Delta RMC101 User Manual download pdf (Page 362)

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collision domains, each with two devices competing for its bandwidth. These smaller collision

domains will yield a more deterministic and higher performance network. This is why we

recommend using a switch instead of a hub.

• A network has four RMCs and one 1756-ENET connection to a switch. The switch is then

connected through a router to the rest of the plant network. The control network will have six

collision domains: the five in the previous example plus one between the switch and router. All

traffic on the plant network will be in separate collision domains.

Full-Duplex Ethernet

Full-duplex Ethernet occurs between exactly two devices (usually a switch and an end-device

such as the 1756-ENBT) when both devices support full-duplex (the RMC100 does not support

full-duplex Ethernet). Each device in a full-duplex segment can send and receive data at the

same time, thereby doubling the bandwidth, and more importantly, eliminating collisions. The

1756-ENBT supports full-duplex Ethernet. Therefore, use of a full-duplex-capable switch is highly

recommended to eliminate collisions between the 1756-ENBTs and the switch, which are the

most heavily trafficked segments. The 1756-ENET and RMC ENET do not support full-duplex,

and therefore will be susceptible to collisions.

Network Utilization

The probability of a collision occurring depends primarily on the utilization of the network in a

collision domain. Utilization is the relationship between the available bandwidth (10 or 100 Mbps)

and the actual amount of data vying for that bandwidth. The amount of data is a function of the

number and size of frames being sent. We will represent utilization as the percent of time the wire

is active.

Utilization vs. Collisions

The probability of collisions in a given collision domain depends on the network utilization. As is

described below, the maximum delay per frame depends on how many collisions a frame

experiences, therefore a good approach is to choose how long of a delay is acceptable and then

determine how many collisions per frame are acceptable.

The maximum acceptable collisions per frame, and therefore the maximum acceptable delay per

frame will vary from application to application. Typically the absolute limit will be the timeout time

for an I/O connection. The ControlLogix sets up its I/O connections to time out if no packets are

received within 32 RPIs. For example, for an RPI of 5.0 ms, the connection will reset if a packet is

not received in 32 x 5.0 ms or 160 ms. Therefore, the number of collisions per frame should be

kept below 11 (using the chart below). When a connection times out, the connection is closed and

may take several seconds to be re-established. During this time the PLC and RMC will execute

any broken connection handling they have implemented.

Notice that with half-duplex Ethernet, there can be no guarantee that a frame will reach its

destination by any given time. However, the probability of such an event may become so low that

they are effectively masked by other more-probable events such as a cable being cut or a plant

fire. Again, it should be noted that with full-duplex Ethernet, collisions do not exist, and as such,

the Ethernet media becomes deterministic.

The following table shows the percentage of the frames that collided n or more times at various

network utilizations in lab tests at Delta. These tests were done with a single ControlLogix 1756-

ENBT/A module requesting data from multiple RMCs with RPIs of 5.0 ms using a switch. The

same utilization with a hub would result in higher collisions since more than two devices can

compete for the same bandwidth at once:

Utilization

Collisions:

7.6%

15.1%

22.7%

30.2%

37.8%

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