Delta RMC101 User Manual download pdf (Page 361)

100

101

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

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