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This is a real 4x4 bit (P)ROM!
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Every ROM (Read Only Memory) is made out of an adress decoder (logic gates upper right) and a diode matrix just like this!
Each diode represents a logical ¨1¨.
In early computers, those ROM units were either soldered by hand or ¨mask programmed¨ while creating the chip. For prototyping or small series purposes, PROMs were available. Those were in fact ¨blank¨ ROMs just like in this demonstration. To program them, you simply increased the supply voltage (12V instead of 5V), put on an adress and then connected the outputs which should be zero to ground and literally burned out the diodes. The intact diodes give you ¨1¨s and the burned out ones ¨0¨s.
That´s exactly why it´s called ¨to burn a ROM¨. You literally had to burn it.
To ¨burn¨ this simulation, just tap a diode you want to ¨burn¨ and then tap the delete button to remove it. Then start the simulation and then use the two switches (Off = ´1´, On = ´0´) to check out your programmed (burned) data.
Later, EPROMs were invented. Instead of burning the diodes, the diodes were disabled by an electric charge. By shining UV light onto the chip, the charge could be removed to recycle the chip. Those EPROM chips had a highly expensive Germanium glass window on top to shine the light through (ordinary glass blocks UV) so they were rather expensive. As an alternative, OTP - One Time Programmables - were used. Same technology but no window.
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To all the teachers:
It also works in RL, use 1N4148 diodes, they blow up nicely when you connect a big capacitor in parallel to the PSU. But don´t use a real adress decoder, use push buttons for each column instead.
You can also use this as a student experiment. Use buttons for the columns ¨Monday¨ to ¨friday¨ (or Saturday) and label the lamps with¨Math¨, ¨History¨, ¨PE¨, ...
;-)
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