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.
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