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A unified global and local closed-orbit feedback system has been implemented at the Advanced Photon Source in order to stabilize both particle and photon beams. Beam stability requirements in the band up to 50Hz are 1 7~ in the horizontal plane and 4.4pm vertically. Orbit feedback algorithms are implemented digitally using multiple digital signal processors, with computing power distributed in 20 VME crates around the storage ring. Each crate communicates with all others via a fast reflective memory network. The system has access to 320 rf beam position monitors together with x-ray beam position monitors in both insertion device and bending magnet beamlines. Up to 3 17 corrector magnets are available to the system. The global system reduces horizontal ms beam motion at the x-ray source points by more than a factor of two in the frequency band from lOmHz to 50Hz. SYSTEM DESCRIPTION I . I Hardware ArchitectureThe real-time orbit feedback system is implemented digitally using multiple digital signal processors (DSPs). A total of twenty VME crates are distributed around the circumference of the 40-sector APS storage ring to manage interfaces to beam position monitors (BPMs) and correctors, and to compute orbit corrections. Sofrware ArchitectureThe 68040 EPICS processor interfaces the DSP(s) to the control system and provides the means to monitor and control feedback algorithms running on the DSP. Data structures residing in dual-ported RAM on the DSP board are used to communicate commands and return status and data. The EPICS processor uses state programs to interface database variables with the shared data structures.The reflective memories contain a shared data structure that is replicated at each node and is accessible to all processors in the feedback system. At each sample tic the DSP fetches BPM data from the BPM interfaces, computes the error in position, and writes the error values to assigned locations in the reflective-memory shared data structure. When all other DSPs have written their BPM errors, the DSP reads in the complete BPM error vector from the reflective memory and computes and writes new corrector values.The master feedback crate provides global controls to the other 20 feedback crates by writing to specific locations in the reflective-memory shared data structure. In addition, it provides analysis tools such as 'dspscope' which functions like a digital scope, and 'ac voltmeter' which performs a sliding discrete Fourier transform in real time, both tools working on 40 channels of data. RF Beam Position MonitorsThe APS storage ring contains 360 rf BPMs, of which 320 are available to the real-time feedback system. In order to improve spatial resolution and minimize measurement error, it is preferable to use as many of the available BPMs as possible in the global correction algorithm. However computational resources are limited, and t h e n is a r of BPMs in the lkHz [l]. X-ray Beam Position MonitorsIn addition to the rf BPMs, the APS is implementing x-ray BPMs on all the insertion device ...

A bunch clock timing module has been developed for use by Advanced Photon Source beamlines. The module provides bunch pattern and timing information that can be used to trigger beamline data collection equipment. The module is fully integrated into the control system software (EPICS) which automatically loads it with the storage ring fill pattern at injection time. Fast timing outputs (1 ns FWHM) for each stored bunch are generated using the storage ring low-level rf and revolution clock as input references. Fiber-optic-based transmitters and receivers are used to transmit a 352-MHz low-level r f reference to distributed bunch clock modules The bunch clock module is a single-width VME module and may be installed in a VME crate located near beamline instrumentation. A prototype has been in use on the SRI CAT beamline for over a year.The design and integration into the control system timing software along with measured performance results are presented.

4. Measure position at each BPM on each turn ( 3.6 mi-This paper describes the Beam Position Monitor (BPM)croseconds for the storage ring). data acquisition scheme for the Advanced Photon Source 5. Provide averaged beam position for higher accuracy.(APS) storage ring. The storage ring contains 360 beam posi-6. Provide a beam history for each BPM.