A serial communication system for communicating data over a Controller Area Network CAN bus comprises a security slave device located between each system node 8 and the CAN bus 10. The system characterised in that: • Said Security slave device further comprises a tagging means for inserting data indicative of said first source node (e.g. ID or address) into a CAN frame received from said first node; • Said system further comprises a Security master device 4, located between said CAN bus and a second system node 2; • Said Security master device further comprises a means of extracting said data indicative of first said node from a received data frame; and • Said system further comprises a means 1 of checking the validity of received CAN frame associated to said extracted data indicative of said first node (e.g. source node ID or address check). A security supervisor 1 may also be included to co-ordinate the operation of the security master device and check the validity of the data indicative of the first node. If the data is not valid the CAN frame may be deleted. Further if not valid the connection of the first node to the CAN bus may be removed 9.
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