An image forming apparatus including a main body and a processing unit detachably mounted in the main body. The processing unit includes at least one of an image bearing member and developing device. The number of image forming repetitions of the processing unit are counted by a counter in the main body....http://www.google.ca/patents/US4851875?utm_source=gb-gplus-sharePatent US4851875 - Identification and monitoring of image forming process units
