Upgraded from Readout Control Unit in ALICE/PHOS experiment, the Scalable Readout Unit has more stable point to point readout and up to 20 MB/s readout rate per Front-end Electronic Card. Based on the test results of newly designed 90 FECs in SRU system, analysis focused on readout control mechanism, the results of testing signal chain of FEC and evaluated the influence of pedestal shift. Energy resolution of FEC was proved under 5MeV by plentiful test...