10
To map the PM174 register address to the range of the Modbus holding registers, add a
value of 40001 to the PM174 register address. When a register address exceeds 9999, use
a 6-digit addressing scheme by adding 400001 to the PM174 register address.
2.6 Data Formats
The PM174 uses three data formats to pass data between a master application and the
instrument: 16-bit short integer, 32-bit long integer and 32-bit modulo-10000 formats.
Binary values and counters are always transmitted in 32-bit registers, while analog values
can be read both in 32-bit and in 16-bit scaled registers.
Analog registers 256 through 308 and 6656 through 10935 contain scaled 16-bit values.
2.6.1 16-bit Scaled Integer Format
16-bit scaled analog data is transmitted in a single 16-bit Modbus register being scaled to
the range of 0 to 9999. To get a true reading, a reverse conversion should be done using
the following formula:
LO
9999
)
LO
HI
(
X
Y
+
−
×
=
where:
Y
- true reading in engineering units
X
- raw input data in the range of 0 to 9999
LO and HI
- data low and high scales in engineering units
The engineering scales are indicated for every scaled 16-bit register. Refer to Section 4
“Data Scales and Units” for applicable data scales and measurement units.
The default voltage scale in the device is 144V (120V+20%). It can be changed through
register 242 (see Section 3.1, Device Data Scales), or via the supplemental PAS software.
The recommended voltage scale is 120V+20% = 144V for using with external PT’s, and
690V+20% = 828V for a direct connection to power line.
CONVERSION EXAMPLES
1. Voltage readings
a) Assume device settings (direct wiring): PT ratio = 1; Voltage scale = 828V (690V + 20%).
Voltage engineering scales (see Section 4):
HI_ENG = Vmax = 828.0
×
PT ratio = 828.0
×
1 = 828.0V
LO_ENG = 0V
If the raw data reading is 1449 then the voltage reading in engineering units will be as follows:
Volts reading = 1449
×
(828.0 - 0)/(9999 - 0) + 0 = 120.0V
b) Assume device settings (wiring via PT): PT ratio = 14,400V : 120V = 120; Voltage scale = 144V.
Voltage engineering scales (see Section 4):
HI_ENG = Vmax = 144.0
×
PT ratio = 144
×
120 = 17,280V
LO_ENG = 0V
If the raw data reading is 8314 then the voltage reading in engineering units will be as follows:
Volts reading = 8314
×
(17,280 - 0)/9999 + 0 = 14,368V
2. Current readings
Assume device settings: CT primary current = 200A.
Current engineering scales (see Section 4):
HI_ENG = Imax = CT primary current
×
2 = 200.00
×
2 = 400.00A
LO_ENG = 0A
