Remote-to-master data transfer uses the Type 2 transaction. The master terminal message contains the standard 4-octet Information field.
Each Data Unit may contain any number of octets, but the complete remote terminal message shall contain at least 3, and no more than 27, data octets together with the Remote Terminal Protocol octet. The effective internal address of each transferred Data Unit is implied by its position in the message.
Data types that may be acquired by the master terminal using this procedure include
1) Current condition of binary (status) inputs to the remote terminal.
2) Current values of analog inputs to the remote terminal.
3) Values of analog, binary, or other inputs previously stored at the remote terminal in response to a master terminal “Store Data Type (N)” command. This is commonly referred to as “Data Snapshot Reporting” and is used to minimize time skew between measurements made at multiple remote terminals.
4) Values of analog, binary, or other inputs that have changed significantly since last reported. This is commonly referred to as “Data by Exception Reporting” and is used to minimize the total volume of data to be transferred to the master terminal. Each Data Unit in this case includes both the new value and an identification of its internal address.
5) Sequence of Events Data. Each Data Unit of this type of data contains the internal address identification of a binary input that has changed state since previously reported, a flag defining its new state, and data indicating the time at which the remote terminal detected the state change.
6) Values of accumulators, e.g., input contact cycle counters, which may either be their current values or values previously stored in the remote terminal in response to master terminal or local “Accumulator Freeze” commands.
7) Data values calculated and stored within the remote terminal from combinations of its external inputs.
8) Data values previously transferred to the remote terminal from the master terminal that are being requested for verification purposes.
9) Data values obtained from remote terminal internal diagnostic processes that provide information on the operational performance of the various remote terminal subsystems.
Both Sequence of Events and Data by Exception Reporting involve the transfer of an unknown volume of data to the master terminal. Each master terminal request for the transfer of these data types therefore specifies the transfer of 27 data octets.
These requests are normally repeated until the corresponding protocol octet flags are reset by the remote terminal. Unused data octets are set to zero in the last message of a set of these data transfer transactions.
For Data by Exception reporting of analog values, the difference between the current measured value and the value previously reported to the master terminal for each analog input point is compared with a change limit for that point, which is stored at the remote terminal.
When this deadband limit is exceeded, the new input signal value and its address identification are added to a table stored for transfer to the master terminal. The new value is then used as the reference value for subsequent tests for significant input signal change.