Abstract: A control system for an automated storage/retrieval system (AS/RS) machine includes a programmable, general-purpose logic unit that operates under the control of a stored program. The program is functionally divided into two distinct blocks, including a host control program that is encoded in an elementary program language, and a second, application program that is encoded in a second, non-elementary program language. The second, non-elementary language is chosen to be emulatable by the first, elementary program language so that the host control program may implement the application program. In preferred form, the application program is formed of a plurality of independently programmed routines having a hierarchal relationship to one another. A feature of the host control program permits an operator to selectively trace through an execution of one or more of the routines of the user application program to permit diagnosis of the program logic of the user application program.