appears that a previous owner has taken the card cage from a DEC chassis such as a BA-23 and mounted it in a 19″ cabinet. There is space above the card cage for a couple of full-height 5.25″ drives.

06deb3285e7f43917474be0687d2ae7dAt the back.

I haven’t followed the internal wiring through yet and so I haven’t found out why there are two mains sockets. When I run the machine I connect to the socket on the main PSU and use the switch on the back.

The card cage is completely exposed as you can see. Currently there is plenty of space for expansion.

Above the card cage is the space of drives and to the left of that an aluminium plate with 2, 25 way serial connectors. These are connected to the two serial ports on the processor board. One for the console the other is labelled “Printer” but is a general SIO. These are known as SLU0 and SLU1.

The Cards.

My machine has the following cards…

  • M7554 – CPU – KDJ11-DA Q J11 CPU 15MHz, 512-Kbyte RAM, 2 SLUs, LTC
  • M7551 – CA – Memory – MSV11-QC Q 4-Mbyte 22-bit parity/CSR MOS RAM
  • M7555 – Disk controller – RQDX3 Q MFM Winchester and floppy disk controller (RX50/RX33/RD50-54/RD31/RD32/RD33)
  • M7516 – Ethernet – DELQA-M Q Ethernet interface (replaces DEQNA)
  • M7546 – TK50 – TQK50-AA Q TMSCP controller for TK50 tape unit.



PDP-11/53 – Introduction


The beginning

Towards the end of 2013 I started to think getting hold of a PDP11 of some form might be fun. I know there are excellent emulators out there but nothing is the same as actually using the real thing. It’s partly the “getting”. It’s partly the intangibles. Flicking the power switch and hearing the clunk of the switch, maybe relays, the fan. Watching the self test go through for the first time. On an emulator it will pass. On the real machine it might not. All or these things and more make me want to use a real computer rather than an emulator.

That’s not to say I won’t use an emulator. Let’s be pragmatic here. I’m sure as I write this that one will be an important part of getting the PDP-11 up and running. The easiest way of migrating files between different disk and tape formats seems to be to use an emulator such as SIMH.


Where to begin?

I am a long time fan of the Vintage Computer Forum and also the CCtalk mailing list. Every now and then something will come along on one of these two sources. Every six months to a year, someone’s collection will reach “critical mass” and they reach the point where a major clear out is needed. In the last few of these I’ve read about, the people concerned has accumulated large amounts of “Big Iron” and it seems as if they were just at the right place at the right time to get shed loads of stuff given to them.

So it seems with my PDP.

I saw a posting from someone with an industrial unit full of piles of stuff, Sinclair, BBC, Atari, etc. etc. and so on. There were a couple of things on my wish-list and so I got rid of a few pieces in my collection to make space and raise some funds and set off on a bit of a road trip.

I’d already inquired about the possibility of a PDP-11 but it was not my primary mission but when I got there and saw piles and piles of stuff it became clear that I’d probably have a DEC in the boot when I left.

I’ve just picked up a PDP-11

This is a bit strange isn’t it?

I haven’t used any DEC kit for nearly 20 years and here I am sitting a few feet away from my latest addition, a mid ’80’s DEC PDP-11/53 (Pictures to follow).

There was a bit of confusion as to what model it really was. It was an 11/23 for a while. Then an 11/73, 11/83 but now I’ve been through the boards one by one I’m pretty sure it’s an 11/53.

It’s not in the original rack or more properly the original rack has been boxed in to another rack. I was given another rack by the previous owner though on my way out which seems very generous.

I’m now trying to learn about PDPs and how to find boot images for the 5.25″ floppy disks, no hard drive yet.

I’ll give it a proper entry in the main part of the site when I can.