What is a request-response mode in HART?
Request-response mode in HART (Highway Addressable Remote Transducer) communication is a basic technique of data transfer whereby a master device starts communication by sending a request to a field device, which then replies with the desired data or an acknowledgment. This mode follows a master-slave communication model, meaning the slave device usually a transmitter or positioner only communicates when the master (such as a DCS, PLC, or handheld communicator) prompts it. Communication is strictly one transaction at a time; every master request must be answered before a new one can be started. This guarantees consistent and orderly data transfer, which is absolutely crucial in process automation systems. Reading process variables like pressure, temperature, or flow; accessing device configuration parameters; collecting diagnostic data; and carrying out maintenance activities are all done in the request-response mode. It supports a wide range of standard and device-specific commands described in the HART protocol, including reading the primary variable (Command 1) or device status (Command 48).
Applicable in both point-to-point and multi-drop network topologies, this mode permits two masters (primary and secondary) talking with the same field device in a time-shared manner, hence preserving system stability and data integrity.