Rockwell Automation's EtherNet/IP Performance and Application Guide (Pub. No. ENET-AP001C-EN-P) describes how to compute the 1756-ENET loading in detail. When using the worksheets in that manual, consider each RMC connection a "Rack Optimized Connection." That is, the Frames/Second required for an RMC in the 1756-ENET is computed the same as Rack Optimized Connections:
Frames/Second = (2 x connections) / RPI
The 1756-ENET module's bandwidth is limited to 900 frames/second. Rockwell recommends that no more than 90% (810 frames/second) be allocated to I/O connections. From our independent testing, we recommend allocating no more than 80% (720 frames/second) for I/O connections.
If a 1756-ENET will be used only for controlling RMCs and each RMC has the same RPI, then the following chart can be used to determine the minimum RPI allowed for the number of RMCs:
|
RMCs |
Min RPI |
|
1 |
5.0 ms* |
|
2 |
6.0 ms |
|
3 |
9.0 ms |
|
4 |
12.0 ms |
|
5 |
14.0 ms |
|
6 |
17.0 ms |
|
7 |
20.0 ms |
|
8 |
23.0 ms |
|
9 |
25.0 ms |
|
10 |
28.0 ms |
* The 1756-ENET has enough bandwidth for a single connection with an RPI as low as 3.0 ms, but the RMC does not support that low of an RPI.
If the 1756-ENET is controlling other non-RMC I/O, the bandwidth required by these other connection will also need to be taken into consideration. The EtherNet/IP Performance and Application Guide covers these more advanced configurations.
In addition to the 1756-ENET and RMC ENET load limitations, the effect of collisions needs to be considered. See Predicting the Effect of Collisions for details.
Example:
Suppose one ControlLogix will be controlling three RMCs. The intended RPI is 6.0 ms. Therefore the bandwidth in frames/second is computed as follows:
|
Frames/Second |
= |
(2 x connections) / RPI |
|
|
= |
(2 x 3) / 0.006s |
|
|
= |
1000 |
This is over both the 720 frames/second we recommend and the 810 frames/second that Rockwell recommends. Therefore, you have three options:
Increase the RPI of one or more of the RMCs until the bandwidth is below 720. Raising each to 9.0 ms does this.
Replace the 1756-ENET with the 1756-ENBT. The bandwidth on the 1756-ENBT (see Evaluating the Load on the 1756-ENBT) is limited to 5000 frames/second, and easily handles this load.
Use two 1756-ENET modules: one will control two RMCs with RPIs of 6.0 ms (667 frames/second) and the second will control one RMC with an RPI of 6.0 ms (333 frames/second). Both are within the recommended limits.
Example:
Suppose one ControlLogix will be controlling three RMCs again, but this time one RMC needs an RPI of 5.0 ms, and the other two RMCs need an RPI of 15.0 ms. To compute the bandwidth requirement on the ControlLogix, compute the bandwidth required at each RPI, and sum them together:
|
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.
Copyright (c) 1997-2015 by Delta Computer Systems, Inc.