SBAS940 December 2018 DAC8742H
The behavior of the carrier detect or CD pin changes depending on whether the device is in HART mode or FOUNDATION FIELDBUS and PROFIBUS PA mode.
In HART mode the pin operates as a carrier detect pin. When a valid carrier is detected and the modem is receiving data the CD pin is taken high. When the CD pin is high, UART data sent to the device and the request to send, or RTS, pin will be ignored until the carrier is no longer present.
In FOUNDATION FIELDBUS and PROFIBUS PA the CD pin operates as a carrier detect pin when not in transmit mode. When the CD pin is high, UART data sent to the device will be ignored until the carrier is no longer present. When in transmit mode the CD pin functions as an alarm indicator that the jabber inhibitor has triggered and further UART transmission data will be ignored. In general if the CD pin is high the host controller should not be sending transmit data to the device.