Home » PDP-11/70

I was lucky enough to pickup a PDP-11/70 last week, thanks to the generosity of a fellow collector.

The racks

The PDP-11/70 Maintenance and Installation Manual (EK-11070-MM-002) explains that the basic PDP-11/70 system components are located in a double cabinet as shown in Figure 1-1 of that manual. That figure is shown to the right. You can click on it for a larger version.

My system is similarly configured in two racks. The main difference being that in my system the CPU Mounting Box and Memory Frame are in the one rack, with the second rack housing the Unibus expansion box.

Both cabinets are about 50% full. This leaves a reasonable amount of space to add some peripherals to each cabinet.

Further H960-C cabinets can be added to accommodate additional memory or peripherals.


Here are some pictures of the racks (vertical):

… and more pictures (horizontal):

Main Processor Box

The standard decal showing the modules in a PDP-11/70 is shown below (this is Figure C-3 from the Maintenance and Installation Manual):

Figure 4-20 of the Maintenance and Installation Manual shows a more detailed view of the backplane, viewed from the wirewrap side:

Based on a quick check, my 11/70 seems to be pretty much complete.

This machine has the KB11-C CPU, as evidenced by the M8123 in Slot 9. The earlier KB11-B processor would have an M8133 in Slot 9.

There’s no floating-point option fitted to this 11/70.

The PDP-11/70 processor box has capacity for the following peripheral controllers:

  1. 4 x RH70 Massbus Controllers (each controller takes up 4 slots)
  2. 5 x SPC Slots

There are no RH70 Massbus Controllers installed in this 11/70.

The peripheral controllers actually installed in this 11/70 are:

  1. Emulex SC72 SMD controller (in the Massbus Controller C slots)
  2. RL11 RL01/RL02 Disk Drive Controller
  3. Emulex TC13 Tape Coupler (emulates TS11)
  4. DL11-W SLU & Real Time Clock

Here’s the full list of modules in the processor box:

Slot Part No. Description
(Rows E-F)
M9312 Unibus Terminator and Bootstrap Loader and Diagnostic
2-5 Empty Floating Point Processor
6 M8130 11/70 Data Paths
7 M8131 11/70 General Registers and ALU Control
8 M8132 11/70 IR Decode and Condition Codes
9 M8123 11/70 ROM & ROM Control, FP11-C compatible
10 M8134 11/70 Processor Data and Unibus Registers
11 M8135 Trap and Miscellaneous Control
12 M8136 Unibus and Console Control
(rows C-F)
M8139 Timing Generator
14 M8137 System Address Paths
15 M8138 System Status Registers
16 M8140 System Descriptor / Console Cables
17 M8142 Cache Control
18 M8143 Address Memory
19 Empty Not Used
20 M8144 Data Memory
21 M8145 Cache Data Paths
22 M8141 Unibus Map
23 Empty Not Used
24-27 Empty RH70 Controller A – Spare
28-31 Empty RH70 Controller B – Spare
32 SU7010406 Emulex SC72 – C Board
(Rows D-F)
SU7010404 Emulex SC72 – CST Board
34 SU7210403 Emulex SC72 – B Board
35 SU7210401 Emulex SC72 – A Board
36-39 Empty RH70 Controller D – Spare
40 M7762 SPC Slot – RL11 RL01/02 Disk Drive Controller
41 G7273 SPC slot – Spare
42 TU131041 SPC Slot – Emulex TC13 Tape Coupler (emulates TS11)
43 G7273 SPC slot – Spare
44 M7856 SPC slot – DL11-W SLU & Real Time Clock

In addition:

  1. there are G727 grant continuation cards in 27D and 31D and 39D
  2. there are G7273 grant continuation cards in 41C-D and 43C-D
  3. there is an M9302 Unibus terminator in 44A-B

Memory Frame

The memory frame is mounted just above the CPU Mounting box.

This system is fitted with 512KB of memory (2 x M8728 256KB boards are fitted).

Figure 3-30 of the Manual shows the standard configuration of the Memory Frame:

The Memory Frame is populated as follows:

Slot Part No. Description
1 Empty Not Used
2-8 Empty Storage Array (even) #14-#2
9 M8728 Storage Array #0: 256KB
10 M8161 0 Control B (Data & ECC)
11 M8160 0 Control A (Timing & Control)
12 Empty Not Used
13 M8158 Address Buffer
14 Empty Not Used
15 M8164 Data Buffer
16 M8160 1 Control A (Timing & Control)
17 M8161 1 Control B (Data & ECC)
18 M8728 Storage Array #1: 256KB
19-25 Empty Storage Array (odd) #3-#15
26 Empty Not Used

Interestingly the same memory array boards (M8728) were also used in the VAX-11/780.

Expansion Unit

This Expansion Unit is a model H960-D. It has another sticker on it identifying it as a Box Type of “BA11-FE”.

It has a number of 4-slot and 9-slot back planes in it. I’m not sure yet which of these are “SPC” back planes, and which are special purpose back planes.

Here’s the list of the back planes and their contents:

Backplane 1 (4 slot):

Slot Part No. Description
Slot 1 A-B M9680 “DR11-B Test Board”
Slot 1 C-F M7219 “Bus Interface”

Slot 2: A-B empty, C: M208, D: M208, E-F: M7821

Slot 3: A: Small paddle board feeding power in, B: M205, C: M611, D: M611, E: M112, F: M112

Slot 4: A-B: M920 bridge, C-D: M7229, E: M116, F: M239

Backplane 2 (9 slot):

Slots 1 & 9: M920 bridge in A-B

Slots 2, 4, 6 & 8: G727A grant continuity card in D

Slots 3, 5 & 7: Completely empty (previous boards removed).

Backplane 3 (4 slot)

Slot 1: A-B: M920

Slot 2 & 3: D: G727A

Slot 4: Strange hex-width M7084, with small paddle-board above A-B bridged to M9202 in Backplane 4. Labelled “BUS REPEATER FOR PDP11” and “CSS MUNICH SSM 0235”. Also marked “DB11-H”. Has a “digital” sticker on Side 1.

Backplane 4 (9 Slot)

Slot 1: A-B: M9202 bridge from Backplane 3

Slots 2, 5, 8: M7814 (Multiplexer?) [Hex Width]

Slots 1, 3, 4, 6, 7 & 9 C: G727A

Slot 9 A-B: M920 to Backplane 5

Backplane 5

Slot1 A-B: M920 from Backplane 4

Slots 2, 4 & 6: M7814

Slots 1, 3, 5, 7, 8 & 9 D: G727A

Slot 9 A-B: M9302 terminator