A diagnostic system for a field device in a process control apparatus, comprising: at least one sensor associated with the process control apparatus; a computer located on the field device and adapted to receive data from the sensor and to detect an occurrence of a predetermined process event; and a memory device operatively connected to the computer and adapted to store sensor data received by the computer at a time corresponding to the occurrence of the predetermined process event, wherein the memory device is located on the field device, characterised in that: the computer is configured to: temporarily store the received sensor data on the memory device for a predetermined time interval, after which predetermined time interval the temporarily stored data is overwritten; evaluate whether the predetermined process event has occurred; continue to store the received sensor data on the memory device while the predetermined process event has not occurred; and upon occurrence of the predetermined process event: continue to store on the memory device sensor data received during a period after the occurrence of the predetermined process event corresponding to one-half of the predetermined time interval; and maintain sensor data stored on the memory device for a period before the occurrence of the predetermined process event corresponding to one-half of the predetermined time interval at least one sensor associated with the process control apparatus; a computer located on the field device and adapted to receive data from the sensor and to detect an occurrence of a predetermined process event; and a memory device operatively connected to the computer and adapted to store sensor data received by the computer at a time corresponding to the occurrence of the predetermined process event, wherein the memory device is located on the field device, characterised in that: the computer is configured to: temporarily store the received sensor data on the memory device for a predetermined time interval, after which predetermined time interval the temporarily stored data is overwritten; evaluate whether the predetermined process event has occurred; continue to store the received sensor data on the memory device while the predetermined process event has not occurred; and upon occurrence of the predetermined process event: continue to store on the memory device sensor data received during a period after the occurrence of the predetermined process event corresponding to one-half of the predetermined time interval; and maintain sensor data stored on the memory device for a period before the occurrence of the predetermined process event corresponding to one-half of the predetermined time interval the computer is configured to: temporarily store the received sensor data on the memory device for a predetermined time interval, after which predetermined time interval the temporarily stored data is overwritten; evaluate whether the predetermined process event has occurred; continue to store the received sensor data on the memory device while the predetermined process event has not occurred; and upon occurrence of the predetermined process event: continue to store on the memory device sensor data received during a period after the occurrence of the predetermined process event corresponding to one-half of the predetermined time interval; and maintain sensor data stored on the memory device for a period before the occurrence of the predetermined process event corresponding to one-half of the predetermined time interval temporarily store the received sensor data on the memory device for a predetermined time interval, after which predetermined time interval the temporarily stored data is overwritten; evaluate whether the predetermined process event has occurred; continue to store the received sensor data on the memory device while the predetermined process event has not occurred; and upon occurrence of the predetermined process event: continue to store on the memory device sensor data received during a period after the occurrence of the predetermined process event corresponding to one-half of the predetermined time interval; and maintain sensor data stored on the memory device for a period before the occurrence of the predetermined process event corresponding to one-half of the predetermined time interval continue to store on the memory device sensor data received during a period after the occurrence of the predetermined process event corresponding to one-half of the predetermined time interval; and maintain sensor data stored on the memory device for a period before the occurrence of the predetermined process event corresponding to one-half of the predetermined time interval