A multipoint synchronous data network is formed by junction units which provide signal paths that interconnect incoming branch lines to a common outgoing branch. The junction units arranged to combine the branches so that data words on one branch are propagated up the common branch, subject to the condition that only one branch can send to the common branch at a time. Each data word includes a designation whether the word carries message information or control information (such as an idle branch condition). Control words are blocked by the junction unit so that message words from a sending branch are not garbled by control words from idle branches. The junction unit passes each message word from a sending branch to the common branch if the junction unit had not priorly passed any message word to the common branch (presumably because all branches were idle) or if a message word priorly passed was from the sending branch whereby error conditions on idle branches simulating message words are not passed to garble the data from the sending branch. This control is maintained by the sending branch for a plurality of word intervals after it has terminated sending data.

An unslotted radio frequency multiple access bus communications system employing a contention listen-while-talk (LWT) protocol. The system includes a plurality of remotely located terminals coupled to a communications bus. The bus includes a pair of oppositely directed, unidirectional signal paths which are connected at one end so that one path is an inbound path to that end and the second is an outbound path from that end. Each of the remote terminals includes a bus interface unit (BIU) coupled to both the inbound and outbound paths at the remote location. Each BIU is adapted to transmit and receive modulated carrier signals on the inbound and outbound paths, respectively. When a terminal wishes to send a message to another terminal, the terminal's BIU initially performs an rf carrier sensing operation to make an initial determination as to whether or not any other subscriber is transmitting on the inbound path. In the event no carrier is detected, the BIU confirms this determination by first beginning to transmit a message packet modulated on a carrier signal and addressed to the desired receiving terminal on the inbound path, and then monitoring the outbound path during a collision window. If the transmitted message is received intact by the transmitting terminal within this collision window, the transmitting terminal's BIU determines that no collision has taken place between its message and a message sent by another terminal or noise, and that the transmitting terminal has gained access to the bus. Thereafter, the BIU may transmit any remaining portion of its message packet on the inbound path. However, in the event a collision is detected, the transmitting terminal's BIU aborts its transmission and then backs off for a random time period before again attempting to gain access to the bus and transmit its message.

A multi-computer system includes a plurality of data processors and at least one I/O device which is commonly accessible by the data processors. A plurality of serial bus loops are configurated in hierarchy with interbus linkage devices disposed between adjacent layers of the hierarchy. The data processors are connected to a plurality of first layer serial bus loops and the I/O device which is commonly accessible by the data processors is connected to a second layer of serial bus loop. The interbus linkage devices control linkage among the plurality of serial bus loops and carry out routing control for a start command from the data processor to the I/O device, routing control for an interruption to report the end of I/O device operation, routing control for data transfer, routing control for a request interruption and exclusive use control of the shared I/O device.

Methods, apparatus, and systems are presented for communicating structured data in a system utilizing devices having different data processing capabilities. The methods, apparatus, and systems involve transmitting a unit of data from a first device, wherein the unit of data comprises at least a version-specific portion and a version identifier associated with the version-specific portion, receiving the unit of data at a second device distinct from the first device, processing the version-specific portion at the second device if the second device recognizes the associated version identifier, and disregarding the version-specific portion at the second device if the second device does not recognize the associated version identifier. Disregarding the version-specific portion may comprise examining a data length field in the data unit associated with the version-specific portion to determine a length value and skipping an amount of data corresponding to the length value.

A printing system comprises: (x.sub.1) a source of A data and B data to be separately printed; a first printer connected with that source to receive the A data for printing it on a first sheet, such as an envelope, and to pass the B data; and a second printer connected with the data source via the first printer, and to print the B data on a second sheet, such as a letter, the first printer including: (a) a feed plate adjacent which the first sheet is fed in a longitudinal feed direction, (b) a print head for printing the A data on the first sheet, (c) a first drive at one longitudinal side of the print head, and a second drive at the opposite longitudinal side of the print head, the drives adapted to move the first sheet longitudinally forwardly, and then longitudinally reversely, to enable the print head to print on relatively spaced zones on the sheet.