National Semiconductor NS23M QBUS memory

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The National Semiconductor NS23M is a QBUS memory card in dual QBUS card-size format. It can be configured to use either 16Kx1, 32Kx1 or 64Kx1 DRAMs, operating at 150nsec or 200nsec; it has a maximum capacity of 256 Kbyte (using 64Kx1 DRAMs). It provides byte parity.


Unlike the NS23C, the NS23M printed circuit board does not contain any identification which indicates directly that it is an NS23M. The PCB only says "PWB 551103882-003" and "PWA 980103882", both on the component side; one often sees these boards listed on auction sites under these ids.


The board is configured with a combination of jumpers and DIP switches.

Jumper(s) Function
W1-W2, W17-W18 200ns/150ns DRAM
W3-W4 Test
W5-W7, W12 16K/32K/64K DRAM
W8-W9 Test
W10-W11 Internal/External Refresh
W13-W14 32K DRAM Upper/Lower
W15 18-/22-bit QBUS (in/out)
W16 External Refresh / MEMSEL
W19-W20 Battery Backup

Starting Address

Switch Address bit Location
S1-1 020000 8KB
S1-2 040000 16KB
S1-3 0100000 32KB
S1-4 0200000 64KB
S1-5 0400000 128KB
S1-6 01000000 256KB
S1-7 02000000 512KB
S1-8 04000000 1024KB
S1-9 010000000 2048KB


Jumper(s) Function
S2-1 16KB
S2-2 32KB
S2-3 64KB
S2-4 128KB
S1-10 2K/4K I/O Address Space (closed/open)

Note that the size is the value indicated, plus 16KB (the minimum); i.e. 0000 = 16KB, 1111 = 256KB.

Fault Isolation

To aid fault isolation, here is a table which relates bit numbers in memory words to chip numebrs:

Bit Low Bank High Bank
0 U12 U24
1 U11 U36
2 U10 U23
3 U22 U35
4 U9 U34
5 U21 U33
6 U8 U20
7 U7 U32
8 U6 U19
9 U18 U31
10 U5 U30
11 U17 U29
12 U4 U16
13 U3 U28
14 U2 U15
15 U14 U27
P0 U1 U26
P1 U13 U25

Facing the board, with the handles at the top, and the contacts at the bottom, the memory chips are numbered from U1 to U9, from left to right, in the top row; from U10 to U18 in the upper middle row; from U19 to U27 in the lower middle row; and from U28 to U36 in the bottom row.