Abstract: A programmable logic controller (PLC) in an operating system such as a machine includes a software development tool having labels in a first field to identify and describe specific input/output (I/O) points in the PLC. Each label assignment can be used in the formation of rungs in a ladder logic array to facilitate programming of the PLC. The PLC also allows for the entry of descriptive comments in a second comment field associated with each label assignment to assist the programmer. Secondary function instructions replace the descriptive comments in the comment portion of a label, or in the comment field, to generate a diagnostic, or status, indication such as an alarm for the machine when executed by the PLC. Comments for labels are transformed by a translator, with a file containing the labels and associated comments accessed and scanned for label comments with the appropriate syntax. Secondary function instructions are recognized by the use of specific key words in the comment field.

Abstract: A diagnostic system coupled to a programmable logic controller (PLC) via a serial data link monitors the operation of apparatus under the control of the PLC. Upon detection of a first event-initiating signal, a central processor unit (CPU) in the diagnostic system initiates a timer. The CPU then awaits receipt of a second confirming action signal representing completion of the event initiated by the first signal. The timer times out after a predetermined time interval determined by an operation parameter of the controlled apparatus. If the second confirming action signal is not received before the timer times out, the CPU provides an alarm signal to a video display such as a cathode ray tube (CRT) for displaying an alarm message.