ABB REU 610, Техническое справочное руководство

Страница: 68 / 140

With selection code -1...-99, the master can move backwards from the newest
event as many events as defined by the selection code and read that specific
event record. After this, the master can continue reading the following records
using selection code 1, irrespective of whether they have been read before.
*
Special case: If there is not as many events in the buffer as specified by the
selection code, the oldest stored event is read.
Selection code 3
The Modbus event buffer is cleared with selection code 3. Clearing the buffer
does not require any read operation to follow.
Digital inputs
As the master may not detect the state changes of all digital signals when
scanning, an additional change detect (CD) target bit is created for every
instantaneous target point; see the example below.
Momentary
Master reads Master reads Master reads Master reads
Change detect
A040332
Fig. 5.1.15.2.-1 Change detection bit
If the instantaneous value of a target bit has changed two or more times since
the master last read it, the CD bit is set to one. When the CD bit has been
read, it is set to zero.
The instantaneous and the CD bit of a certain target point always occur as a
pair in the Modbus memory map.
Modbus data mapping
There are two types of monitoring data: digital targets and measurands. For
convenience and efficiency, the same data can be read from different data
areas. Measurands and other 16-bit values can be read either from the IR or
68
REU 610
Voltage Relay
Technical Reference Manual - ANSI Version
1MRS755972