How to use status inputs in diagnostic I/O modules

[hatrabbit 0]

Hello... I am familiar with several automation products but very new to PLCs. Today I completed the first 2 Allen Bradley courses but feel a lot was skipped over. One thing I think the instructor was saying was that if an output module has diagnostic capabilities, it can be used to start a load and then receive feedback that it actually started. Can someone here explain a little about this? Is load sensed in the module, or does a CT or some other device need to be wired to an input? Thanks for any perspective on this.

[Shawn Tierney 2]

Hey HatRabbit, When you add your diagnostic module to RSLogix5000, you get extra tags which represent the diagnostic features of the module. These features may include short circuit detection as well as no-load detection. These features a built into the module and no additional hard or devices (like CT's) are needed For specifics on particular modules, you can get the manual for the module in question from www.ab.com/literature Hope this helps!

[daba 4]

What you have been told is incorrect. Diagnostic output modules have the ability to detect if "No Load" is present, and it does this by bleeding a small amount of current through the load, but only when the output is not driving the load. So in effect, it's telling you that when you do want to drive the load, it is present, and stands a good chance of working. It cannot detect a "No Load" condition when the load is actually being driven, which is what I think you said your instructor told you. In addition to "No Load" detection, there is an "Output Verification" capability. Imagine that your output module has an input module strapped to it, piggy-back fashion. When the output is energised, then internal wiring places that output signal onto an (internal) input circuit. The module checks at all times that the output and input are "congruous", and can signal an "Output Verification" fault if it does not see the same "input" as the output state. This verification proves that the power driver circuitry on the module itself is working correctly, but it cannot prove that the load is actually being driven, and working. You can see the "verification" data in the "I" tag coming from the module..... e.g. Suppose you program Local:3:O.Data.6 to be turned on..... You will see that Local:3:I.Data.6 also comes on, for a working module. Nether of the diagnostic capabilities of the module can detect that the device you are driving is actually working. The "No Load" detection could tell you that there is a solenoid valve connected to the output, but that doesn't mean the valve will open, it could be seized, and the "Output Verification" function only tests the module's ability to deliver an output voltage to the load, again the solenoid valve could be seized. "Proper" feedback is mostly provided by limit switches on valves, auxiliary contacts on starters, flow switches, etc., etc. i.e. physical things you can monitor to determine that the piece of equipment is doing its job. Hope this helps you understand the Diagnostic modules better.