Handling Input Data In Plc; Setting The Io Cycle Update Time - Comtrol DeviceMaster PNIO 2000 Series User Manual

Handling Input Data
in PLC
Setting the IO Cycle
Update Time
DeviceMaster PNIO | UP User Guide: 2000639 Rev. A
Input data received by an IO controller is available at the input address range and
can be read directly by the IO controller. Due to the nature of PROFINET IO
cyclical communication, the DeviceMaster sends input data to the IO controller at
every IO update cycle regardless if there is new input data or not. If there is new
data, the DeviceMaster increments the sequence number and sends the new data.
If not, it resends the last data using the same sequence number. Therefore, the
sequence number is the key for the IO controller to determine whether there is
new input data or not.
The IO controller should monitor the sequence number field closely during each
scan. If the sequence number changes, the input data should be processed
immediately or copied to other memory location before the next IO update cycle. If
the IO controller fails to do so, the data could be overwritten in the next IO update
cycle. To prevent that from happening, the IO controller must consume the input
data faster than the DeviceMaster can produce them.
The IO Cycle Update Time setting can be used to control how fast the DeviceMaster
produces input data. Using the above example with the PNIO-2304 with six 220
bytes input modules, assuming the IO cycle update time is 8ms. The DeviceMaster
can produce 1344 bytes of new data every 8ms. If the IO controller does not have
the power to process them in 8ms, you can set the IO cycle update time to 16ms,
which means that the DeviceMaster will send data once every 16ms, giving the IO
controller more time to process the data.
However, keep in mind that the DeviceMaster can only hold a limited number of
packets in the receiving queue for each port/device. Each receiving queue can hold
20 packets for a serial port or 10 packets for an Ethernet device. If packets are
arriving at ports/devices at a rate faster than the rate that DeviceMaster is
configured to send data to IO controller, packet losses may occur due to overflowed
queues.
Use the following procedure to set the IO Cycle Update Time.
1. Double-click the DeviceMaster module to open the Device View.
2. On the Properties | General tab, click the PROFINET interface [X1] | Real time
settings, which opens the Real time settings window.
3. Select the Can be set radio button and select the desired update time. The
fastest IO cycle update time is 8ms.
IO Data Handling
IO Data Handling - 85