MOLEXSST-PFB-CLX Remote Rack Configuration

Edition: 1.0 

Date: August 22, 2003 

This document applies to the SST-PFB-CLX module.

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SST-PFB-CLX Remote Rack 

 Configuration 

Topic Overview 

To design a redundant Profibus I/O control system with ControlLogix architecture, the SST-PFB-CLX card 

must be installed in a remote ControlLogix rack. In this setup, it communicates with ControlLogix CPUs 

via the 1756-CNB family of ControlNet bridge cards. 

Although SST’s documentation explains how to set up the SST-PFB-CLX card and ControlLogix CPU to

communicate through the local rack, no SST documentation describes how to use the card remotely 

(as required for a redundant system). 

About this Technical Note 

This technical note documents a test carried out in Asia in May/June 2003. This document is not a 

specification. Its aim is to help automation system designers estimate the number of SST-PFB-CLX cards 

per remote rack vs. parameters such as byte load. 

Note 

The results presented in this document may vary with changes in the testing

environment, such as the CPU type, RSLogix5000 version, RSNetworx version, 

SST-PFB-CLX firmware version, actual I/O load, and so on. Accurately determining 

how many SST-PFB-CLX cards and how much I/O can be remotely configured may

depend on the relevant system. 

This Technical Note Assumes that

Software Output Switching Redundancy Solution vs. Hardware (1757-SRM) Solution: 

According to document 1756Soft_HBU.pdf, Application Solution “Software Output System” 

(Publication number 1756-AP001A-US-E), it is possible to build a ControlLogix redundant system

without the 1757-SRM modules, using ladder logic to switch control between CPUs. 

Note 

The test documented in this technical note is restricted to the hardware-based 

redundancy configuration, which requires the two 1757-SRM modules (as shown in

Figure 1). As of this document’s writing, there has been no known successful softwarebased redundant implementation for SST-PFB-CLX, nor has SST claimed it possible to

successfully implement software output switching redundancy with SST-PFB-CLX. 

Summary of Test Results 

From the tests performed, it was observed that the number of cards per remote ControlLogix rack, using 

one (1) ControlNet bridge, an SST RPI of 40 msec, a CN RPI of 10 msec and an NUT of 5 msec is:

Detailed Test Results 

Table 1: Parameter Setting on SST-PFB-CLX Card for 

Largest Data Load (Tx=Rx=190 bytes) 

Scan Cycle Times Watchdog Results Remarks 

20 x 100 usec 20 msec Profibus comms. Failed 

40 40 Profibus comms. Failed 

60 60 Profibus comms. Failed 

80 80 Profibus comms. Failed 

100 100 No problem

Increasing the Number of SST-PFB-CLX Cards 

per Remote Rack 

As shown in the previous section, it is possible to increase the number of SST-PFB-CLX cards by

increasing the number of ControlNet bridges on the remote backplane. Whether this is practical is for the 

system’s designer to decide.