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PROCESSOR MODES OF OPERATION The four-position keyswitch on the front panel of the Parallel Link Driver Module determines the processor mode of operation. The 620-25/35 features four modes of operation: PROGRAM, RUN, RUN/PROGRAM. and DISABLE. PROGRAM MODE The system may be placed in the PROGRAM mode by the front panel keyswitch. The Processor Module does not execute the control program. The RUN LED on the Parallel Link Driver Module (PLDM) is OFF when the system is in the PROGRAM mode. When the processor is in the PROGRAM mode, a signal is transmitted to the I/O System which allows individual I/O racks to be selected to freeze or clear outputs. Contacts may be forced if the Force Enable Switch (SW2 switch 4) on the PLDM is CLOSED/ON. Timer/counter data stored in the Register Module may be changed regardless of the state of the Data Change Enable Switch (SW2 switch 5) since the processor is already in the PROGRAM mode. Switching the keyswitch to another processor mode removes the processor from the PROGRAM mode. If the system has been placed in the PROGRAM mode by the Loader/Terminal or a CIM, the software PROGRAM mode request must be removed from the processor causing the system to return to the mode of operation specified by the position of the keyswitch. SOFTWARE PROGRAM MODE The system can be placed in the Software PROGRAM Mode by the Loader/Terminal or a CIM. The programmable controller must be in the RUN/PROGRAM or DISABLE mode, and the on-line programming function enabled according to SW2 switch 6 on the PLDM. The system enters the Software PROGRAM mode only after the scan being executed is completed. When the Loader removes the Software PROGRAM Mode request, the processor leaves the software PROGRAM mode and returns to the original mode, after the system successfully executes the retentive scan and self-diagnostics. Software PROGRAM mode changes are made through the LOADER/TERMINAL Mode change auxiliary menu. This function is particularly useful in the program debug stage for extensive changes. The user may monitor program execution, find a bug, change it, and execute the program from the keyboard. RUN MODE The system is in the RUN mode when the front panel keyswitch is in the RUN or RUN/PROGRAM position. The RUN mode is the main control mode for the processor. The system executes a retentive scan when it first enters the RUN mode. During the retentive scan all nonretentive outputs from 0 to 4095 are turned OFF. The retentive outputs retain the state they were in during the last scan executed prior to being removed from the RUN mode. After the retentive scan is complete, the user program scan begins by verifying that an Input Status Scan (ISS) instruction is located in the first memory location of the user program. While input status is being collected from the I/O system, the processor examines the card fault interrupt. If any card faults are detected in the I/O system, the fault information is inserted in the System Status Table.