1756-RM001B-EN-P, Using ControlLogix in SIL2 ... - Tuv-fs.com

2-4 The ControlLogix SystemData Echo Communication CheckAnother powerful by-product of the p/c communication model andthe implementation of the Control and Information Protocol (CIP)protocol is the Output Data Echo, a communication methodemployed between owner-controllers and every output module in thesystem. Output Data Echo allows the user to verify that an ON/OFFoutput command from the controller was actually received by thecorrect output module, and that the module will attempt to executethe command to the field device connected to it.During normal operation, when a controller sends an outputcommand, the output module that is targeted for that command will“echo” that requested state back to the system upon its receipt. Thisverifies that the module has received the command and will try toexecute it. By comparing the requested state from the controller to theData Echo received from the module, the user can validate that thesignal has reached the correct module and that the module willattempt to activate the appropriate field-side device. Again, it is theuser’s responsibility to establish the course of action appropriate totheir safety application.When used with standard ControlLogix output modules, the DataEcho validates the command up to the system-side of the module, butnot to the field-side. However, when this feature is used in tandemwith diagnostic output modules, the user can virtually verify theoutput command integrity from the controller to the actuatorconnected to the module.Diagnostic output modules contain special circuitry that performsField Side Output Verification. Field Side Output Verificationinforms the user that system-side commands received by the moduleare accurately represented on the power side of the switching device.In other words, for each output point, this feature confirms that theoutput is ON when it is commanded to be ON or OFF whencommanded to be OFF.The capability of comparing the actual state of the field-side of thediagnostic module’s output against what the controller commandsgives the user the ability to make sure that the module is performingwhat the control system is requesting, once that output command hasbeen issued.Publication 1756-RM001B-EN-P - October 2003