Rockwell Automation 1757-SWKIT5100 ProcessLogix R510.0 Installation and Upgrade Guide - Manual (Page 253)

Publication 1757-IN510A-EN-P - October 2003
Performance and Capacity Specifications 11-13
System communication is most effective and efficient if minimum
networks and nodes are involved between communicating devices
within a pyramid. For example, if a transmitter and a valve form a ¼
second control loop, and they are on the same H1 network, latency of
communications will be minimal and traffic in other nodes and
networks will be minimal.
There are tradeoffs, however. A better algorithm set in the
ProcessLogix controller, the need for sequencing logic, or mixture
with conventional or other bus I/O are examples where controller
interaction is required. Further, interlocks with equipment in other
areas of the plant under other ProcessLogix controllers is not
uncommon.
Since ProcessLogix permits almost totally generalized
intercommunications paths, the two devices can be under or on:
1 . the same H1 link,
2. different links of the same 1757-FIM,
3. different 1757-FIMs in the same IO chassis,
4. different IO chassis of the same controller,
5. different controllers of the same server
As the configuration moves from 1, co-residency on the same H1 link,
to 2 to 3 to 4 to 5, more nodes and more networks get involved,
increasing the load on each node and network and adding additional
delays to the signals. Therefore, the interacting equipment should be
co-located in the pyramid when the plant instrumentation and control
networks are laid out. The number of communication circuits between
points of differing controllers should be small (less than 2–5% of the
total number of circuits.)
System Communications Limits
Communications limits are derived from various capacities. Common
constraints are described below.
Node Address Range Limits
Arbitrarily high limits on a network reduce effective bandwidth and
effect recovery time.