A lock mechanism is disclosed for a sheet store adapted for the dispensing or accepting of sheets. The modular lifting beam is equipped with a set of staggered mounting lugs at each end of the beam. According to one embodiment, a method is disclosed in which the reservoir valve is commanded to an open position when flow through the single exhaust duct is large and commanded to a closed position when flowrate through the single exhaust duct is small. The compensation unit is coupled to the scan signal generating unit, and used for compensating the total resistance of each of the output channels, and sequentially receiving and transmitting the scan signal to a display panel.