An EPROM Emulator for the Nascom

by P. Burgess

Introduction.

This article describes the design and construction of a unit of hardware intended to temporarily replace an EPROM during the developement and debugging of firmware for dedicated Z80 CPU based projects. Detail is given to allow the construction of a unit, including an example of a loading program in Z80 assembler code, and connections to a Nascom. The article is also intended to act as a basis for experiment for anyone who might wisj to extend the design.

Design Considerations.

My own computer interests lie in the application of microprocessors to small design projects, often employing a Z80 CPU plus minimal RAM and EPROM configurations. A single 2716 EPROM is often enough for the whole program, ie. 2048 (2K) bytes. A Nascom plus and EPROM programmer (such as the Gemini one) makes quite a fair development system. However, when testing a newly built piece of hardware (or a ‘target’ system, as it is known) it can be necessary to repeatedly blow and erase EPROMS to find faults and debug the program. What is required is a chunk of memory which will look like an EPROM to the target system, but to which a host computer can read and write quickly. This would then replace the EPROM until program development is complete.

Several such emulators are commercially available, but tend to be expensive, as they incorporate their own monitors, keyboards and displays. The Nascom, though, already hay its own powerful machine code facilities, so this design relies entirely on the Nascom to manipulate the data, resulting in a very simple (7 common lsTTL chips) design, at, perhaps, the cost of a little flexibility.

The device is controlled from a Z80 PIO, in this case, ports A and B of a Nascom 1, although, of course, other ports may be used.

As drawn, the emulator plugs in directly in place of the single supply rail 2716 EPROM, although it could he extended to 2732 devices by adding extra RAM and decoding. Components are hot critical; higher density RAM packages such as the 6116 have become available since the prototype was built and could be worth looking at.