Delta RMC101 - Manual (Page 361)

Ethernet 5.2
Communications 5-111
Frames/Second = (2 x connections) / RPI + (2 x
connections) / RPI
= (2 x 1) / 0.005s + (2 x 2) / 0.015s
= 400 + 267
= 667
This load is under the recommended 80% bandwidth (720 frames/second) of the 1756-ENET.
Therefore, this network should work.
5.2.6.3.7.4 Evaluating the Load on the 1756-ENBT
The 1756-ENBT has a total bandwidth of 5000 frames/second. If we reserve 10% of this
bandwidth for non-I/O communications (RSLogix 5000, etc.), then we are left with 4500
frames/second. This is enough bandwidth to control eleven RMCs at the RMCs' minimum RPI of
5 ms. As will be discussed in the next topic, collisions on the RMC-to-switch collision domains
preclude this many RMCs being placed on the same network with such a low RPI in most
applications.
The load on the 1756-ENBT is computed in the same way as the 1756-ENET. However, for the
purpose of controlling RMCs, the collisions will be the limiting factor in network performance and
determinism when using a 1756-ENBT. See Predicting the Effect of Collisions for details. For
additional considerations when setting up large EtherNet/IP networks, see Setting Up Large
EtherNet/IP Networks.
5.2.6.3.7.5 Predicting the Effect of Collisions
Collisions
With half-duplex (full-duplex will be described below) Ethernet, collisions occur when two or more
devices attempt to send a frame at the same time. When frames collide, the frame must be
retried, so each device delays (backs-off) a random amount of time and then retries. For each
successive collision on a frame, the maximum back-off time doubles. If a frame collides sixteen
successive times, the frame will be discarded.
Note: RMCWin provides a histogram of number of frames that have collided a given number of
times. See RMC Ethernet Statistics for details.
Collision Domains
Ethernet networks are divided into collision domains. A collision domain is the set of all devices
that can collide with one another. The more devices in a collision domain: the higher the
probability of a collision. Each collision domain extends through any hubs or repeaters and stop
at switches, routers, or end-devices (PCs, RMCs, PLCs). This is best illustrated with some
examples:
• A network has four RMCs and one 1756-ENET connected to a hub. This network has a single
collision domain with five devices competing for its bandwidth.
• A network has four RMCs and one 1756-ENET connected to a switch. This network has five