This method securely transmits data from a secure control system [110] located on an isolated computer network [100] to a separate computer [210] outside the isolated control network [100]. The method includes several features designed to minimize the risk of outside cyber attack on the control system [110] while ensuring that the data is transmitted correctly and promptly. The system uses a non-routable unidirectional physical data link [300]. Messages [400] are redundantly transmitted to computer [210] without acknowledgement along with checksums [430,450]. The checksum information is used to validate that the message header [420] and the message data [440] have been received correctly. Redundant information contained in repeated message data blocks [440] is discarded after the transmitted message [400] is correctly received and decoded. An ordered transmission sequence is used to minimize the message delay if an individual message [400] was not received correctly on its first transmission.