NORD Drivesystems BU0550 - Manual (Page 10)

PLC logic for NORD SK 54xE frequency inverters
10 BU 0550 GB-0813
In the flag memory, two different classes of variables are stored in the variable section:
[VAR]
Memory variables for storage of auxiliary information and statuses. Variables of this type are initialised every
time the PLC starts. The memory content is retained during the cyclic sequence of the PLC.
[VAR_ACCESS]
These are used to read and describe process data (inputs, outputs, setpoints, etc.) of the frequency inverter.
These values are regenerated with every PLC cycle.
1.2.2 Process image
Several physical dimensions such as torque, speed, position, inputs, outputs etc. are available to the
inverter. These dimensions are divided into actual and setpoint values. They can be loaded into the process
image of the PLC and influenced by it. The required process values must be defined in the list of variables
under the class VAR_ACCESS.
With each PLC cycle, all of the process data for the inverter which is defined in the list of variables is newly
read in. At the end of each PLC cycle the writable process data is transferred back to the inverter. See the
following diagram.
Fig. 2 Process image
Because of this sequence it is important to program a cyclic program sequence. Programming loops in order
to wait for certain events (e.g. changes of input level) does not produce the required result.
This behaviour is different in the case of function blocks which access process values. Here, the process
value is read on call-up of the function block and the process values are written immediately when the block
is terminated.
NOTE
If the blocks MC_Power, MC_Reset, MC_MoveVe locity, MC_Move…, MC_Home or MC_Stop
are used, the process values "PLC_Control_Word" and "PLC_Set_Val1" to "PLC_Set_Val5"
must not be used. Otherwise the values in the list of variables would always overwrite the
changes to the function block.
1.2.3 Program Task
Execution of the program in the PLC is carried out as a single task. The task is called up cyclically every 5
ms and its maximum duration is 3 ms. If a longer program cannot be executed in this time, the program is
interrupted and continued in the next 5 ms task.
Read in
process
values PLC program
execution Save process
values Start of
cycle End of
cycle