The HD67900-A1 are Serial / HART converters.
They allow to monitor HART devices (like Sensors, etc..) with a Serial net (like a PC, etc..).
On the HART side, the HD67900-A1 can manage up to 15 deivces (+1 Secondary Master).
HD67900-A1 supports HART FSK.
HD67900-A1 can be used like Primary HART Master or like Secondary HART Master.
HD67900-5-A1 supports the connection on the Serial side over RS485.