Abstract: Data messages are transmitted between host data processors and a communications processor. Data messages from a communications processor are transformed for the host processor to which the message is assigned. Data messages from a host processor are transformed for the communications processor. Protocol is established between (a) untransformed data messages from the communications processor and transformed data messages for the communications processor, and (b) untransformed data messages from each host processor and transformed data messages for each host processor. A device interface connects a plurality of host processors to a plurality of buses, each bus being connected to one communications processor. The device interface handles the protocol, transformation and control function in parallel so that messages between a given host processor and terminal connected to a communications processor are processed through a device interface.