Fieldbus Controller 750-841 • 45

Operating System

3.1.3 Operating System

3.1.3.1 Start-up

The controller starts-up after switching on the supply voltage or after a hardware reset.

Note

The Operating Mode slide switch must not be in the bottom position during start-up!

The PLC program in the flash memory is transferred to RAM.

This is followed by the initialization of the system. The controller determines the I/O modules and the present configuration. The variables are set to 0, FALSE, or to an initial value given by the PLC program. The flags retain their status . The "I/O" LED blinks red during this phase.

Following a fault free start-up the controller changes over to "RUN" mode. The "I/O" LED lights up green.

3.1.3.2 PLC Cycle

The PLC cycle starts following a fault free start-up when the Operating Mode Switch is in the top position or by a start command from the WAGO-I/O-PRO CAA. The controller starts a PLC cycle by first reading the fieldbus data, I/O modules, and time data. Next, the PLC program in RAM is processed (scanned). After the program is processed, the fieldbus data and I/O modules are updated with new output data. System functions are then preformed (i.e., system diagnostics, communications, time calculations, etc). At this point, if a STOP command is not present, the cycle starts over again with the reading of the fieldbus data, I/O modules, and time data.

The change of the operating mode (STOP/RUN) is only made at the end of a PLC cycle.

The cycle time is the time from the start of the PLC program to the next start. If a sizeable loop is programmed within a PLC program, the PLC cycle time is extended correspondingly.

The inputs and outputs are not updated during the scanning of the PLC program. I/O updates only occur at the end of the PLC program scan. For this reason, it is not possible to wait for a physical I/O change from within a program loop.

WAGO-I/O-SYSTEM 750 ETHERNET TCP/IP

46 • Fieldbus Controller 750-841

Operating System

WAGO-I/O-SYSTEM 750 ETHERNET TCP/IP