MSV11-Q QBUS memory

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MSV11-QA card, later etch

The MSV11-Q (M7551) is a quad-height QBUS DRAM main memory card. Initially it used 64Kx1 DRAM chips, later ones use 256Kx1 DRAMs. The memory is arranged as 8 banks, each 16 data bits (1 PDP-11 word) wide, with 2 additional bits for parity (1 per byte).

It holds 1 MByte with 64K DRAMs; 4 Mbytes when fully populated with 256K DRAMs, or 2 Mbytes when half-populated (the only partially-filled configuration allowed). Three versions exist:

  • MSV11-QA, 1 Mbyte (64K DRAMs)
  • MSV11-QB, 2 Mbytes (256K DRAMs)
  • MSV11-QC, 4 Mbytes (256K DRAMs)

all are Q22, and support block mode. The -QA comes in two etch revisions; the latter version supports battery backup. The -QB and -QC are the same etch as the later -QA.

Control Register

Each board has a single control register, which can be configured in the range 172100-172136.

In the register contents (below), all the bits can be read and written by software; most are cleared by power up and bus INIT. Bits which can only be modified by the CPU are shown in normal font, and those which can also be set by the hardware in italics.

Parity Error Extended Error Address Enable Reserved Error Address Reserved Write Wrong Parity Reserved Parity Error Enable
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

The 'Error Address' field contents depend on the setting of the 'Extended Error Address Enable' bit; when it holds the low address ('Extended Error Address Enable' is 0), it holds address bits 11 through 17; when it holds the high address ('Extended Error Address Enable' is 1), it holds bits 21 through 18 - bits 11-9 of the register are unused.


The MicroPDP-11/53 System Supplement Manual (below) has information on how to configure the MSV11-Q, in Section 4.3.1, "MSV11-Q Memory Module".

Technical information

As far as is known, there are no copies of the engineering drawings extant for the MSV11-Q. However, some technical information, enough to repair boards with faulty DRAM chips, has been gathered on it, and that is made available here.

As described above, each board has 8 (sometimes 4) banks in the array of DRAM chips; with 64K chips, each bank is thus 128KB; with 256K chips, they are 512KB. (Note that when writing data, the MSV11-Q sends a 'write' signal to all the banks, and selects the one to actually use by use of the RAS signal. It's not certain why DEC did this, but since there is no explicit 'read' signal to the chip, and likely the outputs from all the banks are wire-OR'd together, use of RAS to select the desired bank works for read as well as write.)

The chips in each bank run across the board (when it is oriented with the chip side facing the viewer, with the metal edging holding the handles at the top). Bit 0 (value 1) is on the left hand edge of the array; bit 15 (value 0100000) is on the right, with the two parity bits in the middle. The banks run down the board from the metal handle edge; bank 0 is next to the handles, and bank 7 is down near the contact fingers.

Thus, for example, bit 0 of bank 0 is in chip E197. The full table is:

Bit Bank 0 Bank 1 Bank 2 Bank 3 Bank 4 Bank 5 Bank 6 Bank 7
01 E197 E196 E195 E194 E193 E192 E191 E190
02 E189 E188 E187 E186 E185 E184 E183 E182
04 E180 E179 E178 E177 E176 E175 E174 E173
10 E172 E171 E170 E170 E169 E168 E167 E166
20 E164 E163 E162 E161 E160 E159 E158 E157
40 E155 E154 E153 E152 E151 E150 E149 E148
100 E147 E146 E145 E144 E143 E142 E141 E140
200 E139 E138 E137 E136 E135 E134 E133 E132
400 E114 E113 E112 E111 E110 E109 E108 E107
1000 E105 E104 E103 E102 E101 E100 E99 E98
2000 E97 E96 E95 E94 E93 E92 E91 E90
4000 E89 E88 E87 E86 E85 E84 E83 E82
10000 E81 E80 E79 E78 E77 E76 E75 E74
20000 E72 E71 E70 E70 E69 E68 E67 E66
40000 E64 E63 E62 E61 E60 E59 E58 E57
100000 E56 E55 E54 E53 E52 E51 E50 E49

The following 64K DRAM chips have been observed to be used: M5K4164ANP-15P (Micron Technologies), NEC D4164C211 (NEC Electronics).

Further reading

  • Microcomputer Products Handbook (1985), Chapter 15
  • MSV11-QA, MSV11-QB, and MSV11-QC Field Maintenance Print Set (MP-01931 - not online)

External links