FOFB Implementation

In contrast to the SOFB, which runs on a central PC, the global FOFB is decentralized and integrated in the twelve BPM stations of the SLS storage ring. The feedback will operate with an update rate of 4 kHz. Each of the twelve stations handles six BPM inputs and six corrector magnet outputs. The design of the FOFB at an early stage in the SLS project made it possible to combine the feedback with the digital BPM system without much additional expenditure. The hardware implementation of one of the twelve BPM stations is shown in Fig. 5. The six BPM position readings ($x$,$y$) of the sector are already available in a digital format on a first digital signal processor (DSP 1). They are transfered over a multiprocessor bus to a second processor (DSP 2) which is devoted to the orbit feedback. Since the coefficients of the inverted response matrix only have significant values around the diagonal it is sufficient to calculate the new corrector magnet settings with the adjacent BPM readings [7]. The required beam positions from the neighboring sectors are exchanged over a dedicated fibre optic point-to-point network. It uses the SHARC link ports to which a fibre optic module is attached. Separate channels for transmit and receive have been chosen between each sector thus avoiding any network collisions. The whole processing chain is driven by the data acquisition of the digital BPM system which in turn receives hardware triggers from the timing system. An interrupt is generated on DSP 1 as soon as data arrives from the digital receivers. After transferring the position readings to DSP 2 an interrupt is triggered to start the data exchange over the fibre links with four parallel DMA processors. Data packets of 56 Bytes are transmitted containing additional start and stop bytes. A timer monitors whether the adjacent sectors have responded within a predefined time window. Measurements in the laboratory between two VME crates have shown that the full data exchange requires in the order of 8 $\mu$s. Once the full set of BPM data of three sectors is available the predefined reference orbit is subtracted resulting in the final beam displacement. The sub-matrix multiplication yields the incremental corrector magnet changes which are then written over the VME bus to the power supply controller interface board. The VME bus master capability of the DSP board avoids any necessary intervention of the control system IOC. From the interface boards the new corrector settings are transfered star like to six digital power supply controllers over fibre optic links operating at 5 Mbits/s.