Patent GB-1363254-A (GB)
Title: Data communication terminal
IPC: H04L25/05
Filed: 1971-09-17
Abstract: 1363254 Digital transmission XEROX CORP 17 Sept 1971 [21 Sept 1970] 43467/71 Heading H4P A buffer between a terminal, particularly a teleprinting machine, and a network has a circulatory store in which data can be entered continuously and extracted, when polled, periodically so that information can be transmitted in blocks during the same period it is being entered. The buffer is normally polled more frequently than the time to fill the store but should the store not have a remaining capacity equivalent to one line of type, the machine can be locked. An automatic retransmission feature may be incorporated in the event of erroneous reception. Generally messages are partitioned into blocks of kn characters, kn being selected at 512, the partitioning being identified by marker bits. Each block of kn = 512 need not consist of a single message but may comprise multiple messages suitably identified. Such short messages may be transmitted individually. Stored blocks are not erased under retransmit mode of operation until a correct reception signal is received. In a receive mode the circulatory store receives data from line which if error free is read to a printer while the next block is received. Preferably each terminal operates half duplex. A terminal is preferably arranged with a common input and output bus, control being exercised by a central control unit CCU which polls all in/out (I/O) and storage devices within a terminal. Responses to a CCU poll command may take the form of a status/command (S/C) response which tells the CCU that a particular addressed terminal has a command or status which is to be translated to the output bus of the CCU and subsequently to the input bus of other modules. A S/C response may be directed to a specific module or several modules with appropriate response. Commands may include such information as to whether data is to be entered from a low speed random device, i.e. a keyboard, by a high speed device in blocked format, e.g. from a remote computer, also whether the printer must operate in a low or high speed mode. The CCU polls all devices and looks for a response on its input bus and acts if the response indicates that information is to be translated between modules and/or I/O devices. The CCU directs data flow and acts as a pair of transfer gates directing information from a device addressed to the other modules so information is available to any device. The storage control module SCM stores and formats information from the input device and forwards it to the output device and also acts as a speed translator between I/O devices. All data transfers go through the SCM. The communication channel module CCM is interfaced between module bus and data set interface normally transmitting to a remote station, i.e. a computer; the CCM synchronizes information with the terminal also to and from such computer. As information is keyed in, i.e., from a keyboard, it is monitored by SCM and stored in a circulatory store and when sufficient information is available to complete a block, which is defined by partitioning bits, SCM attempts to set up a link to the output device. The keyboard control module KCM is allocated control on a printer, a line at a time, to enable an operator to modify characters and effect corrections before storage. Storage and retrieval of information, also interposition of marker codes, is described with reference to Fig. 2 (not shown). Either a complete block (mode 2) or parts of a partitioned block (mode 1) which whole or parts is stored temporarily in a silo register (214), Figs. 2a and 6 (not shown), may be transmitted and information is not erased until a correct reception signal is received. The silo is continuously interrogated so the circulatory store can be emptied at the earliest time to ensure maximum capacity available for inserting new information.  Detailed circuit arrangements are described in the Specification.