Option TTL factory configures the WattNode® Modbus meter for a 5-Volt logic level communications interface.

This is a special order item. Contact the factory for availability and price.

This option eliminates the need for extra circuitry (RS485 or RS232 transceiver ICs) when the master host device is only equipped with a 5V CMOS logic level port.

The WattNode meter tri-states the transmit line when it is not transmitting so that the interface can be shared by multiple slave device connections to the same master.

CMOS logic level signals should be kept shorter than a few inches, unless special shielding is used. It is less noise tolerant than RS485 or RS232 but adequate for direct “board level” connections, such as when used in conjunction with the WattNode Modbus – Option X5 for interfacing the meter directly to a wireless transceiver without the need for an external power supply. When interfacing with 3V or 3.3V logic levels, a level translator IC is required to ensure that the interface functions properly under all operating conditions.

The table below summarizes the WattNode logic level thresholds:

Symbol Threshold Description Condition
VIH 3.5 V Minimum input high level VDD = 5.18 V
VIL 1.12 V Maximum input low level VDD = 4.82 V
VOH 4.12 V Minimum output high level IOH = -3.0 mA
VOL 0.6 V Maximum output low level IOL = 8.5 mA

Notes

  • This is a hardware option that cannot be changed in the field.
  • The term, “TTL”, is commonly taken to mean “logic level” since obsolete TTL logic has been superceded by CMOS and other logic families.
  • WattNode Modbus – Option X5 can supply nominal 5 VDC power at 60 mA and is regulated to within +/-3.5% (4.82 V to 5.18 V).
  • The WattNode receive line is a Schmitt trigger input with hysteresis so it is more noise tolerant than normal logic level inputs.

ESD / EMI Precautions

The Modbus CMOS logic lines are not high-voltage ESD (electrostatic discharge) rated, although there are 430 ohm series resistors protecting both transmit and receive lines. Use proper grounding precautions when making connections to the A and B terminals.

  • The transmit line A terminal could conduct some radio frequency noise from the meter. We recommend keeping this line short if possible. You should verify the FCC performance of the final configuration.
  • The receive line B terminal may be more susceptible to ESD and other forms of electromagnetic interference. Try to keep the line short and protect it from ESD and severe electric or magnetic fields. Use a shielded wire for longer lengths.

See Also