Controllogix 1756 & 1492-IFM40F-F120-2: Force a bit

leonlai · Sep 17, 2015

folks,
I have a controllogix 1756, DO Card (OW16) connect to 1492-IFM40F-F120-2. I did a test at a pin on 1492-IFM40F-F120-2, Force a bit on, and measure the voltage out on the pin, there is only around 4-6 Volts different, is it suppose when I force a bit, it should go from 0 to 120V ?
There are multiple unused pin on F120, I did 3 different unused pins and get the same result when I force those DO output and expect to see 0 to 120V, but only around 116V to 120V.

Thanks

phuz · Sep 17, 2015

leonlai said:
folks,
I have a controllogix 1756, DI Card connect to 1492-IFM40F-F120-2. I did a test at a pin on 1492-IFM40F-F120-2, Force a bit on, and measure the voltage out on the pin, there is only around 4-6 Volts different, is it suppose when I force a bit, it should go from 0 to 120V ?
There are multiple unused pin on F120, I did 3 different unused pins and get the same result when I force those DO output and expect to see 0 to 120V, but only around 116V to 120V.

Thanks

A couple things. You say DI card. This would be a digital input, so you aren't going to read any output voltage on it. Did you mean DO? If so, what type of DO? OA16? OW16?

leonlai · Sep 17, 2015

oops, my bad, DO card, it's 1756-OW16

phuz · Sep 17, 2015

And you have 120v feeding the card?

leonlai · Sep 17, 2015

Yes, other pins with relay hook-up already are working fine. These pins which I test dont have relay hook-up yet. I just wanna do test before put 120V relay in.

phuz · Sep 17, 2015

I'm sorry as I am having trouble understanding you. I don't think I fully understand what you are asking.

If you force on the output, and forces are enabled, the voltage will be whatever the supplied voltage is. If you are getting varied readings, you could have a bad module or IFM.

leonlai · Sep 17, 2015

Sorry to confuse you guys,
I have 1756 with a DO connect to 1492-IFM40F-F120-2. first 6 pins on F120 have 120Volt relay, the pins #7,8,9 don't have any 120V relay yet. When I force bit 7,8,9 (pin 7,8,9 on F120) I expect to see 0 to 120V, but I don't, I only see 116V to 120V. While other pin 1 to 6 work fine which I measure 0 to 120V. So my question is what wrong with pin 7,8,9 or I am missing something here ?

danatomega · Sep 17, 2015

If your DO is triac (as I suspect it is) AND you are saying that forced ON = 120 VAC and forced OFF = 116 VAC then you are seeing the leakage current present a voltage to your hi impedance meter and is normal.
You would need to put a load on the output for the leakage current to be sucked down by the load - then you would see ~0 VAC when forced OFF.

leonlai · Sep 17, 2015

Thanks for your answer, it help me to understand more about Panel & Controllogix hardware, I have no formal training in Control Panel, my background is Mechanical. Appriciate. I'll put a load on Monday to see because I cannot get online to force on until Monday.
If there is any explain about why/where leakage current present a voltage, please educate me !?
yes, DO are triac 120VAC

Thanks

danatomega · Sep 17, 2015

You could look at this Wikipedia article for some info but it's kind of technical and not so interesting:
https://en.wikipedia.org/wiki/Leakage_(electronics)

If you look at the datasheet for your output module you will see it in the specs : max leakage current.
It apparently happens in most all semiconductors.

Controllogix 1756 & 1492-IFM40F-F120-2: Force a bit

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