EPROM emulator
What is it
An EPROM emulator capable of emulating the 2716, 2732, 2764, 27128 and 27256. It is operated from a bi-directional LPT-port of your PC.Supporting other types
The 2364 is not supported directly But that can be solved by using an adapter. The same adapter if you want to replace the 2364 of a C64, 1541 or VIC-20 with a 2764 or bigger EPROM.Other types are supported as well if they forfill the following condition: take a 2732 or 27256 as base. As long as it is only one or more address-pins replaced by enable-pins, the EPROM is supported. The emulator doesn't even have to know if it is a positive or negative enable-input. By mirroring an smaller EPROM all over the memory, the data always pops up at the right place.
The hardware
The core is a 57256 32KB*8 SRAM (Uxx). Two systems must be able to approach this SRAM so we must provide provisions for the address, data- and control bus. A 32KB SRAM has:* 15 addresslines
* 3 controllines
* 8 datalines
This means we need three 541 buffers for the address- and datalines, Uxx and Uxx, and one half of a 244 (Uxx) for the controllines.
The LPT-bus does not supply addresslines and therefore we have to supply them. Two 573s, Uxx and Uxx, take care of them. They get their input from the databus of the LPT-port. The second half of Uxx takes care of the controllines.
A 139 2-to-4 demultiplexer, Uxx, takes care of selecting the 573 you want to update. The third output selects the SRAM. A pin from the LPT-port tells the SRAM whether it is an read- or write-operation. The last pin of the 139 controls whether the 573s or 541s are active and which halve of the 244 is active.
The selectlines of the 139 are fed by two outputs of the LPT-port.
As you can see there are no provisions taken to avoid datacollisions: this is purely a matter of software. The moment the SRAM needs to be read by the host or PC, the databus of the LPT-bus must operate in inputmode.
2732 and smaller EPROMS have only 24 pins. This means that the power supply is at pin 26 of our design. A diode routes the power to pin 28.
To make sure that the pins 1, 2, and 27 have a defined level, three resistors pull these line to +5V.
