National Semiconductor NS23C QBUS memory

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The National Semiconductor NS23C is a QBUS main memory card in dual QBUS card-size format, with a maximum capacity of 256 Kbyte (using 64Kx1 DRAMs), or 1 Mbyte (un-documented, using 256Kx1 DRAMs). (The card may be configured to provide less memory thanis physically present.)

It provides byte parity, and supports QBUS Block Mode. (Note that block mode must be disabled on older LSI-11 systems, including the LSI-11/2.) Internal or external refresh may be used


Unlike the NS23M, the NS23C printed circuit board does contain identification which indicates that it is an NS23C; in addition, the PCB says "PWB 551010105-002" on the solder side, and "PWA 980010105" on the component side.

256KB to 1M Upgrade

It is very easy to upgrade these cards from 256KB to 1MB, as it has all the necessary traces, jumpers, etc for this capability built into the card - even though the NS23C documentation says nothing about this capability!

Looking at the prints, the memory chips have all 9 address lines wired (unlike the very similar NS23M card), and there are several jumpers that adapt the card to 1MB operation. The chips are all in sockets, so pulling the 64Kx1's and replacing them with 256Kx1's is easy.

There are three 'jumpers' one has to remove/move; alas, they are in the PCB on the top surface, although there are jumper pins there - us built, there is a trace running between the two pins - so those traces have to be cut.

The first two 'jumpers' one has to remove are W23 and W24 (right next to the other memory size jumpers), which allow one to increase the maximum memory size to 1MB.

The other 'jumper' one has to do is to move the 'jumper' from W40 to W41; this moves the pickup point for the 'RS0' signal, which indicates which bank of chips (there are 2x18 banks, i.e. 16 data, and separate byte parity) to activate, from address line 17 to 19.

Control and Status Register

The NS23C has an optional Control and Status Register, which may be configured, and can be placed at one of eight locations. The layout is:

Parity Error Extended CSR Enable Unused A17-A15 A21-A18 or
Unused Write Wrong Parity Unused Parity Error Enable
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00


NS23C configuration jumpers location diagram

Since the 1 Mbyte option is un-documented, here is updated information on how to configure the board.


First, the low-order size bits, which are the same as the un-modified board:

W22 W21 W20 W19
16KB 000000 I I I I
32KB 010000 I I I R
48KB 020000 I I R I
64KB 030000 I I R R
80KB 040000 I R I I
96KB 050000 I R I R
112KB 060000 I R R I
128KB 400000 I R R R
144KB 440000 R I I I
160KB 500000 R I I R
176KB 540000 R I R I
192KB 600000 R I R R
208KB 640000 R R I I
224KB 700000 R R I R
240KB 740000 R R R I
256KB 1000000 R R R R

and now the high-order size bits, which were not previously documented:

W24 W23
256KB 1000000 I I
512KB 2000000 I R
768KB 3000000 R I
1024KB 4000000 R R


The starting address is also entirely the same as the un-modified board; first, the low-order starting address bits:

W10 W11 W12 W13 W14
0KB 000000 R R R R R
8KB 020000 I R R R R
16KB 040000 R I R R R
24KB 060000 I I R R R
32KB 100000 R R I R R
40KB 120000 I R I R R
48KB 1400000 R I I R R
56KB 160000 I I I R R
64KB 200000 R R R I R
72KB 220000 I R R I R
80KB 240000 R I R I R
88KB 260000 I I R I R
96KB 300000 R R I I R
104KB 320000 I R I I R
112KB 340000 R I I I R
120KB 360000 I I I I R
128KB 400000 R R R R I
136KB 420000 I R R R I
144KB 440000 R I R R I
152KB 460000 I I R R I
160KB 500000 R R I R I
168KB 520000 I R I R I
176KB 540000 R I I R I
184KB 560000 I I I R I
192KB 600000 R R R I I
200KB 620000 I R R I I
208KB 640000 R I R I I
216KB 660000 I I R I I
224KB 700000 R R I I I
232KB 720000 I R I I I
240KB 740000 R I I I I
248KB 760000 I I I I I

and then the high-order starting address bits:

W15 W16 W17 W18
0KB 00000000 R R R R
256KB 01000000 I R R R
512KB 02000000 R I R R
768KB 03000000 I I R R
1024KB 04000000 R R I R
1280KB 05000000 I R I R
1536KB 06000000 R I I R
1792KB 07000000 I I I R
2048KB 10000000 R R R I
2304KB 11000000 I R R I
2560KB 12000000 R I R I
2816KB 13000000 I I R I
3072KB 14000000 R R I I
3328KB 15000000 I R I I
3584KB 16000000 R I I I
3840KB 17000000 I I I I


CSR Address:

Address Jumpers
W9 W8 W7
772100 I I I
772102 R I I
772104 I R I
772106 R R I
772110 I I R
772112 R I R
772114 I R R
772116 R R R

Extended QBUS address type:

Address Type Jumpers
W47 W48 W49 W50
18-bit R R R R
19-bit I R R R
20-bit I I R R
21-bit I I R I
22-bit I I I I


Jumper Function Jumpers
In Out
W1 Battery backup None Use
W2 Use None
W3 Refresh Internal External
W4 External Internal
W5 BREF Inhibit Non-Inhibit
W6 Non-Inhibit BREF Inhibit
W27 CSR Enable Disable
W28 Disable Enable
W30 Block Mode Disable Enable
W31 Enable Disable
W35 Parity Enable Disabled Enabled
W36 I/O Page 1 KW 2/4 KW
W37 1/2 KW 4 KW

All other jumpers are manufacturing-related; leave as they are.

Fault Isolation

The way the address logic works on the card is slightly odd; if the card is not on a 'natural' boundary [e.g. a 256KB boundary, if it's a 256KB card], the memory contents are scrambled; the low memory, in bus address terms, is at the top of the card, in chip terms, and the high memory, in bus terms, is at the bottom of the card.

They did it that way because that's the most economical way to do it in logic/traces, etc but it's something one would have to remember when looking for a bad memory chip, if the card is set to an address which is not a multiple of its size.

Chip Table

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

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 U12, from left to right, in the top row; from U13 to U24 in the middle row; and from U25 to U36 in the bottom row. (See diagram in the previous section.)