Rockwell Automation T8xxx Trusted Communications Interface - Manual (Page 58)

Trusted
TM
Communication Interface T8151B
Issue 21 Apr 10 PD-T8151B 58
If the above happens, the communication interfaces will be overloaded and may crash on releases
before 3.5. From release 3.5, although the communication interfaces will continue to operate, peer to
peer network timeouts are likely to occur instead, which will cause a trip in the application if safety
related data is being carried.
The communication interfaces will not transfer packets between networks. However, it is possible for
PCs to transfer packets through IP forwarding/routing. To disable this, set the following registry entry to
zero.
\HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters IPEnableRouter
6.4.3. Network Load
It is also possible to overload the communication interfaces by using too fast a poll rate on the OPC
server. The poll variables are divided into two groups: those that have SOE or PH enabled (Sequence
of Events Update & Process Historian Update), and the Modbus data that does not have SOE (Global
Ungrouped Tag Update). The product description for the OPC Server describes loading and timing
calculations.
An Ethernet link should be designed to be less than 30% loaded.
For communication interfaces before release 3.5, allow about 3ms per Modbus / data packet for the
OPC server if the communication interface is not running peer to peer. If the communication interface
is running peer to peer, allow about 6 mS per OPC packet. The non SOE Modbus data may have
several packets, each update depending how the Modbus variables are distributed. Sequential Modbus
addresses are the most efficient.
The network statistics are available by typing 'tcp ns' on the communication interface diagnostic (see
section 5). By waiting 10 seconds and requesting another reading it is possible to work out how many
packets are passing over a 10 second period and hence the time per packet. The situation can be
made worse if there are a lot of writes to the system, because they are individually passed to the
processor and can take longer.