didn't see this suggested or maybe it was and I glanced over it but you can initiate the following.
this will try to power down the server as long as your acpi supports it. But yes when your system says it is halted it can be manually powered down.

Running init 0 as root will call rc.0 effectively shutting down the machine but the powerdown problem will be the same as if running poweroff or shutdown -h now had been ran. The problem is usually with a kernel setting which may be fixed by passing a boot time parameter.

If the computer BIOS and power supply do not support APM or ACPI then you will not be able to enjoy automatic power down. I have a box with a 1997 Asus TXP4 motherboard (Socket 7) that supports APM but not ACPI. The power supply does not support either therefore I cannot power down the box automatically. When the screen displays 'system halted' I then manually press the power button.

If you believe the power supply and BIOS support power management, and the box was built before the year 2000, then as previously suggested consider disabling acpi (acpi=off) and use APM. If I recall correctly, however, the stock Slackware huge kernel does not have APM built in. You'll have to recompile the kernel to obtain APM features.

If you are interested in using APM then perhaps the following might help:

Configuring the APM Daemon

1 members found this post helpful.

Am I missing why I can't just use
?

I have a PIII Dell Optiplex GX110 from 1997, though not with me right now, but it should at least support APM. I remember using Slackware >=10 and some other distros on it and automatic shutdown was working.

My Dell XPS P3-800 works just fine under slackware. When I switch off, I just logout, press the power button on the front panel and it runs shutdown for me. When I tried OpenBSD on this same hardware, I was getting the same problem as you describe with slackware. I get the feeling these early APM systems were a bit hit and miss.

Try to find out if your BIOS supports acpi or apm:
dmesg |grep acpi
dmesg |grep apm

I too, would most likely try using apm on an older machine. In that case you'd need to make sure the apm kernel module is loaded before shutting doen with 'poweroff'. I use a home-made shutdown GUI that mdoprobes the apm module before shutting down, if ti isn't already loaded.
You may alos need to adjust your BIOS settings. Some BIOS'es can use either apm or acpi, so you may be able to choose which way to go.

I've been struggling with this matter on one machine (P4, 2005) for quite some time now and haven't solved it so far.
As far as I know, you cannot get APM to work on a SMP-kernel (correct me if I'm wrong). Building my own kernel didn't work either, so I'm curious, has anyone gotten this to be solved by using the non-smp-kernel?

Edit:
Just tried that, no such luck...