DEC PDP-11/35 #2

Donated on 5/25/22 by the family of Spider Boardman.

To the Restoration Blog for this PDP-11/35.

The PDP-11/35 and the PDP-11/40 are 16-bit processors that share the same backplane and boards. The PDP-11/35 has a smaller 20-slot 10.5" chassis and the PDP-11/40 has a larger 36-slot 21" chassis. These processors have 16-bit registers and memory, and can directly address a 28 kW of memory. With the optional KT11-D Memory Management Unit installed it can address 124kW of memory. The upper 4kW of address space is reserved for Unibus I/O devices. 

This system has almost all of the processor options installed. The KE11-E Extended Instruction Set option adds instructions for fixed-point multiplication, division, and shifting, and can handle double-precision 32-bit data. The KE11-F Floating Point Instruction Set option adds instructions for addition, subtraction, multiplication, and division of floating-point data. The KT11-D Memory Management Unit increases the address space from 32kW to 128kW. The MMU also adds the ability for the processor to run in Kernel or User modes, and support for multi-user operating systems. When in the User mode the processor cannot access the peripherals and cannot execute certain instructions. This prevents a program running in User mode from crashing the operating system. The KJ11-A Stack Limit option is normally installed with the MMU option. It limits the depth of the stack address, and will stop program execution with a processor trap.

The system looks like it was assembled from available parts. The Unibus chassis is a BA11-KA, which the DEC documentation says is an empty chassis without a power supply. This style chassis was used for the much later design PDP-11/34 systems. It will be a challenge to mount the 11/40 front panel to the BA11 chassis. The RICM has a nearly empty PDP-11/35 chassis, so it might be easiest to use the boards and backplanes from this system and the the other PDP-11/35 chassis.

The right cabinet contain the PDP-11/35 processor chassis and a VR14 XY display S/N RK03507, the same model that is used in a PDP-12. There is a RX02 dual 8" diskette subsystem and nothing else in the left cabinet. There is plenty of room in the processor cabinet, so we could move the RX02 diskette subsystem and then only have a single cabinet system.

We received a DEC LK11 button box with this system, but didn't know what it was. Al Kossow @ CHM told us that it connects to the M7060 LK10 VT48 push button control board that is in the system. This button box was also used with a VS60 graphics terminal. We might be able to modify the Lunar Lander and SpaceWar! games so that they can be controlled by the LK11 button box.

We should be able to run the RT11 operating system from diskette using the RX02 diskette drives that are connected to the RX211 diskette controller. That would provide enough storage to run some games. We could also add an RL02 10 MB disk drive to the system.

The complete system is similar to a DEC GT44 Graphic Display System. To make a complete GT44 we would need to add two RK05 disk drives. We will probably add two RL02 disk drives that each have 5x the capacity of an RK05 disk drive.

The chassis slots are numbered from right to left. The 9-slot KD11 backplane is installed at the right. A generic 9-slot Unibus backplane is installed in the middle. A special 4-slot VT11 backplane is installed at the left. An M981 Unibus Jumper connects the right and middle backplanes, and a more modern M9202 Unibus Jumper connects the middle and left backplanes. A M9312 Bootstrap/Terminator board is installed at the end of the Unibus in the VT11 backplane.

The ribbon cables from the front panel are disconnected to get a better view of the boards.

The installed boards are:

M7237   KJ11-A  Stack Limit

M787    Kw11-l  Line Time Clock

M7800   DL11   Asynchronous Line Interface Board 

M7228   KW11-P  Programmable real-time clock