A data group transaction (sometimes referred to as a DDS transaction or Tx), is a record of a single instance of communication between a CygNet host and a field device. Such communication includes both reads and writes. The single instance, a read of AGA data for example, can include multiple messages. How many messages a transaction includes depends on many variables, including manufacturer protocol, data group structure, and data size. For example, sometimes protocols place a limit on maximum message size or require a handshake before communication can occur, or a data group structure demands that a user must provide information before a field device can return data.
Most data group data dialog boxes provide a drop-down list of recent transactions called Transaction. The number of transactions displayed here (and saved) is configured on the Data Group Properties dialog box in the Transaction Retention group box. The drop-down list displays the most recent transactions and their status. Selecting a transaction from the drop-down list changes the data displayed on the rest of the dialog box to reflect data applicable to that transaction.
Transactions displayed in the drop-down list are saved only in the Device Definition Service (DDS) unless you've mapped one or more data group elements from this data group to UDCs and points. You can then record current-value data for the data group element in the associated Universal Interface Service (UIS) and (if the data group element's point is configured to record history) to the associated Value History Service (VHS).
The following is a list of the various ways you can view data group transaction data:
The transaction data you view varies by EIE type, data group, and other variables. Each transaction includes metadata about the data group, as well as human-readable data in XML or in bytes in hexadecimal.
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