Option X5 factory configures the WattNode® Modbus meter to provide 5 Vdc on the X terminal on the “MODBUS” connector (upper left corner). The C terminal provides the ground return. The meter can provide up to 60 mA of current, provided you do NOT have a terminated RS-485 bus. This 5 volt supply can be used to power a low-power RF transceiver, a data logger, or other low-power devices.
Most RS-485 buses have biasing resistors—these are commonly built into RS-485 adapters for PCs or into monitoring equipment like data loggers. The biasing resistors put a small load (a few milliamps) on the network, and won’t affect the 5 Vdc output.
Some RS-485 networks, especially with long wiring or high baud rates, also have two 120 Ω terminating resistors, one on each end of the network. These terminating resistors will draw 40-60 milliamps of current from the RS-485 bus when a device is transmitting.
If you are using a terminated RS-485 bus AND drawing power from the X terminal, the combined load could exceed the capacity of the WattNode meter’s power supply. This won’t damage the meter, but it may cause it to reset. Therefore, Option X5 is only recommended for unterminated RS-485 networks.
The meter may be able to provide more than 60 mA in some cases or for short durations, but there is a greater risk that the power supply will drop below 5 volts due to the extra load and may cause the WattNode meter to reset. Therefore, if you need more than 60 mA of supply current, we recommend an external power supply or some mechanism like a large capacitor to provide extra current when needed.
Short circuiting the 5 volt output will cause the voltage regulator to current limit at between 100 mA and 150 mA and the meter to appear dead until the short-circuit is removed. Once the short is removed, the meter should power up and operate normally, so there is no need for a fuse or over-current protection.
ESD / EMI Precautions
The normal Modbus RS-485 lines are rated to handle 15 kV of ESD (electrostatic discharge). Furthermore, the WattNode Modbus meter without Option X5 has passed all FCC Class B and CE Mark EMC (electromagnetic compatibility) tests. However, the Option X5 version has not been separately tested.
- The 5 volt line could conduct some radio frequency noise from the meter. We recommend keeping this line as short as possible. You may also want to verify the FCC performance of the final configuration.
- The 5 volt line may be more susceptible to ESD and other forms of electromagnetic interference. Try to keep the 5 volt line short and protect it from ESD and severe electric or magnetic fields.
- Output Voltage: 5 Vdc ±5%
- Minimum Output Current: 0 mA
- Maximum Output Current: 60 mA
- Maximum Short-Circuit Output Current: 150 mA
- Recommended Maximum 5 Volt Wire Length: 1 foot (30 cm)