Delta RMC151 - Manual (Page 483)

6 Communication
• Use two 1756-ENBT modules and divide the RMCs between the two. For example, one
could control 15 RMCs (3000 packets/second), and the second could control 10 RMCs
(2000 packets/second), putting both within the recommended limits.
Considerations for Network Performance
The maximum amount of information that 100 Mbps Ethernet can carry in one direction is
100 million bits per second. If all packets on the network were EtherNet/IP I/O packets of
the largest size supported by the RMC (125 four-byte REALs), then this would be a
maximum of 21,600 packets/second (notice that packet overhead such as headers and
inter-frame spacing are included in this estimate). The network utilization should be kept
well below this maximum. As the utilization approaches the maximum, it becomes more
and more likely that a packet will have to wait to get onto the wire, introducing small
delays.
In addition to the limits of the wire itself, many devices have a maximum sustained rate
that they can receive packets without falling behind in packet processing and eventually
dropping packets. The RMC is one such device, since priority must be given to controlling
motion above handling packets. No pre-determined maximum rate is currently available,
but by reviewing the Ethernet Statistics in the devices, you can look to see if packets are
being discarded.
Notice that EtherNet/IP multicast I/O connections can greatly impact the performance of
an Ethernet network since—unlike unicast packets, which are sent directly to a single
device—multicast packets are sent to all devices on the network by default. This increases
the load on every device, including the switches, and each individual network segment.
If your network has a high utilization and/or network components are dropping packets
due to high packet rates, you will want to look at ways of reducing the network
utilization. There are three ways to reduce the network utilization:
• Change multicast I/O connections to Unicast
As described above, multicast I/O connections can significantly increase the network
load. Most applications do not require multicast I/O connections, but unfortunately
because Rockwell did not support unicast I/O connections until RSLogix 5000 v18,
most current EtherNet/IP installations use multicast. If you are using a EtherNet/IP
controller that supports unicast and are not using multiple I/O connections per RMC,
then you should make sure that all I/O connections are set up as unicast.
• Increase the RPI for one or more EtherNet/IP I/O connections
The simplest way to reduce network utilization is to increase the RPI of EtherNet/IP
I/O connections. Of course, this is only a possibility if the RPIs are unnecessarily low.
This topic has been discussed above, so no more needs to be said here.
• Use multiple EtherNet/IP modules to divide the network
By adding one or more additional EtherNet/IP communication modules to the PLC,
each with its own isolated network with a smaller number of RMCs, the utilization on
each sub-network is reduced. Notice that if multicast I/O connections are used, then it
is important that the individual networks are not connected to one another directly or
through a switch, as this will largely defeat the benefits of dividing the network. It is
acceptable to connect the networks using routers. However, it is important that
someone with IT experience is consulted before doing so.
• If using multicast I/O connections, use smarter switchgear
As noted above, currently EtherNet/IP I/O connections generate a lot of multicast
Ethernet traffic. This multicast traffic is treated like broadcast traffic by default, which
means that every port on every switch in the network will send out a copy of each
multicast packet, even though in most cases only one device on one port is interested
in the packet. However, it is possible to be more efficient than this default behavior by
using smarter switchgear to direct the multicast traffic only to those ports that are
listening to the multicast traffic.
The current way of doing this uses a protocol called IGMP (Internet Group
Management Protocol). Using IGMP to direct packets in switches to only the
interested ports requires that (1) the switch supports IGMP Snooping, and (2) at
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