1. A processor support system for providing maintenance and software development for an application system which is controlled by an application processor via data, address, and control buses, wherein said processor support system comprises: computer means for running an operating system to generate and store software for said application system; field test unit means connected between said computer means and said application system for interfacing said computer means with said application system, wherein said field test unit means comprises: program store means for storing control instructions generated by said computer means for regulating the operation of said field test unit means; processor means for controlling the operation of said application system; memory means for storing said software generated by said computer means and written in said memory means by said computer means; interface means responsive to said processor means and directly connectable to said application system data, address, and control buses for connecting said application processor to said memory means; field test unit bus means connected to said program store means and to said processor means for applying said control instructions to said processor means; and wherein said processor means is responsive to said control instructions for directing said application processor via said interface means to execute said software stored in said memory means.