Failure detection method and apparatus

A failure detection apparatus detects the existence of an abnormal circuit condition in a circuit which causes a subsequently transmitted data byte to be transmitted from one integrated circuit to another integrated circuit out of sequence relative to a previously transmitted data byte. Even and odd data bytes are received by the first integrated circuit with odd parity. However, the even data byte is transmitted from the first integrated circuit to the second integrated circuit, along existing interface lines extending between the integrated circuits, with odd parity. The parity bit of the odd data byte is inverted, the odd data byte being transmitted along the existing interface lines with even parity. An exclusive OR gate in the second integrated circuit receives the parity bit of the even data byte and passes the parity bit without inversion in response to a first state of a clock signal from an odd latch; however, the exclusive OR gate, upon receipt of the odd data byte, re-inverts the parity bit of the odd data byte in response to a second state of the clock signal. A parity checker compares the data bits of the incoming even and odd data bytes with the parity bit generated by the exclusive OR gate and generates an error check signal representative of the receipt of the subsequently transmitted data byte transmitted out of sequence relative to the previously transmitted data byte when the combined parity of the data bits and parity bit at the input of the parity checker is not odd.