NI-DAQ OPC Server Troubleshooting Wizard: Not All Channels Functioning

MAX shows correct readings for my AI signal. Server Explorer reads different values.

This is probably a hardware issue (involving SCXI AI modules, in most cases) that falls under the category of single-channel versus multiple-channel acquisition. In the MAX test panel, you test one channel at a time. In OPC, if there are multiple tags in the system, you scan all the tags. To verify if this is the issue, create a new MAX configuration with a single virtual channel and compare the readings from MAX and OPC; they should be identical. Due to some NI hardware limitations, the settling times or gains used might be different in the two cases, thereby causing this behavior. See if you can duplicate the same behavior by scanning multiple channels in LabVIEW using the following VI. Make sure you match the update rate on the panel with the OPC client update rate (use the default rate if you are not sure).

See Also:
Virtual Channel Test (AI) VI

Thermocouples are drifting after running for a few hours. Restarting the OPC Server fixes the drifts.

Some SCXI modules, such as the SCXI-1100, have non-scannable channels, for example, non-scannable CJC (cold-junction compensation) or a non-scannable CALGND. CJC channel provides additional information to compensate for the ambient temperature in the measurement circuit. CALGND compensates for input amplifier offsets. In some modules, these inputs are non-scannable, meaning they cannot be read simultaneously with the acquisition. They are read only when the DAQ OPC starts. If your environment (temperature, noise) is changing, the startup values become obsolete after a while and they need to be reacquired. You must manually enable the recalibration in Measurement & Automation Explorer (MAX), following the steps below.

1. Right-click on the DAQ board under Devices and Interfaces>>Traditional NI-DAQ Devices, and select Properties.

2. Go to the OPC tab and enter the number of seconds for the recalibration period.

The AI Recalibration tab does NOT perform a device self-calibration. It is only meant for use if you have any non-scannable channels. Only a few pieces of NI hardware fall under this category; most of the hardware does not have the channels at all or have scannable CJC or CALGND. Be aware that the acquisition has to be restarted. If you do not have any hardware with these types of channels, then this setting does not apply to you and you should leave it disabled. Also, making this value too low can overload your system with a frequent acquisition restart.

I can update Digital Output channels from MAX, but am unsuccessful via NI-DAQ OPC server.

The OPC specification defines two methods to perform an I/O operation: asynchronous and synchronous. There is a known issue with the NI-DAQ OPC server where asynchronous Digital Output updates don't work. You must instruct your client to use synchronous operations instead. For the LabVIEW DSC Module, the following steps apply.

1. Make sure both the Tag Engine and DAQ OPC server are not running.

2. In LabVIEW, select Tools»DSC Module»Advanced»Server Browser...

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3. In the Server Browser, click OPC Client Settings.

4. In the OPC Client Settings, click View All.

5. Highlight National Instruments.DaqOpc and click the Add button. Your OPC Client Settings window should look similar to the window shown below.

6. Close the remaining windows and test the digital outputs.

Cannot read channels from an SCXI-1122 module.

DAQ OPC, when used with an SCXI-1122, does not allow proper settling time. With either the 4 Hz setting or the 4 kHz filter setting on the SCXI-1122, DAQ OPC scans too fast and does not allow the filter to settle properly between channels. The effect is much more noticeable with the 4 Hz setting than the 4 kHz setting, but both are affected. This issue was fixed in NI-DAQ version 6.9. Make sure you have the latest NI-DAQ driver for your DAQ board by following the link below.

See Also:
Download the Latest NI-DAQ Driver

DAQCard-1200 produces a very noisy signal under DAQ OPC.

The DAQCard-1200 produces a very noisy signal under DAQ OPC when reading both thermocouple and 0 to 4 V signals. The thermocouple looks correct in MAX, but when using DAQ OPC to read values, the thermocouple values fluctuate. The problem occurs due to a hardware limitation in the 1200 Series devices that the same gain is applied to all the channels. A low gain is applied to accommodate the 0-4 V signals, and as a result the small thermocouple signals appear very noisy. Use of an E Series board is recommended for this type of mixed acquisition.

Low-quality readings with the 4350/4351 devices.

Using a 4350/4351 board with DAQ OPC is not recommended. The 435X boards need special low-level parameter settings, such as notch filter frequency, open thermocouple-detection, ground referencing, and so on, to ensure the most accurate measurement. These settings are not exposed through the DAQ OPC interface, which results in low-quality readings.

Using Counters with DAQ OPC

See Also:
Can I Use Counters with NI-DAQ OPC?

Using Remote SCXI with DAQ OPC

See Also:
Does the NI-DAQ OPC Server Support Remote SCXI?

Using Remote Device Access (RDA) Server with DAQ OPC

Virtual channels that are connected to an RDA Server are not exposed by the DAQ OPC Server. The server includes only local virtual channels in the OPC configuration. If you need to access virtual channels on a remote computer, you must use Remote OPC.

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