A programmable controller senses the status of input devices and controls the state of operating devices in accordance with a stored ladder logic control program. A memory of the programmable controller contains message data and data defining a set of expected transitions of the status of selected input...http://www.google.com.tw/patents/US5319783?utm_source=gb-gplus-share專利 US5319783 - Programmable controller with an operator messaging function
