MS780 Memory System

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The MS780 Memory System was a MOS DRAM main memory system for the VAX-11/780. It was 64 bits wide, to interface to the -11/780's Synchronous Backplane Interconnect, and provided ECC (8 bits/double-word), which could correct all single-bit errors.

An MS780 was comprised of a custom backplane, a small group of control cards, and a variable number of array cards (between 2 and 16 per MS780), holding the DRAM chips. There were two generations; the first included:

  • MS780-A (sometimes given as MS780A): 4Kx1 DRAM chips, 64KB per array card
  • MS780-C: 16Kx1 DRAM chips, 256KB per array card

(The MS780-B and MS780-D are additional array cards for the MS780-A and MS780-C, respectively.) The second generation includes:

  • MS780-E: 64Kx1 DRAM chips, 1MB per array card
  • MS780-H: 256Kx1 DRAM chips, 4MB per array card

(Similarly, the MS780-F and MS780-J are additional array cards for the MS780-E and MS780-H, respectively.)

For the original release of the -11/780, with the MS780-C, this produced a maximum of 512 KB per MS780. The -11/780 supported up to 4 MS780 subsystems, for a maximum of 2MB with fully populated MS780-C's.

It was possible to interleave two MS780's to improve performance.


All versions were implemented on super hex cards (DEC's first use of this form factor), mounted in a custom 20-slot backplane.

The initial versions, the MS780-A and MS780-C, included:

Slot Board Acronym Function
20 M8214 MSB SBI Interface
19 M8213 MCN Control and Timing
18 M8212 MDT Data Paths

Slots 2-17 held 256-Kbyte M8210 (MS780-C) or 64-Kbyte M8211 (MS780-A) array cards; a minimum of two, in slots 16 and 17, were required. The backplane was DEC part number 70-13625.

The later versions, the MS780-E and MS780-H, were implemented using a different card set mounted in a different custom 20-slot backplane (DEC part number 70-19729-00):

Slot Board Acronym Function
10 M8375 MCN Control (lower)
11 M8376 MSB SBI Interface
12 M8375 MCN Control (upper)

Slots 2-9 and 13-20 held 1-Mbyte M8373 (MS780-E) or 4-Mbyte M8374 (MS780-H) array cards; a minimum of two, in slots 9 and 13, were required.

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