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VAX 11 750 Trondheim.jpg
VAX-11/750 (CPU cabinet on the left)
Announcement date: 1980
FRS date: October 1980
Codename: Comet
OS support (VMS): VMS V2.0
CPU Details
CPU name (VMS): KA750
CPU name (console): None [1]
Number of processors: 1
SID: 02006878
CPU cycle time: 320ns
Instruction-buffer: 8 bytes [2]
Translation-buffer: 512 entries [2]
Control store: 6K 80-bit words [2]
Writable Control Store: 1K 80-bit words [2]
Cache: 4KB [2]
Compatibility mode: Yes [2]
Console processor: CPU
Console device: TU58 [2]
Minimum memory: 1MB [2]
Maximum memory: 14MB
Physical address lines: 24
Memory checking: 7-bit ECC/longword [2]
Max I/O throughput: 5.0MB/s
MASSBUS: 3@ 2.0MB/s
UNIBUS: 1 @ 1.5MB/s
LAN support: optional
VUPs: 0.65

The VAX-11/750 (also known as the VAX/750) is a slower, more compact, but less expensive successor to the initial VAX-11/780. It was primarily implemented in gate array chips holding 400 NAND gates; they were configured at manufacture time into the 39 different types used in the /750.

It is built around the KA750 CPU. It supported several pre-existing DEC I/O buses (UNIBUS and MASSBUS), so there are almost no VAX-11/750-specific peripherals.

One of the first OS's available for the 750 was 4.1 BSD. According to 'Quarter Century of UNIX', the DoD could purchase 6 750's for about $150,000 USD.

Internal structure

VAX-11/750 system block diagram, showing the major /750 functional units and how they connect

The /750 has a plethora of internal data paths; many of them are contained entirely within the KA750 CPU, and will not be detailed here. The one significant one which connected all the major functional units, including the CPU, main memory, and I/O bus adapters, was the CPU/Memory Interconnect bus (CMI). The mandatory major CMI sub-systems (and components thereof) were:

Optional sub-systems included:

CMI Options

A variety of optional bus adapters and device controllers were available for the CMI:

Physical structure

The VAX-11/750 comes in an H9645 cabinet and has one large backplane (DEC part number 50-13821/70-16486/54-13822), into which plug:

  • KA750 boards
  • CMI option boards (RH750, etc)
  • memory array boards

Although it is one physical unit, the backplane can be seen as divided into several logical sections; indeed, the connectors used in the memory array board section are different from those used in the 'main' section of the backplane (where all the other boards plug in).

However, the biggest difference the sections has to do with which buses are connected to each slot; many slots are thereby customized to a particular board, which has to go in that slot - and that slot can't hold anything else.

The 'main' (i.e. non-memory) section of the backplane can be logically further divided into two sub-sections: one holding optional CMI cards, and another (specifically, slots 1-6 and 10) holding the main memory controller, and the CPU (and optional ancillaries thereof). (Those latter slots have internal buses, such as the MBus and WBus, running between them.)

The CMI appears on the backplane (and nowhere else), and goes to some (but not all) slots/cards. (E.g. the FPA is not a 'CMI' card; it's part of the KA750, and plugs into the /750 backplane which carries the CMI to other slots, but that's all.)

So, one shouldn't put much weight on whether boards plug into the 'main' slots (1-10) on the /750 backplane, because that section holds a mix of special-purpose slots, and 'CMI option' slots (7-9). Although cards that the CMI goes to (MIC, UBI, RDM, WCS) are in some sense 'CMI' cards... because they have dedicated, customized, slots, one should reserve the term 'CMI card' for the cards that can go in the CMI option slots: RH780, etc.

Backplane configuration

This diagram:

9 8 7 6 5 4 3 2 1   19 18 17 16 15 14 13 12 11   10 9 8 7 6 5 4 3 2 1
  M9313                     M9202  MS750-C 1 MB (optional) MS750-C 1 MB (optional) MS750-C 1 MB (optional) MS750-C 1 MB (optional) MS750-C 1 MB (optional) MS750-C 1 MB (optional) MS750-C 1 MB (optional) MS750-C 1 MB   Memory Control (MCM) CMI Option (optional) CMI Option (optional) CMI Option (optional) Remote Diagnostic Module (RDM) CPU Control Store (CCS) UNIBUS Interface (UBI) Memory Interconnect (MIC) Data Path Module (DPM) FP750 Floating-Point Accelerator
                    M8750 M8750 M8750 M8750 M8750 M8750 M8750 M8750 L0011       L0006 L0005 L0004 L0003 L0002 L0001
UNIBUS DD11-DK Extended Hex CMI Backplane

shows the UNIBUS, main memory, and CPU backplanes.

An L008 Patchable Control Store (PCS) may be substituted for the L0005 CPU Control Store. Similarly, the later L0016 and L0022 Memory Controllers may be substituted for the L0011 Memory Controller.

VAX-11/750 images

View into a VAX-11/750 card cage

Left of card cage:

  • Top: 54-13489 - TU58 controller
  • Bottom: H7112 - VAX-11/780 / VAX-11/750 Battery Backup Unit

Inside card cage:

  • Left: DD11-DK Unibus backplane (with cards)
  • Center: Memory backplane section (with cards)
  • Right: CPU/CMI backplane section (with cards)

View of VAX-11/750 backplanes from behind


  • Left: CPU/CMI backplane section
  • Center: Memory backplane section
  • Right: DD11-DK - UNIBUS backplane
  • Lower left corner: 5V and 2.5V Power Supply cables (as thick as a finger!)
  • Bottom: 5V and 2.5V Power Supplies

VAX-11/750 Power Supply Units

Power Supplies:

  • Left: H7104-C 2.5V, 85A max.
  • Right: H7104-D 5V, 135A max.

Cable routing inside VAX-11/750 (left side open)

Round cables, top left:

  • Console
  • Remote Diagnose Modem

Cables, bottom left to top right:

  • DMF32 UNIBUS Multi-Function Communications Interface (3 flat cables)
  • TU80K TU80 Tape Adapter (2 flat cables, one behind the other)
  • DEUNA Ethernet Adapter (round cable)

Bottom right:

  • H7112 - VAX-11/780 / VAX-11/750 Battery Backup Unit


[1] CPU Upgrade Guide. Spring-Summer 1992.
[2] VAX Hardware Handbook Volume 1 - 1986.
[3] Systems & Options Catalog. European Edition. Spring 1990.

See also

External links