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Integrating COTS components in critical real-time systems is challenging. In particular, we show that the interference between cache activity and I/O traffic generated by COTS peripherals can unpredictably slow down a real-time task by up to 44%. To solve this issue, we propose a framework comprised of three main components: 1) a COTS-compatible device, the peripheral gate, that controls peripheral access to the system; 2) an analytical technique that computes safe bounds on the I/O-induced task delay; 3) a coscheduling algorithm that maximizes the amount of allowed peripheral traffic while guaranteeing all real-time task constraints. We implemented the complete framework on a COTS-based system using PCI peripherals, and we performed extensive experiments to show its feasibility.