DEC PDP-11/40 #2

This DEC PDP-11/40 was assembled from a PDP-11/40 donated by Dr. Douglas Lyon of Fairfield University, a PDP-11/35 donated by Spider Boardman, and other parts from the RICM collection.

The PDP-11/35 and the PDP-11/40 are 16-bit processors that were introduced in 1973. They share the same KD11-A processor backplane and processor boards. They also share Unibus I/O boards with other PDP-11 systems. The PDP-11/35 uses 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 so you can't use all of the possible memory.

This system has all of the possible 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 that is 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 if the stack size is exceeded. The KW11-L Line Time Clock provides a 60 Hz interrupt to keep the Date and Time.

The system was assembled from parts from three systems. The Unibus chassis is a BA11-FC, which the DEC documentation says is an empty chassis without a power supply. There are two revisions of the BA11-FC, the later revision has an improved power distribution system.  We started with an early BA11-FC, but found that the power connectors were burned, so swapped it for the later design. This style chassis was also used for the much later design PDP-11/45 and PDP-11/70 systems.

The power supply is an H742 that a little +15VDC and sockets for voltage regulators. We currently have 3x H744 +5VDC @ 25A, and one H745 -15VDC @ 10A installed. We will add another H745 regulator when we add more peripheral controllers. There is an 861C Power Controller in the bottom of the chassis that provides switched and unswitched 115VAC power and is controlled by the key switch on the PDP-11/40 console.

The right cabinet contain the PDP-11/40 processor chassis, a TA60 DECassette tape drive, 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, a TC11 DECtape controller, and a TU56-H DECtape drive in the left cabinet. We will add two RL02 10 MB removable disk drives to the left cabinet.

This system includes a VT11 Graphic Display Processor connected to the VR14 Display. The VT11 can use DMA to retrieve Graphics Program Words from PDP-11 memory, and then execute the Graphics Program. We will connect a 375 Light Pen to the VR14 display, which is needed to run the Lunar Lander game.

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 this 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 will be able to boot the RT11 operating system from a diskette using the RX02 diskette drives that are connected to the RX211 diskette controller. That would provide enough storage to run some games. When we add two RL02 10 MB disk drives we will be able to run the RT-11, RSX-11, RSTS/E, DOS, Ultrix-11, and several versions of UNIX. Since the RL02 is a removable platter pack drive, we can just swap the disk pack and boot a different operating system.

The complete system is similar to a DEC GT44 Graphic Display System. To make a GT44 we would need two RK05 disk drives. The RL02 drives have 5x the storage of an RK05, so there is no point to adding the RK05 controller and drives.

To the restoration blog.