Delta RMC151 - Manual (Page 482)

RMC70/150 and RMCTools User Manual
requirement of each RMC. Usually the RPI for all RMCs will be set the same, and each
RMC will have a single connection, so the total utilization can be calculated as shown:
Total Required Bandwidth = ( # of RMCs ) x ( 2 / RPI )
Example
For 12 RMCs with an RPI of 20 ms each, the total required bandwidth is 12 x ( 2 /
0.020 ), which is 1200 packets/second.
Using this formula, we can also calculate the maximum number of RMCs that can be
supported by your controlling EtherNet/IP module:
Maximum Supported RMCs = Total Bandwidth x 90% / ( 2 / RPI )
Example
How may RMCs can the 1756-EN2T support at a 10 ms RPI? The Total Bandwidth
for the 1756-EN2T is 10000 packets/second, so the maximum number of RMCs is
found by 10000 x 90% / ( 2 / 0.010 ), which is 45 RMCs.
If the total bandwidth required exceeds 90% of the master EtherNet/IP I/O controller's
bandwidth, then you will have to consider one of the following options:
• Increase the RPI for one or more of the RMC connections
Increasing the RPI reduces the required bandwidth. For example, doubling the RPI will
cut the bandwidth in half. Notice that in many cases, increasing the RPI comes at no
cost to the system performance because the PLC may be unable to scan its ladder
logic as frequently as the RPI.
• Replace the EtherNet/IP controller with a faster model
If an EtherNet/IP controller with higher bandwidth is available for the same platform,
then you may be able to upgrade the EtherNet/IP controller. For example, on the
ControlLogix family, the 1756-ENET/B can be replaced by the 1756-ENBT or 1756-
EN2T, or the 1756-ENBT can be replaced by the 1756-EN2T.
• 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 bandwidth
requirement on each device is reduced. Notice that 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.
Example
Suppose one ControlLogix 1756-ENBT module will be controlling twenty-five (25) RMCs.
The intended RPI is 10.0 ms. Therefore the total bandwidth in packets/second is
computed as follows:
Packets/Second = Number of RMCs x (2 / RPI)
= 25 x (2 / 0.010s)
= 5000
The maximum allowable I/O load on the ENBT is 90% of 5000, which is 4500. Since 5000
is greater than 4500 packets/second, one of the following alternatives must be
considered:
• Increase the RPI of one or more of the RMCs until the bandwidth is below 4500.
Raising each to 12.0 ms or higher does this.
• Replace the 1756-ENBT with the 1756-EN2T. The bandwidth on the 1756-EN2T is
10,000 packets/second, which easily handles this load.
462 Delta Computer Systems, Inc.