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Thread: Can someone please clarify ECC vs. Parity?

  1. #1

    Default Can someone please clarify ECC vs. Parity?

    Hello All,

    I am looking to buy SIMMs and I am trying to differentiate between parity vs ECC RAM. I understand that parity and ECC use a different checksum calculation code and that ECC can even do corrections so it is more sophisticated. Both also need 9 chips (vs. . So is there physically any difference between ECC and parity SIMMs?

    Put another way is the checksum calculations a function of the SIMM or the mother board? i.e. if I take a SIMM with a parity chip and plug it into a regular motherboard the system does simple parity however if I plug it into an ECC motherboard it will use the extra chip for ECC checksumming? Or must I get a special SIMM with extra ICs on it (aside from the 9 memory chips) to make it work as an ECC SIMM in an ECC motherboard?

    TIA!
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  2. #2
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    The answer is rather complex. First of all, there are 39-bit (32 data + 9 ECC) ECC 72-pin SIMMs and 40-bit (32 data + 10 ECC) ECC 72-pin SIMMs. The latter are only common on IBM RS/6000s AFAIK, but don't quote me. There are also EOS (ECC On SIMM) 72-pin SIMMs which don't present ECC data via the 72-pin socket, appearing as Parity SIMMs to the motherboard. There is a single "presence detect" bit in the 72-pin interface which indicates a true ECC SIMM as opposed to a Parity SIMM.

  3. #3

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    Quote Originally Posted by WBST View Post
    The answer is rather complex. First of all, there are 39-bit (32 data + 9 ECC) ECC 72-pin SIMMs and 40-bit (32 data + 10 ECC) ECC 72-pin SIMMs. The latter are only common on IBM RS/6000s AFAIK, but don't quote me. There are also EOS (ECC On SIMM) 72-pin SIMMs which don't present ECC data via the 72-pin socket, appearing as Parity SIMMs to the motherboard. There is a single "presence detect" bit in the 72-pin interface which indicates a true ECC SIMM as opposed to a Parity SIMM.
    Hi and thanks for the quick response.

    OK that doesn't sound too bad. Assuming 10 ECC is only found on IBM RISC systems one can assume the majority of the systems used 9 ECC and that ECC calculation was handled by the MB. While I can EOS being used to provide ECC on a cheaper motherboards or maybe older motherboards it seems as if it would increase costs greatly per SIMM.

    So now given that how does one go by visually determining if a SIMM has presence detect? I.E. when looking at pics of eBay how can I make sure the advertised ECC part is really ECC and not mis classification by the seller? Or do you actually have to get the SIMM in a system to figure it out? TIA!
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  4. #4
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    Quote Originally Posted by Shadow Lord View Post
    Hi and thanks for the quick response.

    So now given that how does one go by visually determining if a SIMM has presence detect? I.E. when looking at pics of eBay how can I make sure the advertised ECC part is really ECC and not mis classification by the seller? Or do you actually have to get the SIMM in a system to figure it out? TIA!
    Ordinarily it's not obvious, visually, which SIMMs are Non-Parity, Parity, EDO (Extended Data Out), EOS or ECC SIMMs. There are a total of 5 "presence detect" signals, including the ECC indication, 4 for Parity SIMMs. You may see resistors or absence thereof in a group of 4 at one end, or 2 on each side (front and back) which are the Size+Speed definitions - these are different in certain cases for "IBM PS/2" coding and "Industry Standard" coding. They converge around the 8MB 70ns mark.

    8 matching chip SIMMs are often Non-Parity or EDO SIMMs. Parity SIMMs tend to have 8 larger chips plus 4 smaller ones, but there are no hard and fast rules. Have a look here:
    http://ps-2.kev009.com/ohlandl/config/mempresence.html

    Oh, and genuine IBM PS/2 compatible SIMMs will always have gold-plated contacts and should have an "FRU number" which identifies it's Size+Speed. Peter Wendt was, for many years, our guru on the PS/2 systems. He's semi-withdrawn from that now, but his legacy lives on. His PD page is here:
    http://www.mcamafia.de/mcapage0/memorypd.htm
    Last edited by WBST; December 5th, 2019 at 01:37 AM. Reason: Added notes and extra link.

  5. #5

    Default

    Quote Originally Posted by WBST View Post
    Ordinarily it's not obvious, visually, which SIMMs are Non-Parity, Parity, EDO (Extended Data Out), EOS or ECC SIMMs. There are a total of 5 "presence detect" signals, including the ECC indication, 4 for Parity SIMMs. You may see resistors or absence thereof in a group of 4 at one end, or 2 on each side (front and back) which are the Size+Speed definitions - these are different in certain cases for "IBM PS/2" coding and "Industry Standard" coding. They converge around the 8MB 70ns mark.

    8 matching chip SIMMs are often Non-Parity or EDO SIMMs. Parity SIMMs tend to have 8 larger chips plus 4 smaller ones, but there are no hard and fast rules. Have a look here:
    http://ps-2.kev009.com/ohlandl/config/mempresence.html

    Oh, and genuine IBM PS/2 compatible SIMMs will always have gold-plated contacts and should have an "FRU number" which identifies it's Size+Speed. Peter Wendt was, for many years, our guru on the PS/2 systems. He's semi-withdrawn from that now, but his legacy lives on. His PD page is here:
    http://www.mcamafia.de/mcapage0/memorypd.htm
    Hi,

    Thanks again for all the great info! That is what I was afraid of. So basically I have to get the SIMMs here before I can find out if they are ECC. These are actually not for an IBM system. They are for a Compaq Systempro I am currently restoring/rebuilding. The system supports ECC. The Compaq brand sticks are selling for $25 each so at 16 stick a definite no go... Hence my search for cheaper compatible memory!
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  6. #6
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    Quote Originally Posted by Shadow Lord View Post
    Hi,

    These are actually not for an IBM system. They are for a Compaq Systempro I am currently restoring/rebuilding. The system supports ECC. The Compaq brand sticks are selling for $25 each so at 16 stick a definite no go... Hence my search for cheaper compatible memory!
    Well, I have a boxful of NOS IBM FRU 96F9100 16MB ECC SIMMs, if you're interested - asking 5.00 per pair plus P&P.
    I also have an MCA Mafia contact who has the 32MB variety.

  7. #7

    Default

    Quote Originally Posted by WBST View Post
    Well, I have a boxful of NOS IBM FRU 96F9100 16MB ECC SIMMs, if you're interested - asking 5.00 per pair plus P&P.
    I also have an MCA Mafia contact who has the 32MB variety.
    Lets go to PM....
    Current Wish List: 1. IBM 7531 Industrial Series PC 2. NEC MultiSync XL (JC-2001) Monitor 3. MicroSolutions MatchPoint AND/OR UniDOS card 4. Compaq 14" VGA CRT Monitor (the one that came with the SystemPro). 5. Stacker HW CoProcessor Board MCA BUS. If you have any of the above for sale please PM me. Thank you!

  8. #8

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    Quote Originally Posted by WBST View Post
    Well, I have a boxful of NOS IBM FRU 96F9100 16MB ECC SIMMs, if you're interested...
    Do IBM SIMMS work in non-IBM systems? I'm asking because many IBM systems can't use non-IBM SIMMs.
    PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

  9. #9
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    Quote Originally Posted by Stone View Post
    Do IBM SIMMS work in non-IBM systems? I'm asking because many IBM systems can't use non-IBM SIMMs.
    I've no experience of non-IBM servers, so it's up to the user to do the research, from my point of view.

  10. #10

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    When I say non-IBM I mean clones and since IBM memory tends to be proprietary it's not compatible with those clones.

    You offered SL some IBM memory for his Compaq which is what prompted my initial question which amounts to, 'will IBM memory work in a Compaq?'
    PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

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