BECKHOFF BC2000 - Manual (Page 19)

Settings in the TwinCAT System Manager
BC2000 18
PLC Cycle Time
PLC Cycle Time The PLC cycle time determines the program’s repetition frequency. This
time is not deterministic. This means that when the program in the bus
terminal controller needs more time, the PLC cycle time rises. If the
program needs less than the cycle time, it is repeated at intervals of the set
cycle time.
The processing of the Lightbus and the serial interfaces is executed in the
background time. This should be set to approx. 20 % of the PLC cycle time.
The „mean cycle time“ is measured in order to optimise the system. You will
find this item in PLC Control under Online\Coupler. About 20% – 30% is
added to this, and the result entered as the PLC cycle time. The
background time is then set to 20% of the PLC cycle time.
Example of cycle time
optimisation
PLC cycle time = mean cycle time + 20% = 10 ms + 20% = 12 ms
Background time = PLC cycle time * 0,2 = 12 ms * 0,2 = 2,4 ms ≈ 2 ms
Mean cycle time The measured cycle time also includes that required for the terminal bus
update. This results in a connection between the number of terminals that
are inserted and the cycle time. Before initiating the program, the bus
terminal controller executes a terminal bus update, in order to interrogate
the inputs. After the program has been executed, the BC2000 carries out
another terminal bus update, in order to write the current outputs. The
background time follows this. The cycle time can also be shortened, if the
terminal bus update is carried out simultaneously for inputs and outputs
(see Terminal Bus Update).
PLC Variables
PLC variables are variables that are situated in the BC2000 from addresses
%IB128 and %QB128. This offset can be shifted. These addresses are not
assigned to any terminal, so that signals or data can be transmitted to or
received from the master via the PLC variables.
Mean cycle time
10 ms
Back-
ground
time
PLC cycle time