Operating System; Start-Up; Plc Cycle - WAGO 750 Series Manual

44 • Fieldbus Controller 750-841

Operating System

3.1.4 Operating System

3.1.4.1 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.

3.1.4.2 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.
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