EPROM progrogramer does NOT use any device to output a voltage to the PC printer port, only a current sink. All inputs from the printer port are buffered. In this way there is little or no possibility of damage to the printer port, even with most wiring error possibilties during construction. This EPROM burner will handle most standard EPROMS, but some switching may be required by the constructor at the EPROM socket. I will leave that up to you, although I do give some advice on this subject. Please note, the software I have written is NOT suited for the more recent EPROMS that only require 10mS or the shorter “blow until burnt” programing pulses.
If you were to fit a 40-pin socket then there is no reason why you cannot program the 271001, 271024, 272001 and the 274001 EPROMs. Please note that some of the above packages and programing voltage may not be available, but the possibility to program them exists with this burner. I will give you some common chip manufacturer and programing voltages further down in the article. Here is a photograph of the prototype main board. As you can see I have used a prototyping “blob board”.
Now we come to the interesting ‘nitty gritty’ EPROM burner card circuit diagram. As you will see in the circuit below, safety of the computer was my main concern. I have therefore used transistors to buffer signals between the card and the PC. NO voltage is fed from the burner card to the computer. The price is a few transistors, but they are cheap enough. All the connections to the left of the circuit and prefixed with a “B” are the D-25 printer port terminal pins on your computer.
Now that we have addressed the EPROM we need to ENABLE the EPROM and read the data from it. As I mentioned earlier, there are only four input pins to the computer (D25 pins 15, 13, 12 and 10). First we read the EPROM D0, D1, D2 and D3, then we read the other nibble; D4, D5, D6 and D7 by changing the diode switches into the buffer transistors. The diode data selector is controlled from the computer output data D0 (D25 pin 2). There is no output data from the computer during an EPROM read operation, so the eight data pins are free for signalling.