Abstract: A transaction system comprises a plurality of components each performing a respective function, the components being interconnected by a serial data bus. Each component can initiate an information exchange operation with any other component. A component may transmit a message consisting of either a general instruction for causing the component receiving the message to carry out one of a plurality of different types of operations depending upon the nature of the instruction, or a message in the form of a direct memory access request identifying a specific location in a memory of the receiving component to which data is to be written or from which data is to be read.