reboot after manual shutdown

[Anonymous user]

Hi,

I have a system which includes a SMT1500RU. The main switch is providing power to the UPS, which then protects a PC.

If I cut the power to the UPS, it switches to battery mode and correctly shutsdown the PC after 60 sec. So far so good.

If I leave the main switch on, shut down PC manually (windowsà shutdown) and then cut the power, the UPS goes also into battery mode. Since the PC is already down no further steps are required.

No here is the problem:

If I now switch the main power back on, the UPS goes to utility mode , BUT  it will not reboot the PC. I have to go to the UPS and manually switch on the PC (SMT1500: press ON button, reboot with delay.) Then the PC starts. So the PC understands the reboot command from the UPS, but this process is not triggered automatically when the main power comes back on. I made sure the BIOS settings is set to “always on”.

My question: Is the UPS able to reboot the PC, after the power comes back on AND the PC was previously shutdown manually?

Thanks

[BillP]

Hi,

A UPS cannot restart a computer when it is powered off. The BIOS is responsible for restarting the computer. Assuming the UPS is cutting output to the outlets during the outage when AC is restored the UPS will re-energize the outlets. At that point the BIOS would be responsible for powering the PC on. 

[Anonymous user]

Thank you for following up.

So if all settings are correct in the BIOS of the PC, what exactly makes the BIOS to reboot?

The re-energizing of the outlets of the UPS (rising edge of input voltage, off to on) 

or 

a wake up signal over the USB connection (UPS-->PC) ? Does the UPS send wake-up signals via USB?

Patrick

[BillP]

Hi,

On 9/5/2019 1:35 PM, Thomas said:

So if all settings are correct in the BIOS of the PC, what exactly makes the BIOS to reboot?

The re-energizing of the outlets of the UPS (rising edge of input voltage, off to on) 

When the BIOS see that AC has been restored it restart the OS.

On 9/5/2019 1:35 PM, Thomas said:

a wake up signal over the USB connection (UPS-->PC) ? Does the UPS send wake-up signals via USB?

The UPS does not send a signal through USB to wake the OS. 

[Brad_C · Accepted solution]

On 5/9/2019 3:48 AM, Thomas said:

If I now switch the main power back on, the UPS goes to utility mode , BUT  it will not reboot the PC. I have to go to the UPS and manually switch on the PC (SMT1500: press ON button, reboot with delay.) Then the PC starts. So the PC understands the reboot command from the UPS, but this process is not triggered automatically when the main power comes back on. I made sure the BIOS settings is set to “always on”.

This is always a bit of a conundrum. A PC can be configured to boot up on power up.

Is this what is happening ?

Mains fails. UPS goes onto battery. UPS signals PC to shut down. PC shuts down. UPS is still on battery. Mains comes back. UPS comes off battery. PC does not power back up.

The issue is unless the UPS goes flat or is programmed to forcibly disconnect the load after the PC shuts down, it never actually removes power from the PC supply. So the PC has no idea the mains is back. As far as it is concerned it was never without power.

By forcing a reboot from the panel on the UPS, you are physically power cycling the PC.

There can be that "window of opportunity" between when the PC shuts down and the UPS shuts down where the mains comes back and leaves you in that intermediate state where the UPS is still going but the PC is shutdown.

Most management applications can be configured to mitigate this but you need to get an understanding of the issue first.

[vannucchi.giac]

Very interesting indeed. Because power is never cut, the PC's "Power On after power failure" BIOS setting never triggers.

It's ironic that if the current comes back early enough for the UPS to switch back online, then you have a worst outcome (based on this use case) than when the UPS completely shuts down.

Can I ask you what do you mean by "Most management applications can be configured to mitigate this" ? Is there a way to configure the UPS to "forcibly disconnect the load after the PC shuts down" as you mentioned?

I have APC Back-UPS BX950MI

Thanks