From RHESSI Wiki
The four thresholds – front slow, front fast, rear slow, and rear fast – are controlled by DACs that can be set from 0x00 to 0xFF (i.e., 0 to 255 in decimal). The slow thresholds were calibrated in detail prior to launch, but it is likely that the calibrations have drifted over time.
Front slow threshold
Here is a conversion table of selected DAC settings to keV according to the pre-launch calibration. The calibration is a linear function, and the nominal DAC setting is 0x0C, which is ~3 keV.
Front fast threshold
The nominal DAC setting is 0x22, which is ~6 keV.
Rear slow threshold
Here is a conversion table of selected DAC settings to keV according to the pre-launch calibration. The calibration is a linear function, and the nominal DAC setting is 0x30, which is ~20 keV.
Rear fast threshold
The nominal DAC setting is 0x45, which is ~20 keV. The step size is ~0.28 keV/step.