A mixed workload management system and associated operating method modify a shortest job first by service levels. Each of plural image supporting members supports a single color toner image thereon, and abuts onto the slack side of the belt member to define a transferring position at which the toner image is transferred onto either the belt member or a recording medium held by the belt member, while being rotated. A bus signal abnormality processing unit compares an output bus signal output from the bus signal control circuit to the address bus and the control signal line with a feedback bus signal fed back through the bus diagnosis line to determine the presenceabsence of a difference. A negative output of the power supply comprises a first capacitor and a second switch which switches electrical current from the first terminal to the negative output. The sub-pixel resolution permits utilization of smaller cost-effective focal plane arrays by giving the small focal plane array a resolution associated with much larger focal plane arrays.