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Section 3
Vendor-specific implementation
3.3.5.1
3.3.5.2
3.3.5.3
20
All frequently updated data are readable from a sequential data area. Additionally, there is
a separate sequential data area for measurands and counters with a slow update rate.
Primary and per-unit values
Measurands originating from CT measurements can be obtained from the IED in two
ways. They can be viewed either as primary values or as per-unit values.
The primary values are represented internally as decimal numbers. The primary units are
[A] for current. The internal representation of the per-unit values is always 1.0 at nominal
current. A typical range for a per-unit value is 0.00...40.00, that is 0 to 40 times nominal.
If the primary value representation is selected but no CT ratio parameters
are configured in the IED, the Modbus values remain as per-unit values.
Register sizes
In most cases the measurands or counters are located in single 16 bit registers. The
measurands are either unsigned or signed two's complement values while the counters are
always unsigned values.
In some rare cases the measurands or counter values can be located in two consecutive
registers, thus forming a single 32 bit integer value. The 32 bit value is always coded so
that the high word part, that means the higher 16 bits, is located first in the lower register
address. The low word part, that means the lower 16 bits, is then always in the next register
address.
Register sizes and types are clearly stated in the Modbus memory map list.
Time of update
Some Modbus values may have a time structure attached to their values in the Modbus
memory map. This is often the case with demand measurement values. The time structure
shows the time when the value was last updated.
1MAC052634-MB D
615/620 series ANSI
Communication Protocol Manual
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