There are two IEEE 1451.4 interfaces:
- Class 1 Device - Shared wires for analog and digital
In class 1, the digital signal is shared with the analog signal on the same lines, although it is time multiplexed based on the biasing of the sensor. The sensor would include the TEDS EEPROM and the circuitry for switching based on bias direction, which is usually only a resistor and a diode or two.
- Class 2 Device - Independent Digital TEDS Interface
In class 2, the sensor only has the addition of the EEPROM and the two additional wires, while the DAQ instrument has a digital interface in addition to the analog.