BECKHOFF BC7300 - Manual (Page 38)

Bus Terminal Controller
38 BC7300
Bus Terminal Controller
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 MODBUS 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 K-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 K-Bus update, in order to interrogate the inputs. After
the program has been executed, the BC7300 carries out another K-Bus
update, in order to write the current outputs. The background time follows
this.
PLC Variables
PLC variables are variables that are situated in the BC7300 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. The
default setting is for 16 words of input and output. These can be changed
with the KS2000.
Remanent Variables
Retained or remanent variables are data that retain their value when
voltage is not present. In PLC Control this data is placed in the allocated
flags area. Following the default setting, this means that all the variables
from %MB0 - %MB63 are RETAIN data. The maximum setting is for 512
bytes, which can be set using the KS2000 software.
Mean cycle time 10 ms
Background
time
PLC Cycle Time