A number of bus stop channel messages have a property named Acquisition type, which determines how a channel message receives data from or sends data to it's peers. This property defines the following acquisition types, which may or may not be supported by all channel messages.
Please check the channel/channel message documentation for the supported acquisition types for each channel.
Interval Acquisition (Upload only)
All tags contained in a channel message are read every x seconds where x is defined by the Acquisition Settings property.
Trigger Tag Acquisition
The values of the tags are updated when the value of the selected trigger tag changes. This concept is used to prevent data from being updated without the other end approving it. The trigger is configured by the Acquisition Settings property and it allows to configure the following triggers:
For Upload Messages
| Trigger Type | Description |
| Simple handshake upload | The peer (device/service) prepares the data in the device tag and sets the trigger. ATS Bus reads the data and resets the trigger. This option requires the Trigger Tag to be specified. |
| Double handshake upload | The peer prepares the data in the device tag and sets the trigger. ATS Bus reads the data and acknowledges the trigger by setting the Acknowledge Tag. The PLC resets the trigger and ATS Bus resets the Acknowledge Tag. This option requires the Trigger Tag and Acknowledge Tag to be specified. |
| Rising Edge | Start message exchange when the trigger tag goes from zero to non-zero |
| Falling Edge | Start message exchange when the trigger tag goes from non-zero to zero |
For Download Messages
| Trigger Type | Description |
| Value download | This downloads the channel message directly to the PLC without using a handshake. |
| Simple handshake download | ATS Bus prepares the data in the device tag and sets the trigger (or ack). The peer (device/service) reads the data and resets the trigger (or ack). This option requires the Trigger Tag to be specified. |
| Double handshake download | ATS Bus prepares the data in the device tag and sets the trigger. The peer (device/service) reads the data and acknowledges the trigger. ATS Bus resets the trigger and the peer resets the acknowledge. This option requires the Trigger Tag and Acknowledge Tag to be specified |
The Trigger Tag and Acknowledge Tag can be entered manually or you can click the search button (
) to locate it. In the Browse OPC tags dialog click Refresh to view the possible tags.
On Change Acquisition (Upload only)
All tags are read when any of them changes value but there is one exception. The tags in the OPC client channel have a property named Monitored item, all tag values are read if the value of any monitored items changes.
On Change Acquisition does not have any Acquisition Settings parameters that must be configured.
Receive Response (Upload only)
This channel message is triggered by a channel message that has its Direction set to ‘Download’, Process Response checked and its Response Channel Message property set to the name of this channel message.
Example
A channel message with its Direction set to Download can be used to perform a HTTP GET request to get something from a web service. The response can be captured and passed to the channel message identified by the Response Channel Message property. This eliminates the need of a channel message with its Direction set to Upload and the Acquisition type set to Interval to poll for the data. This acquisition type is not supported by all channels.
Scheduled Acquisition (Upload only)
Scheduled acquisition uses the Quartz Cron Scheduler to schedule the execution of channel messages. The scheduler uses a cron-like expression to configure the time at which to execute the channel message.
The Linux cron schedule format differs from the cron expression used by the Quartz Cron Scheduler. Seconds cannot be configured in Linux whereas they can in Quartz Cron Scheduler.
For further information on the Cron expression format used by Quartz Cron Scheduler, please click here.
When setting the acquisition type to Scheduled, the user can select an example/template from the Cron example dropdown and import it into the Cron expression text box by clicking the Use expression button to further define the expression.
