The structure of an RS232 monitor intended for use in online system maintenance of real-time, industrial process control systems is presented. The monitor is written in PLM86 for use in an Intel 8086 based system in which system software is organized into a set of foreground interrupts and an infinite background loop. The monitor, which resides in the background loop, utilizes a programming technique referred to as psuedo-concurrent programming to eliminate the lockout problem associated with the background loop structure. The psuedo-concurrent technique is explained. Maintenance procedures are described and possible implementations utilizing the monitor are suggested. / Master of Science