SMAR manufactures pressure transmitters since 1980. The initial version used the strain gauge technology for the pressure sensor and the device was completely analog.
In the early of 1984, SMAR changed the sensor technology to use a capacitance as a way to measure pressure. In this way, pressure is measured by the change of the capacitances in a double capacitor sensor.  The final result is that we can determine the differential pressure applied.
Initially the transmitter was analog providing straight 4-20 mA output proportional to the applied differential pressure. However, around 2 years later SMAR created their own digital protocol.  SMAR was determine to give the transmitter the capability of remote configuration.  The new transmitter was able to provide the legendary 4-20 mA and get connected in a network to interact with a DCS (distributed control system) or to a hand-held device for operation and configuration.
To make the line of transmitters more universal/compatible with others DCS, SMAR implemented the HART protocol. In 1989.  Consequently, SMAR was the second company in the world to have a Pressure Transmitter working with HART protocol and be completely configurable with a small hand-held device.