The first EPROM ( Electrically Programmable Read Only Memory ) 1702A changed computing forever. The 1702A is Nonvolatile memory chip and does not lose its contents (program) when power is turned off. RAM (Random Access Memory) used at the time was volatile - the contents was lost every time power was turned off. Being able to program the microcomputer and have the program or data stay resident when power is turned off and on was essential.
The nonvolatile EPROM's made the microcomputer a practical computing machine.
|1702A EPROM Integrated Circuit|
EPROM's did have limitations and were somewhat complex to use - yes it was nonvolatile and could be reprogrammed however;
1. To program the EPROM you had to remove it from the circuit and use a special programming circuit to enter the program.
2. To erase the program or data in the 1702A you had to shine UV light thought the little clear quartz window on the integrated circuit (chip). Looking though the quartz window in the photo of the 1702A you can see the integrated circuit ( the actual silicon chip is called a die ) inside and the little gold wires the take the electrical signals in and out of the die.
These two requirements were somewhat complex however once you had the program finalized and working correctly you could make as many copies of the EPROM as needed, place the chips in the microcomputer and the program would reside for 20 or more years. The EPROM contents did eventually become corrupt as it is not a perfect device however most devices do not need to work more then 20 years unless you are sending them to the moon or outer space. EPROM's were not sent out on space missions - other forms of long term nonvolatile memory were used.
Here is a video I made several years ago about the 1702A EPROM - a nice way to take a look at the Chip.
We are talking about the programs that need to be operational every time the computer is turned on. In the early days that perhaps would be the monitor program that allows the computer to operate -take in information from the keyboard and some kind of display - video monitor or LED lights. If the computer was dedicated to being a controller the program needs to be available and running every time the computer is turned on.
The EPROM made the development of control programs possible because the program could be modified until it was operating as the developer wanted - then it could be duplicated by programming more EPROMS for addtional computers to sell.
The alternative was to load the program each time the computer was powered up or use PROM's. The first microcomputers did not have any fast or easy way to enter programs like floppy disk drives or hard drives. One of the first automated method of entering programs or data was to use a modified audio cassette tape player. This method of data entry was slow and often did not load correctly the first time. The PROM chip is manufactured with the program or data in the chip and it could not be modified by the user. PROM's were expensive to manufacture in small quantities . The PROM was used for producing computers in large quantities and the program or data in the EPROM needed no additional upgrades.
EPROM's were used for only a few years as much better technology soon became available. The EPROM did progress to be much larger then the 250 bytes in the 1702A - I think they made it to megabyte or more in a single EPROM memory chip before the technology became obsolete.
Minicomputers of the 60's and early 70's used ferrite core memory - a nonvolatile memory and is 1000's of times physically larger then EPROM's and was never really considered for microcomputers because of the cost, size and complex circuitry to make it function. The microcomputer needed to use all solid state integrated circuit technology to be miniature and practical and the EPROM was answer at that time. Now we have much more effective memory and don't need the EPROM technology - this was the start of practical microcomputers in 1971.
This ferrite core memory module is about 3 inches square and
contains 1024 bits of information - that is 128 bytes. The electronic circuitry to program and read these ferrite core's is not shown and is very complex. The 1702A EPROM contains 256 bytes of information.
|S100 bus card with EPROM Chips 1978.|
EPROM memory cards were normal in the first generation of microcomputers - like the Altair 8800 microcomputer and most S100 bus computers.
Now - Forty two years after the 1702A EPROM's computer memory technology is very good and we have RAM memory that is nonvolatile and we can be assured the program we entered is ready for us when the computer is turned on. A special use of nonvolitale RAM is the USB memory stick or jump drive - it is programmable - nonvolatile and currently available up to 256G Bytes.
Today the 1702A EPROM is a collectable item and the white ceramic chips with gold pins are selling for $25 to $50 on Ebay.
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