Rockwell Automation 1757-FFLDC4 FOUNDATION Fieldbus System User Manual - Manual (Page 165)

Rockwell Automation Publication 1757-UM012A-EN-P - July 2011 165
Appendix
D
Connections
Introduction This appendix describes how to count the connections on an EtherNet/IP
network (used by a 1757-FFLD linking device) or on the ControlNet network
(used by the 1757-FFLDC linking device).
Counting Connections The 1757-FFLD and 1757-FFLDC linking device each uses one Class 1
connection for each Logix Block instantiated through RSFieldbus software and
configured in the Logix controllers via an RSLogix 5000 project. This is a 1-to-1
relationship, whereas if you have 10 Logix Blocks in your program you need 10
minimum connections.
The difference between the two linking devices is the 1757-FFLDC device on
the ControlNet network also uses two Class 3 unscheduled connections for
configuration and monitoring.
The number of linking devices that are allowed on a either network depends on
the following:
• The bandwidth of the network (number of nodes)
• The number of connections that the communication bridge or interface
module supports
• The number of connections that the controller supports
Logix controllers configured for HSE transmissions can support 20 linking
devices and 250 connections. But the connection restriction resides in the
communication module you use for the connection.
For example, a 1756-EN2R supports 256 connections, while a 1756-ENBT
module supports 128 connections. A four-port FFLD linking device requires a
minimum of 80 connections (20 units x 4 ports = 80) so the 1756-EN2R
provides more spare connections.
The number of linking devices that are allowed on a network also is not
dependant on the number of H1 channels. A 2-channel linking device can be
used to instantiate and schedule 16 Logix Blocks, but all of the inputs and
outputs cannot be used due to the VCR limitations per channel.