43
ALL THINGS MODBUS
M
ODBUS
D
ATA
S
CALING
—I
NTERPRETING THE
P
OWER
P
ATROL
R
EGISTERS
The use of Modbus protocols limits the data registers to a maximum of two bytes (16 bits) or a
maximum decimal value of 65535. Modbus requires that the data be unsigned (positive) integer values.
To overcome these limitations some measured (and stored) values must be
scaled
to fit into the Modbus
registers. The raw value read from the Modbus registers is multiplied by a scalar to convert the raw
data. The following table lists the data scalars and the respective values for the Power Patrol.
Data Scalar
Scalar Value
kW/kWh
Demand
kVAR/kVARh
kVA/kVAh
Power
Factor
Amps
Volts
0
.00001
.00001
.00001
.01
.01
.1
1
.001
.001
.001
.01
.1
.1
2
.01
.01
.01
.01
.1
.1
3
.1
.1
.1
.01
.1
.1
4
1
1
1
.01
1
1
5
10
10
10
.01
1
1
≥6
100
100
100
.01
1
1
Table D-1: Data Scalars and Values for Registers 44001 thru 44061
The data scalar is stored in register 44602.
When selecting a data scalar, the following guidelines need to be considered:
If the data scalar selected is too low, an incorrect data result is returned from the register.
If the data scalar selected is too high, the significant digits following the decimal point are
removed.
After selecting a data scalar, the formula for calculating the actual value is:
register value X scalar value = actual value
Or, another way to state this formula is:
actual value/scalar value = register value
D
ID
Y
OU
K
NOW
:
The
register value must
be less than 65,535.
