An accessory device for a field transmitter comprising an enclosure housing an electronic processing unit. Said device comprises a portable supporting body removably couplable with the transmitter and on which there are provided at least one activation unit which can be operated by a user, and an electronic processing circuit which is operatively interlocked with said activation unit and is suitable to be operatively connected to the electronics of the transmitter so as to transmit one or more signals thereto following actuation of the activation unit.