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AD5700/AD5700-1 & Full Duplex Operation


Can the AD5700/AD5700-1 support Full Duplex Operation?



HART is, by nature, half duplex. It uses one channel (one pair of wires) for both directions. If one side is transmitting and the other tries to transmit at the same time, the signal will get corrupted and neither side will be able to decode/demodulate it correctly.

Full duplex operation means that the modulator and demodulator of the AD5700/AD5700-1 are enabled at the same time. This is a powerful feature, enabling a self-test procedure of not only the HART device but also the complete signal path between the HART device and the host controller. This provides verification that the local communications loop is functional. This increased level of system diagnostics is useful in production self-test and is advantageous in improving the application’s safety integrity level (SIL) rating. The full duplex mode of operation is enabled by connecting the DUPLEX pin to logic high.

The full duplex mode only makes sense for diagnostic purposes. The modem starts transmitting and while transmitting, it receives its own message back. It checks the signal path up to the point where transmit and receive signals are connected. In the HART “high impedance” device, this will be connected at the LOOP(+) terminal, so checking the HART modem output, coupling to the 4-20mA transmitter, the 4-20mA transmitter itself, the receive path back to the HART modem. In the ”low impedance” device it will be checking the capability of the HART output driver, signal passing through an external RTS switch, the coupling capacitor, and receive path, back to the HART modem. In both cases, the connected network (cable and the receiver on the other side) is checked for shorts.

General Duplex/Half-Duplex Example:

Half duplex: Only one side can talk at any given time e.g. a walkie-talkie. You press the button and talk, then release the button and listen to check if the person on the other side talks. If you press the button while the other side is talking, the other side will not hear you.

Full Duplex: Both sides can talk at the same time e.g. a phone. You can interrupt the other side by saying something, and the other side will hear you.