Rockwell Automation 1753-DNSI DeviceNet Safety Scanner for GuardPLC Controllers - Manual (Page 49)

Publication 1753-UM002A-EN-P - July 2005
Configure DeviceNet Nodes and Connections 5-7
Increasing the ISD time causes a longer network scan, which
adversely affects overall input-to-output performance. However, the
increase allows lower priority messages to get more network access.
These lower priority messages include those used to do network
browsing and configuration upload and download functions. So, if
these network functions are sluggish on your system, increasing the
ISD time is one way to make more bandwidth available for lower
priority messages.
In addition, if the last node in your scan list produces a large amount
of polled data, you may want to increase the ISD time to ensure that
the entire response is received before the next poll request is sent to
that node.
Foreground to Background Poll Ratio
Devices set for polled behavior can be polled on every I/O scan
(foreground) or they can be polled less frequently (background).
Setting a device for foreground or background behavior is done when
you configure each device on the Scanlist tab, from the Edit I/O
Parameters dialog. A ratio of 2 means that any nodes included on the
background list are polled every other scan cycle. A ratio of 3 means
they are polled on every third cycle, etc.
Advanced Module Settings
Clicking on the Advanced… button lets you to set the Expected
Packet Rate (EPR) and the Transmit Retries as described below.
• Expected Packet Rate (EPR) – When the scanner opens a polled
or strobed I/O connection, it uses this value as a maximum
timeout (Expected Packet Rate) with the device. If the
device does
not receive a packet from the scanner within 4 times the EPR
value, the slave device drops the connection. If the
scanner does
not receive a packet from the slave within 4 times the EPR value,
it drops the connection and periodically attempts to reopen the
connection.