Device for single line bidirectional data transmission between an intelligent card's microprocessor and a second processor

Claim 1

A signal transmission device comprising first and second data processing stations, at least one of the stations being incorporated in a portable object, said portable object comprising a small plastic card embodying a microprocessor, each station having transmission means for transmitting, when in a transmission mode, a signal to the other station and having receiving means for receiving, when in a reception mode, a message transmitted from the other station; and means for coupling the stations together, the coupling means providing a single bidirectional signal line over which a message is transmitted in bit serial form by variations of a signal on the line; each station including first means for placing the line in a first predetermined state when such station is in the reception mode to advise the other station that it is ready to receive a message; second means, operative when the station is in the transmission mode, for detecting said predetermined state; third means controlled by said second means for generating and transmitting a start bit signal to the other station, the start bit signal preceeding a first bit of the message; fourth means operative when the station is in the transmission mode for calculating from the transmitted message a first parity bit and for transmitting the first parity bit to the other station; fifth means operative when the station is in the receiption mode for calculating a second parity bit from the received message and for comparing the first and second parity bits; and sixth means controlled by said fifth means for placing the line in a second predetermined state different from the first predetermined state upon a difference between said first and second parity bits being detected to advise the other station of a transmission error.