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The AdvancedPhoton Source (APS) control system is based on a distributed topology of microprocessor-based Input/Output Controllers (IOCs). Since the cost effectiveness of placing an IOC near every point where an interface to the control system is required may be prohibitive, U 0 subnets implemented via message passing network protocols are utilized. For greatest flexibility, such a subnet must support connections to equipment via discrete 1/0 points, connections to standard interfaces such as GPIB and RS232, and be a practical network for custom-designed interfixes to intelligent equipment. This paper describes the BITBUS Universal Gateway (BUG), a device which supports the different interfaces mentioned above with a connection to a single BITBUS distributed subnet. The BUG utilizes an interchangeable set of circuit boards, which allow for a commonality among interface points, and the ability to use commerciallyavailable modules for I/O. This approach also circumvents several limitations of GPIB and RS232, which restrict their use in industrial, electrically harsh environments, via an implementation of the BITBUS protocol over optical fibers. I. SUBNETS IN THE A P S CONTROL SYSTEMThe APS control system provides for VME-based Input/ Output Controllers (IOCs) to be distributed throughout the facility and interconnected via Ethernet to one another and also to UNIX-based Operator Interface (OPI) consoles. Although this distributed architecture allows for intelligent processors near the major subsystems, 40 subnets are frequently required to interface directly to the equipment and communicate 1/0 information to the nearest IOC. [ll II. CURRENT INTERFACE PROBLEMSCurrently GPIB (IEEE-488) and RS232 interfaces are being used to interface instruments to the APS control system. These interfaces have severe limitations when used in an industrial, non-office type environment for computer control systems. GPIB offers no error detection mechanisms, no ground isolation, and severe distance limitations. Although both fiber optic and twisted pair extenders are available for GPIB they are cost inefficient. RS232 offers no ground isolation, severe distance limitations, and exists with a single node masterhlave topology. Again, extenders and multidrop RS232 network solutions are available, however, their repeated use can be expensive. ~~ *Work supported by U.S. Department of Energy, Office of Basic Energy Sciences under Contract No. W-31-109-ENG-38.Basic binary and analog 1/0 have obvious distance and noise immunity problems when interfaced to a control system. Currently the Allen-Bradley 1771 series 1/0 modules provide remote interfacing for these types of signals in the APS control system. Although effective, it is best used for a multitude of signals as its use becomes cost prohibitive when used with just a few raw binary or analog signal points. An additional complication with the Allen-Bradley solution is that the network and 40 chassis are proprietary designs of Allen-Bradley. Modules cannot be customized to meet the uniqu...