A readout system utilizing the APV25 ASIC for the Forward GEM Tracker in STAR

We have developed a modular readout system for the 30,720 channel Forward GEM Tracker recently installed in the STAR Experiment at RHIC, BNL. The modular architecture is based on a passive compact PCI backplane running a custom protocol, not PCI, connecting 6 readout modules to a readout controller module. The readout modules provide all necessary functions, including isolated power supplies, to operate up to 24 APV25 chips per module with high-impedance ground isolation. The front-end boards contain a minimal set of components as they are located inside the STAR TPC inner field cage and are inaccessible except during long shutdown periods. The front-end boards connect to the readout modules with cables up to 24 m in length, carrying unbuffered analog readout signals from the APV25 as well as power, trigger, clock and control. The readout module digitizes the APV analog samples to 12 bits at 37.532 MHz, and zero suppresses and buffers the data. The readout controller distributes trigger and clock from the central trigger system, gathers the data over the backplane, and ships it to a linux PC via a 2.125 Gbps optical data link (Detector Data Link (DDL) from ALICE). The PC gathers data from multiple readout controllers and dispatches it to the STAR event builders. The readout modules, controllers, and backplanes are housed in a common crate together with the GEM HV bias power supplies.