I have a IIsi with a very dead psu. So I am looking to replace it with a pico atx. My main question is will any 120vac pico atx work? do I need anything special? I do still have the apple connector but I just need to depin it.
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pico atx Mac IIsi
- Thread starter markgeek36
- Start date
hi mate, the IIsi has software start, CayMac offers a board to do the trick. Others here can describe a “do it yourself” board,
its not software start its a 5v standby that the little daughterboard generates when you press the power button on the keyboard. afaik a generic atx 5v standby does the samething?
not totally true; the power state is latched on the logic board and that latch is latched *off* by software, it's soft shutdown.
the polarity for the "power on" signal is opposite. that's what the transistor on the caymac board (or in other schematics you'll see) is doing, it's acting as an inverter.
"soft" power not software. its a board inside of the power supply that does it called trickle 5v. this senses when you press the button and tells the psu to turn on. at this point the computer is off so no software can run.
Just asserting things doesn't make them more true, you know. It is more complicated than you are saying. Before you start lecturing other people on how this works, I strongly suggest you read up on how power control works in ATX PSUs, and how the II series power control circuitry works (see, for example, here: https://www.downtowndougbrown.com/2015/03/explanation-of-the-macintosh-iiiix-power-onoff-circuit/)
In old Macs, to keep the computer on, the "power on" line to the PSU needs to be pulled high and kept high. When you press the button, that line is pulled high - the daughterboard does not "sense" anything - and then that line is kept high by a latch on the logic board. When you then shut the machine down, that latch is deasserted by means of a line coming out of the VIA, and the power supply turns off. You will note that the "turning off" part of this is absolutely under software control. There is no latching done in the PSU, either in ATX or in the Mac PSU interface.
The 5v trickle from the PSU is useful but not strictly relevant here. Some earlier Mac II models, for example, generated the "power on" signal from batteries - see the previous link.
The reason why you can't just wire an ATX PSU's soft power up to your IIsi, as people upthread have been telling you, is because the ATX "power on" signal is pulled *down*, whereas the Mac "power on" signal is pulled *up*. So you need something to invert the signal coming from the logic board to the power supply.
If you look at the caymac board, you will see a transistor labelled Q1. That is what that transistor is for. When its base is pulled high, it pulls the ATX-side "power on" signal low, turning the PSU on. When the computer is shut down and the latch is deasserted, it stops pulling the ATX line low, and the power supply turns off.
This is why you can't just wire up an ATX PSU to a Mac without that inverter, if you want power control to work. If you don't care about power control, fine - but the power button won't do anything at all, you'll have to unplug it from the mains in order to turn it off.
In old Macs, to keep the computer on, the "power on" line to the PSU needs to be pulled high and kept high. When you press the button, that line is pulled high - the daughterboard does not "sense" anything - and then that line is kept high by a latch on the logic board. When you then shut the machine down, that latch is deasserted by means of a line coming out of the VIA, and the power supply turns off. You will note that the "turning off" part of this is absolutely under software control. There is no latching done in the PSU, either in ATX or in the Mac PSU interface.
The 5v trickle from the PSU is useful but not strictly relevant here. Some earlier Mac II models, for example, generated the "power on" signal from batteries - see the previous link.
The reason why you can't just wire an ATX PSU's soft power up to your IIsi, as people upthread have been telling you, is because the ATX "power on" signal is pulled *down*, whereas the Mac "power on" signal is pulled *up*. So you need something to invert the signal coming from the logic board to the power supply.
If you look at the caymac board, you will see a transistor labelled Q1. That is what that transistor is for. When its base is pulled high, it pulls the ATX-side "power on" signal low, turning the PSU on. When the computer is shut down and the latch is deasserted, it stops pulling the ATX line low, and the power supply turns off.
This is why you can't just wire up an ATX PSU to a Mac without that inverter, if you want power control to work. If you don't care about power control, fine - but the power button won't do anything at all, you'll have to unplug it from the mains in order to turn it off.
soft start, apologies
FYI
FYI
