PMU vs SMU

*The SMU (System Management Unit) is a microcontroller chip on the logic board that controls all power functions for your computer. If your computer is experiencing any issues regarding these functions, resetting the SMU may resolve the issue. The functions controlled by the SMU include:


*The Power Management Unit (PMU) is an integrated circuit (computer chip) typically found on a computer's logic board. As its name implies, the PMU is responsible for managing all aspects of the computer's power. It controls hard disk spin down, sleep and wake, some charging aspects, and how any devices attached to the computer affect sleep.


*Those are both taken from Apple's data base. I did not write them.


Maybe I'm crazy but the two sound exactly like the same thing. So, if I read those two things I see the same thing. I havent ever had to reset either of them so I am not sure what the difference is.

The SMU can be reset on Rev A iMac by opening the back, and pressing a button. That changed with the Rev B/C which allow the SMU to be reset by turning off the computer, unplugging it and restarting, which will automatically reset the SMU. I believe the SMU replaced the functions of the PMU.

Hope this helps,
Miriam

I don't think there is any particular beep to indicate the SMU has been reset, it occurs as a matter of course when you unplug and restart the computer.

Miriam

No beeps or other signs when you reset the SMU, dsjkvf.

One important difference to be aware of between resetting a PMU and resetting an SMU is that a PMU reset will also reset the PRAM/NVRAM. This is NOT the case with an SMU reset. You have to do this seperately with the SMU equipt machines, either by starting up with command option p r keys held down and waiting for the computer to beep three times, or by using the relevant open firmware commands.

This can be important in some troubleshooting situations.


Cheers

Rod

Please be aware that resetting the SMU for the second generation or "rev. B" iMac G5 does not just require you to unplug and then plug your computer back.

You need to hold down the power button simultaneously as you plug the power cord back in. Not doing this will result in an unsuccessful reset.

See this link for more on this:
http://docs.info.apple.com/article.html?artnum=301733

The "second generation iMac" in your profile is referred to here as iMac (Ambient Light Sensor)

You need to hold down the power button simultaneously as you plug the power cord back in. Not doing this will result in an unsuccessful reset.

MMM. Well it says that in the document you mention, d-mo, but elsewhere it indicates that simply removing the power cord for a period will perform the same function. See, for example, http://docs.info.apple.com/article.html?artnum=300908 and http://docs.info.apple.com/article.html?artnum=301283 where it indicates that power cord removal itself is sufficient with the ALS model. http://docs.info.apple.com/article.html?artnum=301733 was actually modified AFTER the iSight iMac came out. The earlier version of this article (when the ALS model was the only one which it applied to) simply indicated the need to remove the power cord.

My suspicion is that pressing the power switch simply speeds up the process by draining residual current. If you wait a bit longer than the suggested "10 seconds" I suspect that the consequences are the same.

I'll see if I can find a definitive answer.

Cheers

Rod

I was not aware of those documents Rod. Thanks for bringing it up.

Strangely enough, the iMac Intel CD Document on resetting the SMC (Yes, that's what it's called on these Models, not SMU) it specifically highlights "making sure the power button is not being pressed at the time".

So it seems to me they were clearly differentiating from the iMac G5 Ambient Light and iSight reset procedure on the link I posted above.

As I have both machines this has been the way I have been differentiating the processes, myself.

Anyway, here's the link to the Intel CD SMC reset for your reference:
http://docs.info.apple.com/article.html?artnum=303446

Yeah, it would be great to get a definitive answer on this...

Yeah, it would be great to get a definitive answer on this...

I've put in a request for clarification of the various imac SMU procedures to the moderators through the level 4 lounge, d-mo.

Might take a while, though. I'll post back if / when I hear anything.

Cheers

Rod

That would be great Rod.

I have been handing out advice to friends, co-workers and even here on these forums, about being particular in pressing the power button as you plug the machine in, as my link above says... Turns out this may potentially be very embarasssing for me (excuse me while I stick my head in a hole...).

Would be nice to know if I should eat my words or continue to give them out.