Delta RMC101 - Manual (Page 369)

Ethernet 5.2
Communications 5-119
Suppose you need to control 40 RMCs from a single ControlLogix 1756-L1. If you use a single
1756-ENBT for this task, the bandwidth required for this system at a 5 ms RPI is calculated as
follows:
Frames/Second = (2 x connections) / RPI
= (2 x 40) / 0.005 s
= 16,000
This is well over the allowed 4500 frames/second on the 1756-ENBT. However, if the system can
get by with a slower RPI, the bandwidth drops dramatically. For example, increasing the RPI from
5 to 20 ms reduces the bandwidth requirement as follows:
Frames/Second = (2 x connections) / RPI
= (2 x 40) / 0.020 s
= 4,000
In many cases, the reduction in bandwidth comes at no cost to the system performance because it
may be likely that the ControlLogix is unable to scan its ladder logic more frequently than every 30
ms, in which case most 5 ms updates are ignored.
Notice that in some applications, the RPI required for different RMCs may be different. For
example, suppose that three of this group of forty require a 5 ms RPI, but the rest can get by with a
25 ms RPI. Since the RPIs are set independently for each RMC, the bandwidth can be further
reduced as follows:
Frames/Second = (2 x connections) / RPI + (2 x
connections) / RPI
= (2 x 3) / 0.005 s + (2 x 37) / 0.025 s
= 1,200 + 2,960
= 4,160
• Divide the Network.
You can further improve the stability and determinism of your system by dividing up the system
into multiple networks. For example, we could reduce the near-maximum load of 4,000
frames/second to a comfortable 2,000 frames/second by adding a second 1756-ENBT module.
By keeping the two networks separate (that is, use a separate switch for each), the collisions are
also reduced drastically. For a large Ethernet system, the additional cost of a second, third, or
even fourth 1756-ENBT module (around $1000 each as of this writing) is often insignificant