For a long time now, I've had some ideas that center around EPROMs. I've been using EPROMS since I started getting into arcade machine collecting. EPROMs are programmable devices where you can store bytes of memory in a fairly stable method. They look like any other microchip, except that they have a transparent window on the top (usually quartz, even though it looks like glass). If you look in the window, you can see the chip die in there, which I usually think looks like a tiny tennis court.
The above image shows a few different size. The one on the right is a 2716, 16 kilobit (2 kilobyte) device. The one in the middle is a 27C256, 256 kilobit (32 kilobyte) device.
Then you take the chip, and plug it into a programmer. A computer tells the programmer what zeroes to write and where, and you end up with a chip that has a program or some data stored in it, so that it can be read out later. Often, the window is then covered up so that they are not accidentally erased over time from ambient UV light. One data sheet I read said that 1 week in direct sunlight, or 3 years in normal office lighting will erase a device. The programmer above is my Needham's EMP-10. It has a 30 pin simm-looking slot on the side with 3 snap-in cards that configure the programmer pins so that it can handle a variety of different target device EPROMs.
This process has fascinated me since I started burning ROMs for use in arcade machines. This is where this project comes in. I've had a few thoughts about things to explore here...
First of all, it should be possible to determine which bytes are in which physical locations on the device by casting a shadow or projecting an image onto the die from the eraser. It should be possible to determine this by projecting different patterns on the die with the UV erasing light. Obviously, it has to be well focused; this has to be quite precise. A project for another day...
Secondly, I thought a fun project might be to read the pattern produced by erasing the device. I would program all zeroes to the EPROM. The chip would be plugged into an Arduino, to read the data out of it. The Arduino would also control a UV LED or two which would illuminate the window on the EPROM, erasing it. It would then alternate between illuminating the window and reading the content out of it. The data pulled out would then be displayed as an image with pixels white if the content is a '1', and black if it is a '0'. This should show all white at the start, and all black at the end, with some amount of dithering in the middle somehow.
I do have a spare Mayhew Labs Mux Shield, (mine is a previous version) so I decided to just run with that. Unfortunately this is where the story goes south. I went from idea to making the board in one day. I really should have spent more time thinking about it, and less time being impulsive and just creating the thing. I was too excited, incorrectly thinking that I had what I needed for the project.
I should note that I often enjoy when a project fails. It really helps me learn what I did wrong, why it didn't work, and then it's like a puzzle to figure out how to make it work. Sometimes, things just get frustrating and I will shelve a project indefinitely, but often I figure things out.
I wired up a socket, cobbled together from smaller DIP sockets, onto a piece of strip board. I also threw a few indicator LEDs onto the board for various runtime display. After an evening of work, I came up with the board as seen here:
The Mux Shield is an excellent device. I use it for my Jasper Box to read inputs. For inputs, it's awesome. It'll do 48 IO, even analog input. For output, that's where it gets a bit non-intuitive.
The chips used on it are unbuffered. That is to say that there's no way to specifically sample or store data out or in on it. You pick a line, and then immediately read or write from it. Think of it as a valve, not a view. This is to say that at any one point in time, you're only setting or reading one bit. When reading the EPROM, I want to specify a single address (16 bits of digital output) then read in the data from the chip (8 bits of digital input). I can only do one bit at a time, so it is impossible to set the 16 bits simultaneously.
In order for this to work, I need to add a latch of some kind to the circuit to store the 16 bit address, so that I can then read in the 8 bits of data for that address in the EPROM.
74HC595 serial-in, parallel-out shift registers chained together. The idea here is that I would shift in the address I want to read through both of the '595s, then latch in the data on the 74HC165 parallel-in, serial-out shift register. This is actually taking it one step too far, since the '165 is unnecessary. The thing on the left is the UV LED and its resistor.
I need to order some parts to build this, and once I get them, I will be posting further results on the project.
"Failure is always an option." - Adam Savage
(This article was originally posted at the Interlock blog.)
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