I'd like to know the answer to this too. My board is an S5520SC and I have the same issue. If it's 1-wire, would we expect the mainboard to pull the bus low at some point during initialization? If I look at the "1-wire temperature sensor" pin on the front panel header, it's constantly 3.3V, even when running the SDR update script, which probes for the presence of the temp sensor. If it's looking for an analog thermistor, it also doesn't work with any value of resistor I've tried.
To further confuse, if it is 1-wire, it's likely either a DS18S20 or DS18B20. However, looking in the HC_BMC.sdr file in the BIOS update package, the temperature values/curve defined there don't seem to correlate with either device:
19 // Nominal Reading (19h = 25 degrees C) 28 // Normal Maximum (28h = 40 degrees C) 0A // Normal Minimum (0Ah = 10 degrees C)
Thanks for any help.
The support group just tried to sell me one of the front panel things
However, I've discovered a few things regarding the SDR update. If I select "OTHER" chassis then, instead of taking its temperature measurements from the front panel sensor (which is of course absent on our machines), it uses the on-board sensor.
I then ran into an issue because I'm using much larger CPU coolers, and any given PWM value translates to lower RPMs. If the RPMs go below a predefined level, the system turns all the fans on 100%. So what I then had to do, is locate these definitions in the HC_BMC.sdr file (I think it's called), and adjust them to more suitable values. These are defined within CPUx_TACH sections in the file. I lowered both the lower critical and lower non-critical values, to prevent the system activating failsafe mode (= all fans 100%). Now it's just a matter of fine-tuning that file to get suitable cooling/quietness for my machine, using techniques from "Found the solution to loud fans when using "other chassis" option when updating Firmware on a server board". Any changes of course have to be activated by re-running the SDR update after one modifies the file.
So in summary, we need to tell the system that we want to use the on-board temperature sensor, by selecting "OTHER" chassis. And then make sure there are no conditions activating failsafe mode, and correct as necessary by modifying HC_BMC.sdr. You can look for such conditions with the Intel Active System Console.
I hope this helps!