A header is used to customise the programmer for each EPROM type that may be programmed. Address lines A11, A13, A14, and A15, and other control signals generated by the programmer go to the header. Seven EPROM lines (pins 1, 20, 22, 23, 26, 27 and 28) come from the header.
Note that both the data and the two address latches can be disabled under software control. The data latch is disabled in order to read and verify the EPROM's data but is enabled during programming. The control latch is always enabled.
Any one specific latch on the programmer is written to using the following procedure. First the 138 is setup to select the specific latch (output of 138 is active low). This is done by writing to the parallel port's BASE+2 register and this produces a negative edge to the latch's trigger input and is ignored by the 374. Next the latch's input data is setup by writing to the port's data at BASE+0. Lastly the 138 is written to in order to deselect the latch, and this produces the positive edge which actually latches in the data to the 374.
To program a location in an EPROM the address, as a low and high byte, is first set up in the address latches. The data is then set up in the data latch. Finally the control signals are manipulated to generate a programming pulse. The programming pulse is generated by the 555 timer and this has a duration of 1 mSec. For EPROMS that require pulses longer than 1 mSec (many early EPROMs require 50 mSec pulses) the software must produce multiple pulses. For durations shorter than 1 mSec it is possible for the software to terminate the programming pulse.
Programming verification requires that the data latch is tri-stated (the data 374's output is disabled) and the EPROM's data may then be read through the 257 multiplexor (mux) in two nybles (groups of 4 bits).
Here's the EPROM socket and header pinouts (individual headers not shown).
And here's a photo of the prototype (!).
We don't have a PCB, so, if you're interested in this project, and would buy a PCB from us, then contact us and we may have one soon.
Anyway, if you've read this, tell us. However, don't bother telling us the information is incomplete, or that we haven't made the software or a PCB design available (we know this!). Our final design will also probably have additional parallel port buffering and noise reduction circuitry to ensure reliable operation with long cables (that people insist on using!). This final point is the main reason we have not produced a final design yet.
We also have a design for a 2 chip mini-reader (working),
and a 3 chip programmer (conceptial stage). Details soon...
To ensure the correctness of this document, we greatly appreciated your feedback on the information presented here.
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