using memory chip without a CPU

Discussion in 'General Electronics Chat' started by a1pha, Mar 14, 2010.

  1. a1pha

    Thread Starter New Member

    Mar 14, 2010
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    I would like to build a circuit to put values into a RAM chip (like a 2114) using a bank of switches( say- double pole single throw switches with one pole at ground and one pole at Vcc) for address and a bank of switches for data,
    then toggle a write enable, then manually switch to the next address and toggle in the next data, etc.
    then use a counting chip on a slow clock to feed the address lines and put data out to LEDs.
    Some Engineers have told me it's not possible, that you need a CPU to access a RAM chip, but I'm not sure that's true, especially with a static RAM like a 2114.
    so my question: is it possible to enter data into a RAM chip using only switches, and if so, any suggestions about the circuit.

    Hope this isn't too confusing, Thanks.
     
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    Yes, it is absolutely possible!

    Any static RAM will work with switched address & data lines. You will need to use an octal CMOS buffer chip to drive the LEDs from the data i/o (unless you use a RAM that has seperate data in & out).

    Use pullups on your data switches then reasonably high value resistors, something like 47K, to link them to the RAM data lines. That way the switches can drive the RAM (& buffer) when you are writing data to it, but the RAM drive the buffer without fighting the switches when it's reading data out.

    You could use a binary counter to drive the address lines, with increment & reset switches to control the address while feeding in data, or you could use a counter plus another tristate buffer to drive the address while reading, and switches plus resistors to set the write address.

    Show this to your engineer friends - a PDP8 front panel, complete with switches and lights for the operator to manually load it's bootloader program into RAM...
    http://www.oldcomputers.arcula.co.uk/files/images/pdp8301.jpg
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    With static memory chips it is possible.
    You can store some data in the memory using the switches.
    Put the data on the lines and give a store data command.
    (I have done this 30 years ago on school storing my name in the memory, and reading it out showing it on a oscilloscope).

    With dynamic rams it si not possible, becouse these need a refresh cycles.

    Greetings,
    Bertus
     
  4. csh

    New Member

    Mar 13, 2010
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    I quickly glanced at a datasheet for a 2114, and I see no reason why it can't be used in a manner similar to what you've described. When I was in college, a guy essentially did this for a project, he called it "Address Line Selector." They guy basically just clocked in a value, then displayed the data at that address with some LEDs. However, I think he used single pole single throw DIP switches with pull down resistors to provide the inputs. Other types of RAM require refresh cycles and such, which could be the types of memory that other engineers are telling you require a CPU.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    As long as the specs for the read and write cycle is followed any device can do this. Also a human doing as you described
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    I may be wrong, but was this not exactly the method used on the Altair 8800.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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  8. jgessling

    Active Member

    Jul 31, 2009
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    Is there any particular reason you've settled on the 2114? Amongst the classic computer crowd they are known for their high failure rate. They were common in PET, Altair, Heathkit, S-100 systems, Atari, Commodore, pinball machines and lots of other things in the 70's.
     
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