EPROM Emulator

As opposed to most other EPROM emulators, the design discussed in this article provides a virtually perfect imitation of a ‘real-life’ 27C256 device, which may be programmed in any programmer as well as inserted into the target system.An EPROM emulator is a useful if not indispensable tool if you are into developing software for a system employing an EPROM for storage of executable code or fixed data. Such a ‘programmable imitation EPROM’ obviates the need to erase an EPROM after any programming cycle, before the new (and hopefully improved) program may be loaded into the chip. Erasing an EPROM with the aid of UV light usually takes about 20 minutes, which, many of you will agree, is an annoyingly long time.

Resistors:
R1,R2,R5,R6 = 10kΩ
R3 = 33Ω
R4 = 100Ω
R7,R8 = 1kΩ2
R9 = 8-way SIL array 10kΩ
Capacitors:
C1-C4 = 100nF
Semiconductors:
D1 = zener diode 7.5V
D2,D3 = zener diode 4.7V
D4 = LED, high-efficiency, yellow
D5 = LED, high-efficiency, red
T1 = BS170 or BS107
IC1 = 78L05
IC3 = 62256-70CP
IC4 = GAL 16V89, programmed, order code 024066-31
IC5 = 74HCT245
Miscellaneous:
IC2 = 2 ? 14-way pinheader
S1,S2 = pushbutton
K1 = 9V battery clip with leads
PCB, order code 024066-1
Disk, GAL JEDEC listing, order code 024066-11