Abstract: A device to secure a JTAG type bus in its “scan chain” component chaining mode functionality, when several components are connected in series on the JTAG bus, includes a first interface for receiving JTAG signals and a second interface for the JTAG signals originating from a chain of components. The device includes the following modules: a JTAG frame generator module for verifying the continuity of operation of said Bus and components; a module for monitoring the electrical activity of said Bus and components; an alarm module for sending back an alarm detected by the above modules; an alarm module for managing the operating mode of the device; and a security functions activation module AFS.