OPC UA Read Task

Learn how to configure a read task to acquire data from an OPC UA server.

Read tasks are the primary method for acquiring data from a PLC and storing it in Synnax. In this guide, we’ll walk you through the process of setting up a read task.

Prerequisites

Before configuring the task, you’ll need to have connected to an OPC UA server and created the relevant channels in Synnax. Check out Connect to an OPC UA Server for information on how to do this.

A Short Primer on Tasks

Tasks are the primary method for communicating with devices in Synnax. Tasks can be used for both control and data acquisition purposes. In the context of OPC UA, a read task defines a background process that reads data from your server at a specified sampling rate.

An OPC UA read task can be started, stopped, and re-configured at any time. We permanently store the configuration of the task in Synnax, so it’s easy to set up multiple tasks for different purposes.

Important Rules for Read Tasks

There are a few important concepts to keep in mind when configuring a read task.

Sample Rates & Timing

All channels in a read task are sampled at the same rate. If you’d like to sample different channels at different rates, create a separate task to do so.

Synnax uses software timing to read from your PLC, and typically delivers precision on the order of 100 microseconds. Under heavy load, this timing precision may degrade.

For low rate, sub 200Hz tasks where timing is not critical, we recommend letting Synnax automatically determine the timestamps for index channels (for more information, see correctly configuring indexes).

For high-precision tasks, we recommend reading timestamps from the PLC. More info on this here.

For high rate tasks above 500Hz, we also recommend sampling in array mode, where Synnax will read samples in bulk from the PLC. Array mode is more challenging to configure, but can be far more efficient.

One Running Task Per Channel

Synnax can only accept live data from a channel from one source at a time. If you have a task running that reads from a pressure channel, you cannot create another task that reads from the same channel.

One Running Task Per Index Channel

Index channels are use to store the timestamps for your data channels. When you configure a task to read from a data channel, Synnax will automatically generate timestamps for the corresponding index channel, regardless of whether you’ve explicitly added it to the task.

For example, imaging we two channels, pressure and temperature that share the same index channel, time. If you configure a task to read from pressure, Synnax will automatically generate timestamps for the time channel. If you then configure a task to read from temperature, Synnax will not allow you to start the task, as the time channel is already in use.

Open the Task Configuration Dialog

To configure a read task, open the Synnax Console and click on the Quick Search & Command Palette at the top. You can also open this palette with Ctrl+Shift+P on Windows/Linux or Cmd+Shift+P on macOS.

In command mode (enabled when the first character in the input is ”>”), type “OPC UA”. You’ll see an option called “Create a New Read Task”. Select this option to open the read task configuration dialog.

Configuring the Task - Default Sampling

The default sampling mode reads a single sample from the server for each channel in the task at the specified sampling rate. This is the simplest mode to configure, and is ideal for most applications that don’t require high data rates. For high data rates, see Array Sampling.

Step 1 Select your OPC UA Device

The first step is to select the OPC UA server you’d like to collect data from. Use the dropdown menu in the top left corner to do so.

Step 2 Add Channels to the Task

Next, you’ll need to add channels to the task. These channels define a mapping between the OPC UA node you’d like to read from, and the corresponding Synnax channel you’d like to store the data in.

To add a channel, click on the ”+” button in the channels section. Then, click on the new channel and select the OPC UA node you’d like to read from. Finally, select the Synnax channel you’d like to store the data in. Note that the Synnax channel doesn’t need to be the same as the channel you created when configuring the server.

Step 3 Optional - Read Timestamps from the PLC

If the index channel(s) for your corresponding data channels is not in the task, Synnax will automatically generate timestamps when reading data from the server. This is fine if you’re not concerned about timing precision.

If you’d like to pull timestamps from the PLC, simply add the index channel to the task and select the corresponding OPC UA node.

Step 4 Configure Timing

There are two parameters to control the timing of the task: the sample rate and the stream rate. The sample rate defines how often Synnax will poll the OPC UA server for new data, while the stream rate defines how often the task will stream data into Synnax.

For example, if you set the sample rate to 10hz and the stream rate to 1hz. Synnax will poll the OPC UA server every 100ms, collect 10 samples, and then forward these samples to the server.

For very low rate tasks (sub 50Hz), we recommend setting the stream rate to the same value as the sample rate. For higher rate tasks, lowering the stream rate can significantly improve performance. It’s typically best ot keep the stream rate below 50Hz unless you need to quickly process incoming data in real-time.

Step 5 Configure and Start the Task

Once you’d added all the channels you need, click the “Configure” button. If there are no errors, Synnax will acknowledge the configured task and enable the “Play” button in the dialog.

At this point, feel free to close the dialog. Your task will continue to run in the background until you re-open the window to stop it.

Configuring the Task - Array Sampling

Array sampling is a more advanced method of reading data from an OPC UA server, and is typically only necessary for very high rate tasks (above 500Hz). Instead of reading single samples from the server, Synnax will read many samples in bulk. Array sampling requires a little bit of tuning to set up, and here’s how you can do it.

Step 1 Make sure your PLC is correctly configured

Make sure your PLC is correctly configured to read data in arrays. For information on how to do this, see considerations for high data rates.

Step 2 Enable Array Sampling

To enable array sampling mode for a task, toggle the “Array Sampling” switch in the top left corner. This will alter the “Stream Rate” field to display an “Array Size” field instead.

Set the array size to the same length as the arrays you’ll be reading from the PLC.

Step 3 Add Channels to the Task

In array mode, you’ll only be able to add PLC nodes that are array-based. Otherwise, the process is the same as for default sampling.

Step 4 Configure Index Channels

For high data rates, we recommend reading timestamps from the PLC instead of having Synnax generate them. To do this, add the index channel for any of your data channels to the task, and select the corresponding PLC node that contains the timestamps.

Step 5 Timing

Running a task with array sampling enabled can result in both oversampling and undersampling. Oversampling occurs when the PLC doesn’t completely replace all values in the array before Synnax reads it. Undersampling can occur when the PLC is re-writing the array faster than Synnax can read it.

We recommend setting the sampling rate to the same sampling rate as the PLC. Naturally, we recommend that this rate is an even multiple of the array size. For very high data rates, under and oversampling by a few samples in the array can occur under normal operating conditions. This typically isn’t a problem. If you’re concerned about this, let us know.

As an example, here’s a configuration for a task that reads at 8460Hz from an array of length 320. This means that the task will reads 320 samples and forward them to Synnax 27 times per second.

Step 6 Configure and Start the Task

Once you’ve added all the channels you need, click the “Configure” button. If there are no errors, Synnax will acknowledge the configured task and enable the “Play” button in the dialog.