Synopsis: no start up chime from your PowerMac G5? Suspect the SMU. Unplug your machine, leave it sit overnight and try again the next day. The first restart attempt may fail, but the second should work. Good luck! This post explains the details behind this prescription.
My last two posts have dealt with my attempts to recover bootability (if that is a word!) to my late 2005 Power Mac G5, 2.3 GHz dual core (not dual CPU). The symptoms the machine was exhibiting will be familiar to regular readers of this blog: no start up chime and no signs of intelligent life after powering on the machine. The fans would come on, the disks would spin, but nothing else would happen. Eventually, the fans would ramp up to full speed and stay that way.
All along, I had been assuming that the issue lay with the RAM upgrade I was attempting, but as chronicled in my last post, this was definitely not the case. With that safely out of the way, something told me the issue had to be power related. The behavior was too unrelated to anything else I could determine, and did seem at least temporarily correctable by simply unplugging the machine, leaving it to sit and then trying again. In fact, I got the machine “back on the air” today after just such a sequence of events. For whatever reason, just like the last time, it took TWO restart attempts to get the machine to “catch” and boot, but just like last time, on the second attempt it roared back to life and has been stable ever since.
So, what could it be? I recalled that the PowerMac G5, like many of its predecessors, has a power management unit (PMU) on the motherboard, sporting the famous CUDA (Capacitive Unit Discharge ASIC) switch, which allows an externally initiated reset of the PMU. On the late 2005 PowerMac G5, the PMU has been replaced by a newer, but equivalent, microsystem, referred to by Apple as the SMU (the System Management Unit). It still includes the famous motherboard mounted external reset switch:
and Apple helpfully provides full details on how to use it at this page:
A good resource that I have found related to the PMU/SMU and the CUDA reset button is the following page:
The above page contains not just information about the PMU, SMU and CUDA, but also provides links to pages containing instructions for how to reset the PMU or SMU for just about every PowerMac based machine that has one, starting with the eMac and ending with the XServe G5.
Going back to Apple’s page (above), there is a note on it that says that a reset of the SMU may be helpful for many reasons, one of which is the nebulously worded “not starting up” condition. Well, that was definitely the condition I was experiencing, and so this seemed a logical root cause. Somehow, the SMU had to be getting into a bad state and not allowing power to be applied to the CPU after a restart. This fit all the symptoms I was experiencing, and so it was definitely worth digging into.
Regular readers of this blog will recall that I have in fact been faithfully resetting the SMU as I tried various solutions, so why hadn’t this worked? Well, per Apple’s page (above), it turns out that I was doing two things wrong. Firstly, for the late 2005 PowerMac G5, the machine should be plugged in (not running, but plugged in) when you reset the SMU. Secondly, if instead you are going to follow the “unplug the machine” approach, you need to leave it to sit for at least two minutes (…and I am guessing that quite a bit longer is needed!). I was not doing this either. I was opening the machine up, making my changes, pressing the SMU reset button, and immediately closing it up again and applying power. What I wasn’t allowing to happen was a discharge of the capacitor that seems to provide trickle power to the SMU … at least, that is my hunch anyway.
With the above information in hand, it starts to make sense that when I would unplug the machine and leave it overnight, it would accomplish a reset of the SMU, and the machine would then boot again. Why it always seems to take two boot attempts to get it to “catch” and perform a successful boot is a mystery I still haven’t solved, but I think I can live with this relatively small anomoly! 🙂
So, in short, if you have a PowerMac G5 that is powering up but not providing a start up chime, and is not booting, the PMU or SMU (depending on the model of your PowerMac G5) is a likely cause. The field proven approach for my machine has been to unplug it, leave it unplugged overnight, and then reapply power the next day and attempt a restart. If the first restart attempt doesn’t succeed, try a second. If THAT doesn’t succeed, then perhaps this is the wrong solution for YOUR machine’s problem.
This approach has now worked for me twice, and in fact, probably several other times when I have managed to briefly get the machine going again.I am postulating that the root cause is a firmware bug in the SMU, but I have been unable to find any indications on the web that this is a known problem.
In the meantime, my PowerMac G5 is back in the land of the living, and just to make that point, this post is being authored on the self same machine! I know that there seem to be a lot of people out there with similar problems, and I hope that this post may help a few of you to get your PowerMac G5 back on the air.