Abstract: Radiation dose interlock is necessary for top-up operation in Shanghai Synchrotron Radiation Facility (SSRF). The radiation dose interlock system was based on a programmable logic controller (PLC). On PROFIBUS, the system was built up with an S7-300 master and 4 slaves. With DWORD logic, the inputs were compared with the input masks module by module to tell if there were trip signals. The trip signals and the status of all the detectors were logically synthesized and off-delayed for two interlock outputs. The trip events were located and recorded together with date and time. The record function is proved to be useful for disorder diagnostics of the radiation dose detectors.