Page: 21Service Guide PM3410Chapter 2 Principles of Operation2.1.08 Alarm CircuitsFault Alarm Circuit (in Power Supply Unit)This protective circuit turns off the power supply when a fault occurs in the printhead, line feed, or bailmotor drive circuits, thus preventing secondary component failure.To accomplish this, the circuit monitors the overdrive signal of each drive circuit. If the duration of anydrive circuit exceeds a predetermined length of time, the appropriate signal (EVN COM, ODD COM, LFCOM or BM COM) will be sent to the fault alarm circuit. The fault alarm circuit generates the ALM signalwhich causes the power supply to turn all DC power OFF.Printhead Overheat Alarm CircuitIn order to protect the printhead coils, this circuit monitors the printhead temperature by using a thermistorcontained in the printhead. The CPU senses this circuit every time a line is printed.When printing, the printhead temperature will increase. If the head temperature reaches approximately115° Celsius, a head overheat ALARM 1 is generated. When ALARM 1 is detected by the MPU, theprinter will begin unidirectional printing. If the printhead temperature increases to 125° Celsius, ALARM 2is generated and printing is halted until the head temperature decreases. When the head temperaturereturns below the ALARM 1 threshold, printing is resumed.Cover Open Alarm CircuitWhen the front access cover is opened, the CVOPEN-N signal is sent to the MPU from the cover interlockmicroswitch. The MPU will halt printing immediately and light the ALARM Lamp.Copyright 1997, Okidata, Division of OKI America, Inc. All rights reserved. See the OKIDATA BusinessPartner Exchange (BPX) for any updates to this material. (http://bpx.okidata.com)
