1762sc-IF8u problems?

I was wondering if anyone else has experienced problems with their universal PLC modules by Spectrum? I've had situations where when I connect a thermocouple signal to the module, the signal fluctuates excessively on the HMI, but when connected to a watlow panel meter, it reads rock steady and accurate.

I've had the same problems with 0-10vDC feedback signals and 4-20mA signals. Sometimes changing out the universal module will fix it and sometimes it does not. Sometimes when changing the module out does not fix it, I wonder if i just have another bad module?

These Spectrum universal modules always seem to be the weak link in the PLC racks. What am I doing wrong? Is it really just noise issues?

I'm a technician with limited programming experience, FYI.

Welcome!


I haven't used that card but do you have a process meter that you can connect to the card directly and bypass the field wiring for a test?

Jbotens said:

These Spectrum universal modules always seem to be the weak link in the PLC racks. What am I doing wrong? Is it really just noise issues?

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I also have not used them but I have one customer that lives by them and its his go to, if you have a scope you can check for noise or like Bob has suggested you can generate a signal direct, even if you have a spare device hook it direct and see if its out in the field the issue or the card

The 1762 bus can be problematic when using sensitive signals (I.E. analog). If the PLC is in a noisy environment, the noise can wreak havoc with the bus signals. There are ways to deal with it but the possible causes of what you are seeing are many, so a good diagnostics process is needed. All of Spectrum's I/O products are covered by Rockwell's TechConnect but you don't need an account (they use Spectrum's account). One of the reasons Spectrum does this is because of your type of scenario where the possible issues involve more than just the I/O module(s). If you don't get the needed support from Rockwell, you can contact Spectrum's support (the best method is "[email protected]"). If you do reach out to Spectrum make sure you get your ticket number from Rockwell so Spectrum's support can look up what Rockwell covered.
Having said that, before contacting support (either one) I’d go through both the MicroLogix manual as the IF8U manual and make sure your installation follows the recommended guidelines (including using the filter settings in the IF8U’s configuration). Grounding as well as proper wiring can be critical in reducing or eliminating noise and sometimes just properly grounding the PLC chassis will solve a whole host of issues. I’d also consider having any analog I/O as close to the PLC as possible I.E. as the first expansion module as well as having the wiring for the thermocouple as short as possible with as few connections as possible. In other words, if the thermocouple comes into the panel, goes to a junction block then to the PLC, consider bypassing the junction block and going straight to the PLC. Panel builders typically don’t like this, but it reduces noise exposure.
Lastly, the irony of all of this is you may be seeing this issue because the Spectrum I/O modules are very sensitive making them a good choice when needed very accurate analog I/O. It’s a lot easier for noise to cause noticeable issues when you’re using a 16-bit word for your analog signal than a 10- or 12-bit word. It does however mean that attention to detail during installation is much more important.

I had one site that was installed by others where we kept losing these modules, often when there was lightning in the area. Tracked it down to badly installed field wiring ( no screen termination) and lack of surge arrestors. Also added some sacrificial loop isolators. No further problems.

I have about 20 of these cards in service with no issues in my local area, in small remote water treatment plants.

As others have said, check for grounding and installation issues and follow the install manuals

Friday's (after a long week) aren't very good days for me to be writing long messages. My ability to catch grammar mistakes tends to be gone by Wednesday or Thursday

My ability to catch spelling and grammar mistake ends on the day that ends in "y". Also, the OP hasn't been back so we're just talking to ourselves at this point.