The HD67901-A1 are BACnet / HART converters.
They allow to monitor HART devices (like Sensors, etc..) with a BACnet Master (like PLC, Supervisory System etc..).
On the HART side, the HD67901-A1 can manage up to 15 deivces (+1 Secondary Master).
HD67901-A1 supports HART FSK.
HD67901-A1 can be use like Primary HART Master or like Secondary HART Master.
HD67901-IP-A1 supports the connection on the BACnet side over Ethernet.
HD67901-MSTP-A1 supports the connection on the BACnet side over RS485.