If I power a motherboard without CPU/RAM, what will happen?

First, I know it won't get through POST. No RAM/CPU make that fairly obvious. There is a 2-digit seven segment debug display on the board. (It's an ASRock Z77 Extreme4.) Can I expect any of the following, if the motherboard is good?

The 2-digit 7-seg display to have some diagnostic code, probably something along the lines of "No CPU"
Power LEDs on the board itself (if they exist) to light?
Whirs or full-action from case fans or PSU fans?

...basically, some sign of life.

I'm looking for some way to isolate and test just the board itself.

The PSU looks okay: shorting the green wire to a ground I was able to power fans, and a DVD drive that seemed to be endlessly resetting itself (probably due to there not being anyone on the other side of the SATA cable).

Also good to know: Would powering it up without a CPU/RAM damage the motherboard? I can't see why it would, but you never know...

motherboard

Thanatos

Posted 2013-02-10T01:38:43.127

Reputation: 1 924

Depending on manufacturer, it probably beeps at you a lot if there's an onboard speaker. – Fiasco Labs – 2015-09-27T03:24:39.647

Typically with missing ram and/or CPU, you'll get system beeps (old school -> 3 beeps for bad/missing memory) – kobaltz – 2013-02-10T01:56:41.697

@kobaltz, yep, with speaker attached or an on-motherboard speaker, the motherboard just complains loudly at you, just not as interestingly as R2D2. – Fiasco Labs – 2013-02-10T02:11:17.313

Well, I tried it with just the main motherboard power cable attached (not the CPU power connector) and got nothing. So I went from there and installed the CPU (and connected the power connector) & a single stick of RAM (twice, with two different ones, as I have two RAM modules), and still nothing. Something is horked, but I don't know what. – Thanatos – 2013-02-10T04:31:01.063

Answers

I'm going to answer this as it happened for me:

The 2-digit 7-seg display to have some diagnostic code, probably something along the lines of "No CPU"

For this specific motherboard, sigh, no such luck.

Power LEDs on the board itself (if they exist) to light?

Again, not that I remember.

Whirs or full-action from case fans or PSU fans?

Yes. The case fans did indeed spin up, and the DVD drive started attempting to spin up the nonexistent disc in it. (and appeared to proceed to put itself in a reset loop, presumably because it couldn't find anyone to talk to.)

One more thing: the purpose of this was to find out if my PSU or my motherboard was at fault, as nothing powered up when connected normally.

Despite getting signs of life from the non-mobo case components attached to the PSU, the PSU was at fault.

The fact that I got signs of life when I shorted the PSU's motherboard connection lead me to believe that the motherboard was at fault. (When the whole thing was connected, nothing powered up.) In my case, this was a mistake.

If you're debugging hardware, I highly recommend seeing a component work with a known-good setup in a full configuration before declaring it good; in my case, I didn't narrow in on the PSU until I swapped it out for a known good PSU, and everything else started functioning perfectly.

Thanatos

Posted 2013-02-10T01:38:43.127

Reputation: 1 924

Booting a motherboard without RAM should cause beep codes.

However, from the few motherboards Ive powered that had no processors, no beep codes were generated.

Keltari

Posted 2013-02-10T01:38:43.127

Reputation: 57 019

1@cpast many multi-cpu motherboards use a "blank" to put in those slots. The blank has a few pins on it that tell the mobo there is no cpu here. – Keltari – 2015-08-15T02:49:35.240

1I've done this with a dual-processor server missing processor 1, and it did give a beep code (this was a server motherboard with a second processor installed, but the manual indicated that it was a generic "no processor installed" code - processor 1 [removed] was apparently mandatory, and processor 2 [installed] couldn't fill its role) – cpast – 2013-02-10T02:56:12.153