PDP-11/35 Details

Here’s a little more information about the PDP-11/35.

It came to me with the following peripherals:

  • ECCO Paper Tape Reader
  • Diablo Series 30 disk drive — compatible with DEC RK05
  • Applied Engineering 2200 Disk Controller for the Diablo Series 30
  • Data System Designs DSD 440 floppy disks — compatible with DEC RX02
  • DEC H750 Power Supply

There are three backplanes (or “System Modules”, as DEC called them) installed. The first is a 9 slot KD11-A, the second is a 4 slot MM11-S, and the third is a 4 slot DD11-A. The MM11-S is interesting in that it suggests that the 11/35 originally came with the MM11 16KW core memory option, though somewhere along the line that was upgraded to MS11 MOS memory.

Here’s the layout:

A B C D E F
01
02
03 M7232 – KD11-A 11/40 Micro Word Module
04 M7231 – KD11-A 11/40 Data Paths Module
05 M7233 – KD11-A 11/40 IR Decode Module
06 M7235 – KD11-A 11/40 Processor Status Module
07 M7234 – KD11-A 11/40 Timing Module
08
09 M981 Jumper M7800 Async Serial
10
11 M7847-BD – MS11-EP 16KW RAM 1
12 M7847-DJ – MS11-JP 16KW RAM
13 M920 Jumper
14 DSD 440 Floppy Controller
15 ECCO Paper Tape Reader
16 G772B M7800 Async Serial
17 M9302 Terminator G727A
  1. The MS11-JP module is normally an 8KW board, but this one has been field upgraded to 16KW of MOS RAM, minus parity bits on the upper 8KB.

Rescuing a PDP-11/35

On Sunday, April 28 2012, I picked up a PDP-11/35 from a friend who had been storing it in a shed for many years. The system has been home to generations of spiders and mice, and needs more than just a little bit of TLC.

The PDP-11/35, dirt and all
The PDP-11/35, dirt and all

The provenance of the machine is kind of interesting. The friend who gave it to me grabbed it from the loading dock at ATARI in Milpitas, California, when it was being scrapped. They were apparently using it in some kind of standards or verification engineering role, but it’s not known what. He later moved to Washington state, where the machine was stored in a shed for some time. Unfortunately, the climate and the local wildlife took their toll.

I’ll document ongoing progress on restoration here on the blog. Luckily, my initial inspection suggests that things are not as bad as they look. The electronics are in fair condition, with only very little surface corrosion on some IC leads. The metal chassis and power supply case have some bad surface rust, but can be stripped and re-painted. The front panel and switches are mechanically sound. I think it is definitely worth restoring!