HP 8550mfp Service Manual - Page 161

Black toner level detection and cartridge detection The toner antenna in the black toner cartridge detects the black toner level. The controller board monitors the toner level sensor signal output from the toner antenna via high-voltage PCA 1. The controller board checks the signal at the moment that the black developing AC bias is applied to the black developing cylinder. If the toner level is less than specified, the controller board signals the formatter that the black toner cartridge has no toner. The black toner cartridge is detected by the black toner cartridge switch (SW644) in the printer. Environment change control The controller board judges the temperature/humidity environment based on temperature/humidity sensor signals from the temperature/ humidity sensors on the temperature/humidity sensor PCA and executes the following controls to obtain the appropriate image:
Controls the value of the secondary transfer bias, the transfer drum cleaning bias, and the transfer charger bias.
Performs a color calibration if the environmental change is great enough. The controller board checks for an environment change before starting image density control, after the power is turned on, or every 30 minutes after the image density control is completed. EN Consumable detection mechanisms 159