A security module (434) for a serial communications device. The security module (434) comprising: a receive data, RXD, input interface (436) for receiving data from a serial communications bus (404); and a transmit data, TXD, output interface (438) for transmitting data to the serial communications bus (404). The security module (434) is configured to: receive a message (540) from the serial communications bus (404) via the RXD input interface (436); compare the message (540) with one or more error conditions; and upon detection that an error condition has been violated, output an error-signal (543) to the serial communications bus (404) via the TXD output interface (438), wherein the error-signal (543) identifies one or more parameters relating to the error condition.