Patent GB-1322901-A (GB)
Title: Data transmission systems
IPC: G06F13/374
Filed: 1969-09-22
Abstract: 1322901 Data transmission systems; telephone exchanges PLESSEY CO Ltd 26 Oct 1970 [22 Sept 1969] 46540/69 Headings H4K and H4P A digital transmission system for interconnecting processors and peripheral units control. ling a telecommunications switching centre, consists of a plurality of stations, at which there is a processor or peripheral unit, which communicate with one another via a plurality of ring highways, each station having access to some or all highways depending upon its status, and when wishing to call another station selects and seizes a free highway to which that station also has access and calls up and connects with that station and excludes all other stations on that highway from the connection. The highway stations with access to a particular highway have an order of calling priority for that highway if two or more stations attempt to seize the same highway simultaneously. A receiving station receives data around one side of the highway and retransmits it to the sending station via the other side. The sending station only sends a data signal upon satisfactory return of the previous one. Only one repeated transmission is attempted before the connection is terminated as being faulty. Each ring highway consists of ten data wires for parallel data signals, a strobe wire, and four code wires for parallel coded service signals, for example a station free or busy signal, calling and termination signals, fault and retransmit signals. Each station has a buffer unit, to interface the processor or peripheral device with the ring highway and a highway control circuit for each ring highway to which the station has access. Upon a data processor, say, wishing to call a processor at another station all highway control circuits at the originating station of those rings to which the other station has access are primed and a search is made through the primed highway control units for a free highway in order of their priority. The calling station applies its address and an establishment code to the highway, which is received back at the station if no other station is attempting a call. The establishment code sets the highway control circuits of all the other stations on this highway to a  busy condition, thereby establishing the highway for the calling station. Any station simul-  taneously attempting a call, but of a higher priority, blocks the arriving address and substitutes its own on the highway; any of a lower priority  are disconnected from this highway. The address  of the station to be called is now put on the highway by the calling station and only the called station recognizes the address and connects itself to the highway, returning the address to the calling station accompanied by a return code. The calling station now sends the data message preceded by a destination-signal indieating where in the processor the received message is to be put. An end of message signal disconnects the called station and resets the highway control circuits of the other stations on the highway. A calling station in the process of setting up a connection upon receiving a call to itself will always accept that call. A station may, by use of a particular code signal, call up another station and request transmission of data to itself, a so-called queueempty signal being returned if there is no data to transmit, so terminating the connection. If a called station has no free storage space to accept a data from a calling station a queue-full signal is returned, again terminating the connection. The system may be used for interconnecting the stored programme central control and the peripherals, e.g. line scanners and markers, of the reed relay telephone exchange described in Specification 1,243,464.