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27256 or 27C256 rom chips

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    27256 or 27C256 rom chips

    Greetings again, i'm posting this here as i'm trying to source a few for my AT to update the rom for 1.44mb support. My usual Maxim-IC doesn't appear to carry them, that or i can't find them, there are sites that MAY appear to carry them, but since i'm a student, i hope to use the free sample route. Not much luck though, any suggestions on who still carries them?
    Nothing beats the roar of a 36yr old drive coming to life after a decade in storagg

    #2
    Any particular reason that you don't want to use EEPROM? 28C256s are pretty common.

    But Futurlec has the 27C256s for less than $3 the each.
    Reach me: vcfblackhole _at_ protonmail dot com.

    Comment


      #3
      28c256? didn't know about them, but primarily because my burner is good for the glass window chips only *i think*
      Nothing beats the roar of a 36yr old drive coming to life after a decade in storagg

      Comment


        #4
        Unless it's very old, it should do EEPROMs.
        Reach me: vcfblackhole _at_ protonmail dot com.

        Comment


          #5
          Are the 28c256s pin compatable with the 27c256s? If so, I've wasted a lot of time putting 27c256s under the UV light for erasure.

          Originally posted by Chuck(G) View Post
          Any particular reason that you don't want to use EEPROM? 28C256s are pretty common.

          But Futurlec has the 27C256s for less than $3 the each.

          Comment


            #6
            heh, i scrounged 2 from a dead speedstar 64 and they are under my chip erasure light *yep i spent the cash on one* way back when, so we see how that goes
            Nothing beats the roar of a 36yr old drive coming to life after a decade in storagg

            Comment


              #7
              Originally posted by ibmapc View Post
              Are the 28c256s pin compatable with the 27c256s? If so, I've wasted a lot of time putting 27c256s under the UV light for erasure.
              Close enough--pin 1 on the 28C256 is A14, while on the 27C256 it's Vpp.
              Pin 27 on the 28C is WE/, while on the 27C it's A14.

              Just a simple pin swap--between 1 and 14. Easy enough to work up a simple adapter.
              Reach me: vcfblackhole _at_ protonmail dot com.

              Comment


                #8
                Originally posted by Chuck(G) View Post
                Close enough--pin 1 on the 28C256 is A14, while on the 27C256 it's Vpp.
                Pin 27 on the 28C is WE/, while on the 27C it's A14.

                Just a simple pin swap--between 1 and 14. Easy enough to work up a simple adapter.
                I also compared a datasheet for a 29c256 and found the pins to line up better to the 27c256. One difference I found was that on the 29c256, pin 14 was labeled GND but on the 27c256 it's VSS. Is GND the same as VSS in this case? I'm also assuming that we don't care about WE since we won't be programming it in place? I know that the 29c256s are getting kind of rare, but I think I found an EBAY seller that I can work with. He sells them in quantities of 10 for 39.99 but I'm trying to talk him in to selling two for a simalar per piece price.

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                  #9
                  This is another possible solution.

                  Comment


                    #10
                    For most chips, Vss = Gnd; Vdd = Vcc.

                    The issue is mostly one of terminology. A FET is souce-gate-drain, so Vss, Vdd; A junction transistor is emitter-collector-base, so Vee (usually labeled GND) and Vcc.

                    Regardless, you want a logic high on WE/ so your data isn't inadvertently overwritten.
                    Reach me: vcfblackhole _at_ protonmail dot com.

                    Comment


                      #11
                      Originally posted by Chuck(G) View Post
                      For most chips, Vss = Gnd; Vdd = Vcc.

                      The issue is mostly one of terminology. A FET is souce-gate-drain, so Vss, Vdd; A junction transistor is emitter-collector-base, so Vee (usually labeled GND) and Vcc.

                      Regardless, you want a logic high on WE/ so your data isn't inadvertently overwritten.
                      So... Do ya think the 29c256 will work in place of the 27c256? I don't quite follow about "logic high on WE/"

                      Comment


                        #12
                        The 29C256 should work without any socket mods. Just check pin 1 on the socket with a meter to make sure that it reads somewhere around +5. Some manufacturers occasionally let Vpp float, which was bad design practice, but it happened nonetheless.
                        Reach me: vcfblackhole _at_ protonmail dot com.

                        Comment


                          #13
                          Originally posted by Chuck(G) View Post
                          The 29C256 should work without any socket mods. Just check pin 1 on the socket with a meter to make sure that it reads somewhere around +5. Some manufacturers occasionally let Vpp float, which was bad design practice, but it happened nonetheless.
                          Socket is U18 and U19 on IBM 5160 64-256K Mother board. Do you think IBM did it right?

                          Comment


                            #14
                            Originally posted by ibmapc View Post
                            Socket is U18 and U19 on IBM 5160 64-256K Mother board. Do you think IBM did it right?
                            IBM never intended that EPROMs would be used. According to the schematic that I have, pin 1 is connected to pin 27 (check this with an ohmmeter) and labeled A14. This isn't good. You'll have to lift pin 1 of the EEPROM from the socket and tie it to pin 28. Again, not a difficult mod.
                            Reach me: vcfblackhole _at_ protonmail dot com.

                            Comment


                              #15
                              ...or use an add-in ROM card....

                              Comment

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