Presenting a home-build One-Kilo-Bit Diode-ROM for DEC PDP-11 !

book half open pdp11

The ROM is organized as 4 pages with 256 bits each, organized as 16 PDP-11 words (64 words total).

And since memory has "pages", the whole is build as "book". That's an alternative to the typical DIL case.

eprom 2716

With 1024 Bits, the book is 10 cm high. For 64kByte it would be 41m, and 2.6 km for (max QBUS memory) 4MBbyte , 671 km for 1GByte , 42950km for 64GByte .


In addition to the plain bit-matrices each page has LEDs, which show last accessed data row and data values.
So if you execute a program from ROM, you can follow the instruction trace just by watching active ROM cells.

Both UNIBUS and QBUS controllers were made, so the ROM can be operated in any PDP-11.

unibus controller qbus controller


For better access, the diode-matrix-book can be handled remote from the PDP-11.

The single data bits are set manually by little jumpers ... no chip programmer needed.


This project merges two sources of fascination:

1) "Diode matrices" as from DEC Diode ROM BM792:

BM792 YA

2) "Switchable bits" from the CDC6600 boot ROM:

6600DeadStartPanel cdc6600 dead start from core memory book big


I wanted big toggle switches for the bits, as in the CDC 6600 panel.
But hobbiest budget allows only for cheap bit plugs ... after all, the price for a single switch multiplies by 1024.

Maybe there's a sponsor for the next-gen design?


Final project kick off was caused by discussion with OCM museum members.

ocm people ocm 1134


These guys liked the idea to get visitors in touch with "single bits".
Function of a diode-matrix memory can be explained, indicated by Access-LEDs.
And you can directly see the "Hello world" text in demo program memory and modify it via jumpers while the program runs.
Its an even more direct contact "to the bits" than presenting a paper tape strip.

bits hello world

So the Diode ROM is mainly a fun and educational toy.

However it's also a useful tool, if you want to run test programs on a defective, RAMless, ROMless PDP-11 (the typical "fresh-out-of-crap" state).