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U6 DI State Staying High


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9 replies to this topic

#1 u3cool

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Posted 30 April 2013 - 12:42 AM

I have a relay that has 24 VDC coming into the Common on the relay. I'm attaching the Normally Closed contact to FIO0 through a 22k ohm series resistor. When the relay is off, 24 VDC is supplied to the Normally Closed contact and there is about 5 VDC into FIO0 and the logic is shown as high. When the relay activates and the Normally Closed contact opens, there is about 3.4 VDC on FIO0 and the logic stays high. As I read section 2.8.1.1 - Input: Driven Signals of the manual, when the signal is removed, the volts on FIO0 should drop to 0.8, and the logic should go low. I'm using DAQ Factory Express. The U6 and the 24 VDC share the same 120 VAC connection and there is 24 VDC between the + side of the 24 VDC power supply and GND on the Labjack. I'm not running a wire from the 24 VDC ground to the Labjack GND, as the 24 VDC power supply and the Labjack already appear to share a common ground. What could be causing FIO0 to stay logic high? I thought about increasing the size of the series resistor to achieve a larger voltage drop across the resistor and have only about 1 volt on FIO0 when 24 VDC is on the Normally Closed contact. Would that approach work?

#2 LabJack Support

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Posted 30 April 2013 - 10:54 AM

As I read section 2.8.1.1 - Input: Driven Signals of the manual, when the signal is removed, the volts on FIO0 should drop to 0.8, and the logic should go low.

Not when the signal is removed, but when the signal is driving 0 volts. Very different.

You don't have a driven signal. You have a PNP style signal, where it changes between 24V and OPEN.

First option, is can you connect the relay common to GND from the U3 rather than your 24VDC source? Is this relay output used for anything else, or just to provide a signal to the U3. If you can connect the common to GND, then you have the open-collector or NPN signal described in Section 2.8.1.2:

http://labjack.com/s...s-guide/2.8.1.2

If you need to keep the 24VDC/OPEN behavior, then you just need to add a resistor from FIO0 to GND that overpowers the internal 100k pull-up to 3.3V and holds FIO0 below 0.8V when the input signal is floating.

Keep your 22k resistor between signal and FIO0, and add a 4.7k resistor from FIO0 to GND. When the signal is driving 24V, you should have about 4.2V at FIO0. When the signal is floating, you should have about 0.1V at FIO0.

#3 u3cool

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Posted 30 April 2013 - 12:33 PM

Thanks for the prompt response. The relay is powering a solenoid valve, so I can't change the relay wiring. I'll add the resistor between FIO0 and GND. Actually, I have 17 relays and need 17 DIs of this type. I just used FIO0 as an example. Can I connect several resistors between several DI terminals on the U6 and one GND terminal?

#4 LabJack Support

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Posted 30 April 2013 - 02:01 PM

You will need to add 1 pull-down resistor for each DI. Whether they connect to the same GND terminal or different GND terminals does not matter.

#5 u3cool

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Posted 06 May 2013 - 11:18 AM

You will need to add 1 pull-down resistor for each DI. Whether they connect to the same GND terminal or different GND terminals does not matter.


Adding the pull down resistors took care of getting the DIs to go low. I now have another problem. There are 4 AINs connected to the U6. Most of the time the 17 DIs are all low. Some times all DIs are high and putting about 4.4 volts each to the U6 DI terminals. When this happens, the AIN signals go low.

As an example, one AIN is monitoring the output of a 12 VDC power supply that is powering the other three AINs. I'm using the approach in 2.6.3.6 of the U6 manual to monitor the output of the power supply. Normally, the U6 shows the power supply to have an output of 12.77 VDC (input to U6 scaled). However, when all DIs go high, the U6 shows the power supply output as 11.74 VDC. Checking the power supply output with a DIMM shows that the power supply maintains a constant 12.82 VDC whether the DIs are high or low.

The other three AINs are voltage drops across shunt resistors in 4-20ma loops.

The DIs are 24 VDC outputs dropped through 21k resistors. Are all the 17 DIs when high and putting about 4.4 volts each on the U6 overpowering the U6 or are the 17 21k resistors in parallel dropping the resistance of the AIN legs such that the voltage drops across the AIN resistors are less since the resistance is less?

#6 LabJack Support

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Posted 06 May 2013 - 02:33 PM

Are these connections on the CB37? Perhaps too much current is flowing on ground as mentioned on the CB37 datasheet:

http://labjack.com/s...7-v21/datasheet

To help troubleshoot and solve, use your DMM to measure the following in the good and bad conditions:

AINx versus CB37 GND
AINx versus CB37 AGND
AINx versus U6 GND
CB37 GND versus U6 GND
CB37 AGND versus U6 GND

#7 u3cool

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Posted 06 May 2013 - 03:13 PM

Are these connections on the CB37? Perhaps too much current is flowing on ground as mentioned on the CB37 datasheet:

http://labjack.com/s...7-v21/datasheet

To help troubleshoot and solve, use your DMM to measure the following in the good and bad conditions:

AINx versus CB37 GND
AINx versus CB37 AGND
AINx versus U6 GND
CB37 GND versus U6 GND
CB37 AGND versus U6 GND

The AINs are all on the U6 with their grounds the SGND on the U6. Ten of the DIs are on the CB37 and 7 are on the CB15. All are connected to the GNDs on the CB37 and CB15. Would a possible fix be to connect the AINs to the CB37 with their grounds being the AGND?

#8 LabJack Support

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Posted 06 May 2013 - 03:17 PM

I'm just not sure what problem we need to fix yet. If you are doing single-ended measurements, the U6 is reading from AINx to U6 GND. So if you use the DMM to measure from AINx to U6 GND, do you see the voltage change when all DIs go high? If you use the DMM to measure from SGND to GND, do you see a reading?

#9 u3cool

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Posted 06 May 2013 - 04:40 PM

I'm just not sure what problem we need to fix yet. If you are doing single-ended measurements, the U6 is reading from AINx to U6 GND. So if you use the DMM to measure from AINx to U6 GND, do you see the voltage change when all DIs go high? If you use the DMM to measure from SGND to GND, do you see a reading?

I won't be able to do all the measurements until tomorrow, but I have measured from AINx to U6 GND and do see a voltage drop from AINx to U6 GND when all the DIs go high. I'll make the other measurements tomorrow.

#10 LabJack Support

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Posted 07 May 2013 - 09:04 AM

So you do see the drop using a DMM to measure from AINx to GND. That tells you the U6 is correctly measuring the voltage you are giving it, but the question is why is the voltage dropping? Use the DMM to also look at AINx to SGND. Perhaps too much current is flowing into SGND and making it substantially different than GND.


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