Specifications 57
Modbus Communication
Protocol:
Modbus RTU (binary)
Baud Rates:
1200, 2400, 4800, 9600, 19200, and 38400
Duplex:
Half (two-wire plus common)
Polarity Auto-detect:
will automatically correct swapped A- and B+ terminals provided network
has at least 200 millivolt bias between A- and B+.
Parity:
Standard:
N81 (no parity, eight data bits, one stop bit)
Optional:
E81 (even parity, eight data bits, one stop bit)
Modbus Buffer:
256 bytes
Communication Response Time:
5 - 25 milliseconds (may be longer immediately after a
Modbus write command while values are saved to non-volatile memory).
EIA RS-485 Interface:
RS-485 Output Isolation:
4500 Vac RMS
Driver Output Voltage (Open Circuit):
±6 Vdc maximum
Driver Output Voltage (54 Ω load):
±1.5 Vdc minimum
Driver Output Current (54 Ω load):
±60 mA typical
Driver Output Rise Time (54 Ω || 50 pF load):
900 nS typical
Receiver Common-Mode Voltage Range:
-7 Vdc to +12 Vdc maximum
Measurement
Creep Limit:
0.10% (1/1000th) of full-scale. Whenever the power or reactive power for a phase
drops below the creep limit, the power or reactive power for the phase will be forced to zero.
Also, if the line voltage for a phase drops below 20% of nominal Vac, the output power for the
phase will be set to zero. These limits prevent spurious readings due to measurement noise.
To customize the creep limit, see
in
Configuration Registers (p. 46)
.
Update Rate:
1.0 second. Internally, all measurements are performed at this rate.
Start-Up Time:
Approximately 1.0 second. The meter starts measuring 50-100 milliseconds after
AC power is applied, but requires a full 1.0 second measurement cycle before it starts report-
ing data. The WattNode meter does not respond to Modbus packets during this start-up time.
Current Transformer Phase Angle Correction:
1.0 degree leading. Current transformers (CTs)
typically have a leading phase angle error ranging from 0.2 degrees to 2.5 degrees. The
WattNode meter is normally programmed to correct for a 1.0 degree phase lead to provide
good accuracy with typical CTs. The CT phase angle correction can be changed using the
PhaseAdjustA, PhaseAdjustB, PhaseAdjustC
registers.
Over-Voltage Limit:
125% of nominal Vac. If the line voltage for one or more phases exceeds this
limit, the status LEDs for these phases will flash alternating red-green as a warning. Extended
over-voltage operation can damage the meter and void the warranty. See
.
Over-Current Limit:
120% of rated current. Exceeding 120% of rated current will not harm the
WattNode meter but the current and power will not be measured accurately.
Additional Error:
±0.5% of reading
Condition 6 ‑ Temperature Variation
All conditions the same as Condition 1 except:
Ambient Temperature:
-30°C to +55°C
Additional Error:
±0.75% of reading
