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30212Re: [midatlanticretro] CMOS RAM HM6264LP-15 vs HM6264LP-10

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  • Dave McGuire
    Apr 17, 2013
      On 04/17/2013 11:35 AM, B. Degnan wrote:
      >> 15 nanoseconds is *slower* than 10 nanosecond. Also, the -10 means 100
      > nanosecond in this case. Likewise, -15 means 150 nanosecond. So, your 150
      > nanosecond RAM is slower than the original RAM.
      >> The substitution might work - it just depends on how fast the RAM really
      > needs to be. In any application where, say, you need 200 nanosecond RAM,
      > you can safely substitute 150 or 100 - as long as it's fast enough.
      >> But, it's safe to try - it won't hurt anything if it doesn't work.
      >> What computer is this for?
      > understood, no real way to know unless you try.

      Well, you can, actually. It's pretty easy to calculate the required
      memory access time based on processor datasheet parameters and the clock
      rate. In some cases (like in the case of wait states) the design of the
      memory system is also relevant in the calculation.

      > I did assume 15 was slower
      > too, from the data sheet anyway. Not sure if this was a gold band/
      > silverband resistor analogy and did not really matter in the vintage world
      > compared with newer stuff.

      It does, though. However, RAM chips (and many others) are "binned"
      during production. All of those chips were designed to run at (say)
      100ns. They were all tested, and ones that made the 100ns+margin spec
      were binned as 100ns. Ones that didn't, but did pass for 120ns, were
      binned for THAT speed...etc. Then they're packaged and labeled accordingly.

      In many cases that margin will save you, if you need to get it working
      and you don't have a chip that's fast enough on hand. As Ian said, it's
      safe to try.


      Dave McGuire, AK4HZ
      New Kensington, PA
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