National Semiconductor NS23M QBUS memory
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.
|W1-W2, W17-W18||200ns/150ns DRAM|
|W5-W7, W12||16K/32K/64K DRAM|
|W13-W14||32K DRAM Upper/Lower|
|W15||18-/22-bit QBUS (in/out)|
|W16||External Refresh / MEMSEL|
|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.
To aid fault isolation, here is a table which relates bit numbers in memory words to chip numebrs:
|Bit||Low Bank||High Bank|
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.