What does the BIOS of a motherboard interpret as a wake-up signal on the COM port?

Question

I have a motherboard (Gigabyte 990fxa-ud3) that I want to wake up using a ring event on the COM port, but I am unable to do so. I generate the ring event with an atmega32a.

The BIOS settings are properly set as far as I know: ACPI Suspend Type = S3(STR) Modem Ring Resume = Enabled ErP Support = Disabled PME Event Wake Up = Enabled And the COM port is enabled.

I have tried pushing +5V on the Ring Indicator line for 500ms using atmega32a, but this did not do the trick on this motherboard. On a different motherboard (Asus A7V8X-X) this worked flawlessly.

I tried contacting Gigabyte but I did not get a specific answer. I searched for other methods of generating a ring event and found that one could also generate the characters "RING", but this did not work either.

What other methods can I try?

Thanks in advance,

Sandokansan

PS: I hope this is the correct place for this question.

Answer 1

RS-232 defines a logic 1 as -15V to -3V, a logic 0 as +3V to +15V. On a typical MB serial port these are usually around -12V for a 1, and +12V for the 0. If you're lucky, one of the hardware handshake lines in the same port will default to the level needed to activate the ring indicator (RI) input, though not sure if this would necessarily be true with the system in sleep mode.

So, you could try shorting the RI pin to the other pins in the port - try them one at a time, and you might get lucky. If this happens, it shouldn't require more than a couple of components to interface between the atmega328 and the port.

If that fails, you could try using a 9V battery to try waking the port up. Attach the (+) side of the battery to the PC case, and briefly contact the port's RI input to the other battery terminal. If this doesn't work, try the battery the other way. If this scenario succeeds (and the previous method failed), it means your MB doesn't output the levels you need when the machine is off, and you'll need more than just your atmega328's 5V supply. A simple 9V battery and a couple of components should do the trick.

If neither of those two things work, it's probably something with the MB configuration. If you want to rule that out, you could try scrounging an old modem and cabling that to the port, hook the modem to your land line, and ring it with your cell.

Answer 2

It is probably the RS-232 voltage levels which are causing your problem. Logic 1 is a negative voltage, while logic 0 is a positive voltage. Voltages near zero are not considered to be a valid signal.

The board which works may have acted on the transition to a valid signal ( >+3V, logic level 0), but perhaps this board is expecting it to transition to an active logic level one signal (<-3V) before triggering.

The MAX232 and equivalent chips will drive the appropriate RS-232 voltages from 5V supply and logic level inputs.

If all you need is the ring indicator, you could perhaps be more creative without the need for the MAX232.