Microelectronics | 32K ROM Matrix

Synopsis

At some point, during my digital microelectronics experiments, I thought about creating a larger ROM storage device, based integrally on lower capacity UV-erasable EPROM integrated circuits. And since I already had a lot of K573PF2 EPROM ICs, I designed a schematic. The individual EPROM addressing is done by means of a 4:16 TTL decoder of type 74154.

Schematic Diagram

The circuit looks complicated, but in reality it is not. The individual EPROMs are connected in a matrix-like grid.

Fig. 1: Electrical Principial Schematic

Fig. 2: Top Layer Printed Circuit Board

Fig. 3: Bottom Layer Printed Circuit Board

As I said, the main logic circuit is a 4:16 line decoder. There is also an LED driver, constructed around a PNP small-signal transistor. Each of those sixteen EPROM ICs has a capacity of exactly 2K.

Construction and Pictures

The dual-layer PCB is entirely home made. It looks like 1970s industrial PCBs, because it is manually tin-plated and then polished with sandpaper.

This is the completed PCB assembly. I wanted to experiment a bit with EPROM decoding, and attempted to burn a 4:16 decoder ROM mesh in a K573PF2, with mixed results. Needless to say, the 74154 IC is the way to go.

The command and control circuitry must absolutely contain buffers with sufficient fan-out to drive sixteen EPROM address lines. Otherwise, the circuit will not work at all. The communication is done via a 40-pin parallel bus interface. Not all pins are connected, twelve of them being left open. If I think better about it, these should've been tied to ground.

This is the solder side.

How about real-world uses for this contraption!? Well, anything that screams look at me, I am a weird circuit. I initially wanted to build a mini digital photo frame with a few bitmaps hard-coded in ROM. The CPU would've been an 80286 at 10 MHz, and the display, a mere 128 x 128 px. However, I soon shifted my attention to more meaningful projects and abandoned this idea altogether. But I learned a few lessons along with design and implementation constraints and ideas and gained experience in general.

Thank you for reading this page, and I hope you found it useful.