A bus extender and data transmission method in which a continuous train of message frames of fixed size is sent along a dual serial link in both directions simultaneously. There is no handshaking between extenders unless a transmission error is detected. Message frames are constructed at a rate which is independent of serial link length and the bus data transfer rate. Each extender is divided into a bus handshake layer which sends and receives data and signals from a local bus, a management layer which continuously constructs message frames, responds to commands and receives and decodes incoming messages from a remote extender, and a transport layer. The transport layer has shift registers and encoders to receive messages from the management layer and transmit them over the serial link and has a memory to store up to 30 messages in the event of a retransmission request. The transport layer also receives messages from the serial link, checks for valid error-free messages and sends them to the management layer. It also initiates a retransmission request in the event a transmission error is detected.