Delta RMC101 User Manual

Page 364

RMC100 and RMCWin User Manual

5-114

Computing Utilization for RMC/ControlLogix Ethernet Networks
In order to predict the probability of collisions on collision domains of an Ethernet network used by
EtherNet/IP between a ControlLogix 1756-ENET or ENBT and RMCs, we must compute the
utilization on the various collision domains. The first step in computing utilization is to compute
the bandwidth requirement for the collision domain. That is, how many frames are vying for the
bandwidth each second?

There are two types of frames used in an I/O connection with an RMC: frames consumed by the
RMC, and frames produced by the RMC. Each RMC will produce exactly one frame each RPI,
but it will consume one frame per connection each RPI. The bandwidth requirement on each
collision domain reached by a consumed or produced frame is 1/RPI.

When a hub is used, there is only one collision domain, so all produced and consumed frames
affect the required bandwidth for that domain.

When a switch is used, frames are routed intelligently only to the ports that need them. Frames
consumed by the RMC are sent directly from the 1756-ENET/ENBT to the RMC, and thereby only
affect only the collision domains of the two devices involved (1756-ENET/ENBT to switch, and
switch to RMC). Frames produced by the RMC are multicast. For most inexpensive switches treat
multicast packets the same as broadcast packets by forwarding the frames to all ports on the
switch. Switches that support IGMP (Internet Group Management Protocol) or IGMP snooping do
better by keeping track of which ports have devices that care about the multicast frame and only
forwarding to those ports. However, notice that most of these switches require a router to be
present in the network for the IGMP snooping to work. Therefore, if your system will not always
have a router present, then you should purchase a switch that supports IGMP snooping without a
router present. Compare the first two examples to see the difference this can make on utilization.

Note:

Delta always recommends using a switch over a hub. The price difference is negligible

and the determinism boost is significant. When using a protocol that makes heavy use of
multicasting (as EtherNet/IP does), Delta also recommends using a switch that supports IGMP
snooping, preferably without requiring an additional router. EtherNet/IP is the only protocol that
the RMC ENET currently supports that uses multicast.

The bandwidth required in frames/second for each collision domain is computed by adding up
frames/RPI for each different RPI on that collision domain.

Example:

Using the above information, we will give examples of computing the bandwidth required for the most
common collision domains in an Ethernet network. For each of these examples assume there are two
ControlLogix PLCs (CLX1 and CLX2), and four RMCs (RMC1 through RMC4). Assume that RMC1
and RMC2 are controlled by CLX1 and monitored by CLX2, RMC3 is controlled by CLX1 only, and
RMC4 is controlled by CLX2 only. The RPI is 10 ms for RMC1 and RMC2, 5 ms for RMC3, and 7 ms
for RMC4. This is summarized in the following chart:

CLX1

CLX2

RMC1

10.0

RMC2

10.0

RMC3

5.0

RMC4

7.0

This manual is related to the following products:

RMC100