ML11 MOS Memory System

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The ML11 was a MASSBUS solid state mass storage device. Its basic format was 18-bit words (for use in PDP-10 and PDP-15 machines), but it could be used in PDP-11s and VAXen, which simply ignored the two high bits.

It attached to any MASSBUS controller, e.g. RH11 (UNIBUS), RH70, or RH780. Up to 8 devices total can be attached to any MASSBUS controller.

The initial configuration, using 16Kbx1 DRAM chips, could hold up to 4MB in 1MB increments (composed of groups of 4 storage modules); later, the use of 64Kbx1 chips expanded the maximum to 16MB. An ML11 with the former was denominated as an ML11-A; with the latter, as an ML11-B.

The memory was organized in 36-bit words, with 3 bits per word added to hold a CRC ECC.


In the table below, 'Number' is the MASSBUS register number; the 'Address' is the UNIBUS address (e.g. when using an RH11).

Location Number Name Function
000 00 MLCS1 Control and Status 1
002   MLWC Word Count
004   MLBA Bus Address
006 05 MLDA Desired Address
010   MLCS2 Control and Status 2
012 01 MLDS Drive Status
014 02 MLER Error
016 04 MLAS Attention Summary
020 07 MLPA PROM Address
022   MLDB Data Buffer
024 03 MLMR Maintenance
026 06 MLDT Drive Type
030 010 MLSN Serial Number
032 011 MLE1 ECC CRC Word 1
034 012 MLE2 ECC CRC Word 2
036 013 MLD1 Data Diagnostic 1
040 014 MLD2 Data Diagnostic 2
042 015 MLEE ECC Error
044 016 MLEL ECC Location
046 017 MLPD PROM Data
050   MLBAE Bus Address Extension@
052   MLCS3 Control and Status 3@

@ = RH70 only, not RH11

The addresses shown are based at 0776400 for the first ML11 in a system.


The basic system consisted of a number of hex boards mounted in a custom 22-slot backplane (DEC part number 70-17508), all housed in a BA11-K mounting box. The basic device controller boards were:

  • M7361 asynchronous control module
  • M7362 synchronous control
  • M7363 array data module

Initially, data storage was provided with M7357 boards, used in groups of four, each board holding 256KB; later, these were supplanted by M7358 boards, each board holding 1MB.

Three dual-width M5903 boards provided the MASSBUS interface; flat cables led from Berg connector headers on the M5903's to the MASSBUS controller.

Board locations (as seen from the board insertion side of the backplane, not the wire-wrap pin side, as is common in DEC documentation) are:

Slot A B C D E F
1 Unused M5903 MASSBUS Unused
2 Unused M5903 MASSBUS Unused
3 Unused M5903 MASSBUS Unused
4 M7361 asynchronous control
5 M7362 synchronous control
6 M7363 array data
7 M7357/58 arrray board 0
8-21 Arrray boards 1-14
22 M7357/58 arrray board 15

Further reading

  • ML11 User's Guide (EK-0ML11-UG) - not available online
  • ML11 Technical Description (EK-0ML11-TD) - not available online
  • ML11 MOS Memory System Technical Manual (EK-0ML11-TM-002) - not available online, but in microfiche
  • ML11 MOS Memory System Technical Manual (EK-ML11A-TM-003) - not available online, but in microfiche
  • ML11 MOS Memory System Service Guide (EK-ML11A-SV-001) - not available online, but in microfiche