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Do AT motherboards (286, 386, 486) really need -5V (besides redirecting it to ISA connectors)?

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Do AT motherboards (286, 386, 486) really need -5V (besides redirecting it to ISA connectors)?


Can a standard ATX power supply be used on a Macintosh Performa 476?






.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty margin-bottom:0;








7















The power supply of my 386DX computer (an unknown brand M320 motherboard) is not trustworthy anymore, so I'm planning to use an ATX power supply, plus an ATX to AT adapter for P8-P9 connectors.



The thing is that AT power suuplies provide +5V, +12V, -5V and -12V. ATX power supplies, until not much time ago, supplied the same outputs, plus 3.3V, but it seems that the -5V output has phased out, and some ATX supplies mark that pin as NC (not connected).



So when it comes to chosing the right ATX power supply (otoh, one that does not have any fans in the upper side, but in the back side, because ATX supplies are mounted upside down in an AT case and the fan faces the upper side of the case which is most ofently closed) it's hard to tell if the ATX connector includes the white cable that should carry -5V.



So I would like to know if AT-class motherboards need that supply for its internal things (chipset, CPU, SuperIO, etc). I can say that the very usual UM82C206L chip, used to implement most of the AT system chips (DMA, RTC, PIC, Timer, etc) does use only +5V.










share|improve this question
























  • -5V on ATX supplies was phased out almost 20 years ago now.

    – Ross Ridge
    6 hours ago

















7















The power supply of my 386DX computer (an unknown brand M320 motherboard) is not trustworthy anymore, so I'm planning to use an ATX power supply, plus an ATX to AT adapter for P8-P9 connectors.



The thing is that AT power suuplies provide +5V, +12V, -5V and -12V. ATX power supplies, until not much time ago, supplied the same outputs, plus 3.3V, but it seems that the -5V output has phased out, and some ATX supplies mark that pin as NC (not connected).



So when it comes to chosing the right ATX power supply (otoh, one that does not have any fans in the upper side, but in the back side, because ATX supplies are mounted upside down in an AT case and the fan faces the upper side of the case which is most ofently closed) it's hard to tell if the ATX connector includes the white cable that should carry -5V.



So I would like to know if AT-class motherboards need that supply for its internal things (chipset, CPU, SuperIO, etc). I can say that the very usual UM82C206L chip, used to implement most of the AT system chips (DMA, RTC, PIC, Timer, etc) does use only +5V.










share|improve this question
























  • -5V on ATX supplies was phased out almost 20 years ago now.

    – Ross Ridge
    6 hours ago













7












7








7


1






The power supply of my 386DX computer (an unknown brand M320 motherboard) is not trustworthy anymore, so I'm planning to use an ATX power supply, plus an ATX to AT adapter for P8-P9 connectors.



The thing is that AT power suuplies provide +5V, +12V, -5V and -12V. ATX power supplies, until not much time ago, supplied the same outputs, plus 3.3V, but it seems that the -5V output has phased out, and some ATX supplies mark that pin as NC (not connected).



So when it comes to chosing the right ATX power supply (otoh, one that does not have any fans in the upper side, but in the back side, because ATX supplies are mounted upside down in an AT case and the fan faces the upper side of the case which is most ofently closed) it's hard to tell if the ATX connector includes the white cable that should carry -5V.



So I would like to know if AT-class motherboards need that supply for its internal things (chipset, CPU, SuperIO, etc). I can say that the very usual UM82C206L chip, used to implement most of the AT system chips (DMA, RTC, PIC, Timer, etc) does use only +5V.










share|improve this question














The power supply of my 386DX computer (an unknown brand M320 motherboard) is not trustworthy anymore, so I'm planning to use an ATX power supply, plus an ATX to AT adapter for P8-P9 connectors.



The thing is that AT power suuplies provide +5V, +12V, -5V and -12V. ATX power supplies, until not much time ago, supplied the same outputs, plus 3.3V, but it seems that the -5V output has phased out, and some ATX supplies mark that pin as NC (not connected).



So when it comes to chosing the right ATX power supply (otoh, one that does not have any fans in the upper side, but in the back side, because ATX supplies are mounted upside down in an AT case and the fan faces the upper side of the case which is most ofently closed) it's hard to tell if the ATX connector includes the white cable that should carry -5V.



So I would like to know if AT-class motherboards need that supply for its internal things (chipset, CPU, SuperIO, etc). I can say that the very usual UM82C206L chip, used to implement most of the AT system chips (DMA, RTC, PIC, Timer, etc) does use only +5V.







power pc-at






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 9 hours ago









mcleod_ideafixmcleod_ideafix

12.1k1 gold badge42 silver badges75 bronze badges




12.1k1 gold badge42 silver badges75 bronze badges















  • -5V on ATX supplies was phased out almost 20 years ago now.

    – Ross Ridge
    6 hours ago

















  • -5V on ATX supplies was phased out almost 20 years ago now.

    – Ross Ridge
    6 hours ago
















-5V on ATX supplies was phased out almost 20 years ago now.

– Ross Ridge
6 hours ago





-5V on ATX supplies was phased out almost 20 years ago now.

– Ross Ridge
6 hours ago










3 Answers
3






active

oldest

votes


















4













Anecdotal answer, because I don't feel like googling for lots of schematics:



  • Most AT mainboards only use +5V.

  • In some cases, +12V is used for miscelleanous stuff, I remember an auto-voltage mainboard where the +5V/+3.45V switch for the 80486 processor used the voltage detection pin, and controlled the +5V Vcc connection using a FET that got its gate drive from +12V (via a resistor; pulled down if a 3.45V cpu is installed).

  • If a board has onboard serial ports, the 1488/1489 level shifters also use +/-12V.

  • You should nearly always be fine with missing -5V.





share|improve this answer


































    3













    Looking at the three ATX PSUs I have to hand, all three of them have a bottom-mounted fan, and only one (the oldest, least efficient and noisiest) provides a -5V line. That's a Nexus NX-3500.



    One of the others is actually an SFX-L form factor, which mounts to an ATX case using an adapter plate which leaves a significant gap on all sides relative to a full-size ATX PSU. This may expand your options somewhat, especially as the SFX-L mounting bolts are symmetric, allowing you to mount it with the fan inwards in free air.



    enter image description here



    To fit an ATX PSU to an AT m/board, you'll need to insert an adapter between the plug and socket on the low-voltage side. You could take advantage of this to derive the -5V supply from the -12V one, as not much current is carried on that line to begin with. A 7905-series linear regulator should do the trick; just add a small heatsink on top and a bypass capacitor on each side.






    share|improve this answer



























    • Oh! Just to clarify: when I said "one that does not have any fans in the upper side" I meant the upper side of an AT case, as ATX supplies have to be mounted upside down so the normally bottom faced fan is left facing up. Most ATX supplies I've seen, including the one I have as spare, have a bottom-mounted fan.

      – mcleod_ideafix
      8 hours ago











    • I think SFX-L still solves that part of your problem. I've expanded the answer to explain why.

      – Chromatix
      8 hours ago






    • 2





      As fot he 7905, yes, I considered that. Once I needed to service an ATX supply with a not standard form factor, so finding a replacement was hard and I wanted to give the old one a try. As I opened it, I quickly noticed the footprint of a 7905 regulator in the silk screen, obviosuly to add the -5V which this particular power supply didn't have (but it had the white wire).

      – mcleod_ideafix
      8 hours ago


















    0













    Engineering is Driven by Specifications



    In general, it is always dicey to not implement the full breadth of a spec.



    This is engineering, not magic or wish.



    In the absence of other information, you must assume that all voltages in the spec are required.



    Your Board May Be an Exception



    For a particular AT motherboard, one could examine the schematic or trace the -5 V pin to be sure it doesn't connect with anything.



    Seeming to Work Does Not Equal Actually Working



    The -5 V rail may not be needed at all, or it may only be needed for functions you aren't currently using. Any particular motherboard may or may not require the negative voltage. Add-on cards may or may not require the negative voltage.






    share|improve this answer

























    • While all statements in your answer are true it doesn't answer the question. The question was about the actual necessarity of -5V for the main board. Mcload excluded the need of additional cards. You may want to improve your answer regarding this point.

      – harper
      21 secs ago













    Your Answer








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    3 Answers
    3






    active

    oldest

    votes








    3 Answers
    3






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    4













    Anecdotal answer, because I don't feel like googling for lots of schematics:



    • Most AT mainboards only use +5V.

    • In some cases, +12V is used for miscelleanous stuff, I remember an auto-voltage mainboard where the +5V/+3.45V switch for the 80486 processor used the voltage detection pin, and controlled the +5V Vcc connection using a FET that got its gate drive from +12V (via a resistor; pulled down if a 3.45V cpu is installed).

    • If a board has onboard serial ports, the 1488/1489 level shifters also use +/-12V.

    • You should nearly always be fine with missing -5V.





    share|improve this answer































      4













      Anecdotal answer, because I don't feel like googling for lots of schematics:



      • Most AT mainboards only use +5V.

      • In some cases, +12V is used for miscelleanous stuff, I remember an auto-voltage mainboard where the +5V/+3.45V switch for the 80486 processor used the voltage detection pin, and controlled the +5V Vcc connection using a FET that got its gate drive from +12V (via a resistor; pulled down if a 3.45V cpu is installed).

      • If a board has onboard serial ports, the 1488/1489 level shifters also use +/-12V.

      • You should nearly always be fine with missing -5V.





      share|improve this answer





























        4












        4








        4







        Anecdotal answer, because I don't feel like googling for lots of schematics:



        • Most AT mainboards only use +5V.

        • In some cases, +12V is used for miscelleanous stuff, I remember an auto-voltage mainboard where the +5V/+3.45V switch for the 80486 processor used the voltage detection pin, and controlled the +5V Vcc connection using a FET that got its gate drive from +12V (via a resistor; pulled down if a 3.45V cpu is installed).

        • If a board has onboard serial ports, the 1488/1489 level shifters also use +/-12V.

        • You should nearly always be fine with missing -5V.





        share|improve this answer















        Anecdotal answer, because I don't feel like googling for lots of schematics:



        • Most AT mainboards only use +5V.

        • In some cases, +12V is used for miscelleanous stuff, I remember an auto-voltage mainboard where the +5V/+3.45V switch for the 80486 processor used the voltage detection pin, and controlled the +5V Vcc connection using a FET that got its gate drive from +12V (via a resistor; pulled down if a 3.45V cpu is installed).

        • If a board has onboard serial ports, the 1488/1489 level shifters also use +/-12V.

        • You should nearly always be fine with missing -5V.






        share|improve this answer














        share|improve this answer



        share|improve this answer








        edited 7 hours ago









        Renan

        1255 bronze badges




        1255 bronze badges










        answered 9 hours ago









        Michael KarcherMichael Karcher

        3105 bronze badges




        3105 bronze badges


























            3













            Looking at the three ATX PSUs I have to hand, all three of them have a bottom-mounted fan, and only one (the oldest, least efficient and noisiest) provides a -5V line. That's a Nexus NX-3500.



            One of the others is actually an SFX-L form factor, which mounts to an ATX case using an adapter plate which leaves a significant gap on all sides relative to a full-size ATX PSU. This may expand your options somewhat, especially as the SFX-L mounting bolts are symmetric, allowing you to mount it with the fan inwards in free air.



            enter image description here



            To fit an ATX PSU to an AT m/board, you'll need to insert an adapter between the plug and socket on the low-voltage side. You could take advantage of this to derive the -5V supply from the -12V one, as not much current is carried on that line to begin with. A 7905-series linear regulator should do the trick; just add a small heatsink on top and a bypass capacitor on each side.






            share|improve this answer



























            • Oh! Just to clarify: when I said "one that does not have any fans in the upper side" I meant the upper side of an AT case, as ATX supplies have to be mounted upside down so the normally bottom faced fan is left facing up. Most ATX supplies I've seen, including the one I have as spare, have a bottom-mounted fan.

              – mcleod_ideafix
              8 hours ago











            • I think SFX-L still solves that part of your problem. I've expanded the answer to explain why.

              – Chromatix
              8 hours ago






            • 2





              As fot he 7905, yes, I considered that. Once I needed to service an ATX supply with a not standard form factor, so finding a replacement was hard and I wanted to give the old one a try. As I opened it, I quickly noticed the footprint of a 7905 regulator in the silk screen, obviosuly to add the -5V which this particular power supply didn't have (but it had the white wire).

              – mcleod_ideafix
              8 hours ago















            3













            Looking at the three ATX PSUs I have to hand, all three of them have a bottom-mounted fan, and only one (the oldest, least efficient and noisiest) provides a -5V line. That's a Nexus NX-3500.



            One of the others is actually an SFX-L form factor, which mounts to an ATX case using an adapter plate which leaves a significant gap on all sides relative to a full-size ATX PSU. This may expand your options somewhat, especially as the SFX-L mounting bolts are symmetric, allowing you to mount it with the fan inwards in free air.



            enter image description here



            To fit an ATX PSU to an AT m/board, you'll need to insert an adapter between the plug and socket on the low-voltage side. You could take advantage of this to derive the -5V supply from the -12V one, as not much current is carried on that line to begin with. A 7905-series linear regulator should do the trick; just add a small heatsink on top and a bypass capacitor on each side.






            share|improve this answer



























            • Oh! Just to clarify: when I said "one that does not have any fans in the upper side" I meant the upper side of an AT case, as ATX supplies have to be mounted upside down so the normally bottom faced fan is left facing up. Most ATX supplies I've seen, including the one I have as spare, have a bottom-mounted fan.

              – mcleod_ideafix
              8 hours ago











            • I think SFX-L still solves that part of your problem. I've expanded the answer to explain why.

              – Chromatix
              8 hours ago






            • 2





              As fot he 7905, yes, I considered that. Once I needed to service an ATX supply with a not standard form factor, so finding a replacement was hard and I wanted to give the old one a try. As I opened it, I quickly noticed the footprint of a 7905 regulator in the silk screen, obviosuly to add the -5V which this particular power supply didn't have (but it had the white wire).

              – mcleod_ideafix
              8 hours ago













            3












            3








            3







            Looking at the three ATX PSUs I have to hand, all three of them have a bottom-mounted fan, and only one (the oldest, least efficient and noisiest) provides a -5V line. That's a Nexus NX-3500.



            One of the others is actually an SFX-L form factor, which mounts to an ATX case using an adapter plate which leaves a significant gap on all sides relative to a full-size ATX PSU. This may expand your options somewhat, especially as the SFX-L mounting bolts are symmetric, allowing you to mount it with the fan inwards in free air.



            enter image description here



            To fit an ATX PSU to an AT m/board, you'll need to insert an adapter between the plug and socket on the low-voltage side. You could take advantage of this to derive the -5V supply from the -12V one, as not much current is carried on that line to begin with. A 7905-series linear regulator should do the trick; just add a small heatsink on top and a bypass capacitor on each side.






            share|improve this answer















            Looking at the three ATX PSUs I have to hand, all three of them have a bottom-mounted fan, and only one (the oldest, least efficient and noisiest) provides a -5V line. That's a Nexus NX-3500.



            One of the others is actually an SFX-L form factor, which mounts to an ATX case using an adapter plate which leaves a significant gap on all sides relative to a full-size ATX PSU. This may expand your options somewhat, especially as the SFX-L mounting bolts are symmetric, allowing you to mount it with the fan inwards in free air.



            enter image description here



            To fit an ATX PSU to an AT m/board, you'll need to insert an adapter between the plug and socket on the low-voltage side. You could take advantage of this to derive the -5V supply from the -12V one, as not much current is carried on that line to begin with. A 7905-series linear regulator should do the trick; just add a small heatsink on top and a bypass capacitor on each side.







            share|improve this answer














            share|improve this answer



            share|improve this answer








            edited 8 hours ago

























            answered 8 hours ago









            ChromatixChromatix

            1,2479 silver badges8 bronze badges




            1,2479 silver badges8 bronze badges















            • Oh! Just to clarify: when I said "one that does not have any fans in the upper side" I meant the upper side of an AT case, as ATX supplies have to be mounted upside down so the normally bottom faced fan is left facing up. Most ATX supplies I've seen, including the one I have as spare, have a bottom-mounted fan.

              – mcleod_ideafix
              8 hours ago











            • I think SFX-L still solves that part of your problem. I've expanded the answer to explain why.

              – Chromatix
              8 hours ago






            • 2





              As fot he 7905, yes, I considered that. Once I needed to service an ATX supply with a not standard form factor, so finding a replacement was hard and I wanted to give the old one a try. As I opened it, I quickly noticed the footprint of a 7905 regulator in the silk screen, obviosuly to add the -5V which this particular power supply didn't have (but it had the white wire).

              – mcleod_ideafix
              8 hours ago

















            • Oh! Just to clarify: when I said "one that does not have any fans in the upper side" I meant the upper side of an AT case, as ATX supplies have to be mounted upside down so the normally bottom faced fan is left facing up. Most ATX supplies I've seen, including the one I have as spare, have a bottom-mounted fan.

              – mcleod_ideafix
              8 hours ago











            • I think SFX-L still solves that part of your problem. I've expanded the answer to explain why.

              – Chromatix
              8 hours ago






            • 2





              As fot he 7905, yes, I considered that. Once I needed to service an ATX supply with a not standard form factor, so finding a replacement was hard and I wanted to give the old one a try. As I opened it, I quickly noticed the footprint of a 7905 regulator in the silk screen, obviosuly to add the -5V which this particular power supply didn't have (but it had the white wire).

              – mcleod_ideafix
              8 hours ago
















            Oh! Just to clarify: when I said "one that does not have any fans in the upper side" I meant the upper side of an AT case, as ATX supplies have to be mounted upside down so the normally bottom faced fan is left facing up. Most ATX supplies I've seen, including the one I have as spare, have a bottom-mounted fan.

            – mcleod_ideafix
            8 hours ago





            Oh! Just to clarify: when I said "one that does not have any fans in the upper side" I meant the upper side of an AT case, as ATX supplies have to be mounted upside down so the normally bottom faced fan is left facing up. Most ATX supplies I've seen, including the one I have as spare, have a bottom-mounted fan.

            – mcleod_ideafix
            8 hours ago













            I think SFX-L still solves that part of your problem. I've expanded the answer to explain why.

            – Chromatix
            8 hours ago





            I think SFX-L still solves that part of your problem. I've expanded the answer to explain why.

            – Chromatix
            8 hours ago




            2




            2





            As fot he 7905, yes, I considered that. Once I needed to service an ATX supply with a not standard form factor, so finding a replacement was hard and I wanted to give the old one a try. As I opened it, I quickly noticed the footprint of a 7905 regulator in the silk screen, obviosuly to add the -5V which this particular power supply didn't have (but it had the white wire).

            – mcleod_ideafix
            8 hours ago





            As fot he 7905, yes, I considered that. Once I needed to service an ATX supply with a not standard form factor, so finding a replacement was hard and I wanted to give the old one a try. As I opened it, I quickly noticed the footprint of a 7905 regulator in the silk screen, obviosuly to add the -5V which this particular power supply didn't have (but it had the white wire).

            – mcleod_ideafix
            8 hours ago











            0













            Engineering is Driven by Specifications



            In general, it is always dicey to not implement the full breadth of a spec.



            This is engineering, not magic or wish.



            In the absence of other information, you must assume that all voltages in the spec are required.



            Your Board May Be an Exception



            For a particular AT motherboard, one could examine the schematic or trace the -5 V pin to be sure it doesn't connect with anything.



            Seeming to Work Does Not Equal Actually Working



            The -5 V rail may not be needed at all, or it may only be needed for functions you aren't currently using. Any particular motherboard may or may not require the negative voltage. Add-on cards may or may not require the negative voltage.






            share|improve this answer

























            • While all statements in your answer are true it doesn't answer the question. The question was about the actual necessarity of -5V for the main board. Mcload excluded the need of additional cards. You may want to improve your answer regarding this point.

              – harper
              21 secs ago















            0













            Engineering is Driven by Specifications



            In general, it is always dicey to not implement the full breadth of a spec.



            This is engineering, not magic or wish.



            In the absence of other information, you must assume that all voltages in the spec are required.



            Your Board May Be an Exception



            For a particular AT motherboard, one could examine the schematic or trace the -5 V pin to be sure it doesn't connect with anything.



            Seeming to Work Does Not Equal Actually Working



            The -5 V rail may not be needed at all, or it may only be needed for functions you aren't currently using. Any particular motherboard may or may not require the negative voltage. Add-on cards may or may not require the negative voltage.






            share|improve this answer

























            • While all statements in your answer are true it doesn't answer the question. The question was about the actual necessarity of -5V for the main board. Mcload excluded the need of additional cards. You may want to improve your answer regarding this point.

              – harper
              21 secs ago













            0












            0








            0







            Engineering is Driven by Specifications



            In general, it is always dicey to not implement the full breadth of a spec.



            This is engineering, not magic or wish.



            In the absence of other information, you must assume that all voltages in the spec are required.



            Your Board May Be an Exception



            For a particular AT motherboard, one could examine the schematic or trace the -5 V pin to be sure it doesn't connect with anything.



            Seeming to Work Does Not Equal Actually Working



            The -5 V rail may not be needed at all, or it may only be needed for functions you aren't currently using. Any particular motherboard may or may not require the negative voltage. Add-on cards may or may not require the negative voltage.






            share|improve this answer













            Engineering is Driven by Specifications



            In general, it is always dicey to not implement the full breadth of a spec.



            This is engineering, not magic or wish.



            In the absence of other information, you must assume that all voltages in the spec are required.



            Your Board May Be an Exception



            For a particular AT motherboard, one could examine the schematic or trace the -5 V pin to be sure it doesn't connect with anything.



            Seeming to Work Does Not Equal Actually Working



            The -5 V rail may not be needed at all, or it may only be needed for functions you aren't currently using. Any particular motherboard may or may not require the negative voltage. Add-on cards may or may not require the negative voltage.







            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered 1 hour ago









            cmmcmm

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            • While all statements in your answer are true it doesn't answer the question. The question was about the actual necessarity of -5V for the main board. Mcload excluded the need of additional cards. You may want to improve your answer regarding this point.

              – harper
              21 secs ago

















            • While all statements in your answer are true it doesn't answer the question. The question was about the actual necessarity of -5V for the main board. Mcload excluded the need of additional cards. You may want to improve your answer regarding this point.

              – harper
              21 secs ago
















            While all statements in your answer are true it doesn't answer the question. The question was about the actual necessarity of -5V for the main board. Mcload excluded the need of additional cards. You may want to improve your answer regarding this point.

            – harper
            21 secs ago





            While all statements in your answer are true it doesn't answer the question. The question was about the actual necessarity of -5V for the main board. Mcload excluded the need of additional cards. You may want to improve your answer regarding this point.

            – harper
            21 secs ago

















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