Patent GB-1196824-A (GB)
Title: Data Transmission System.
Inventor: RESTIVO FRANCESCO
IPC: H04L13/08
Filed: 1968-09-16
Abstract: 1,196,824. Data processor. ING. C. OLIVETTI & C. S.q.A. 16 Sept., 1968 [3 Oct., 1967], No. 43987/68. Heading G4A. A central computer and a terminal apparatus coupled for transmission of data contain a store holding a block of characters, a programming unit and a control unit, service characters associated with data in the store serving to cause the appropriate programmed sequences to commence. The message from terminal to computer is always prefixed by a start text character STX and followed by the text proper. The block finishes with character ETB to indicate the end of a block of an unfinished message or ETX to indicate the end of a message. The message from the computer is prefixed by STX and followed by a service character CSN or CSR and the message and ending with the character ETX or ETB. Data entered from a keyboard goes straight to an interface 70 and from the computer goes to the interface via a buffer store constituted by a writing slide on the keyboard. From the interface each character (6 bits in parallel) goes to a store controller 20 which establishes the links between various terminals. The message then enters store 30 complete with data and interposed service codes. The store 30 is a magneto-strictive delay line holding 120 10 bit locations each containing two service bits, a six bit alpha numeric character and a single parity bit. One service bit bs is displaced to indicate which entry is to receive the next character and the second bit b1 is held before the first character to be read. As well as a simple parity check for each character a longitudinal redundancy check of the kind described in Specification 1,086,315 is included. At the start of reception at an accounting machine having a printing mechanism the carriage of the machine is hard up against a stop fixed by the tabulating bar. When an ETB or ETX signal is received the controller causes actuation of a motor key on the machine starting a cycle causing the carriage to move to the immediately following stop of the programming bar. This causes transmission of a signal Po retrieving data from store 30. The first code extracted is CSN or CSR which causes selection of the next stop on the tabulation bar, the accounting machine having a device selecting the next stop by means of a two-position selector which in one position selects predetermined stops and in the other selects other predetermined stops. The stop to which the carriage is brought has a prescribed programme for recalling data. When the data recalled is again CSN or CSR the carriage shifts to a further stop. Dialogue procedures between terminal unit and computer (Fig. 2).-Each sounding and selection by the computer is preceded by dispatch of a service character DOT putting transmission controllers connected to tne terminals from a rest condition (0) to a waiting condition (1). If within a predetermined time the next character is not received the terminal resumes the rest state. The next character is an address TND identifying the required terminal and indicating whether the request is sounding POL i.e. for despatch of messages from the terminal to the computer or selection SEL for transmission of response messages from the computer to the terminal. If the terminal recognises POL it assumes state (2) and transmits code TRI 1 as acknowledgment. If the store, which so far has been under local control is not ready to transmit, the terminal assumes state (13), and returns control to the computer by means of signal EOT and returns to state (0). Otherwise the terminal assumes state (3) to transmit service character STX which precedes the block of information in the store and then assumes state (4) wherein the block is transmitted until character ETX or ETB is obtained indicating end of message or end of block respectively. A waiting state 5 is assumed. If a reply is not received within time TO1 the terminal returns to state (3) and this will occur for three transmissions until logical choice 4RIP1 prevents regeneration of the contents of the store which is placed in free state TEL and then returned to state (13) and state (0). If the service character EOT is received the terminal returns to a waiting state (1). If service character ACK is received the block has been correctly received and the store is cleared A2M and put into the free state. If the terminal line controller recognizes SEL state (6) is assumed and proper address TRŒ 1 is sent. If the store is not ready to receive characters state (13) and then state (0) are assumed otherwise state (7) is assumed and acceptance signal ACK is transmitted and the terminal is put in the assigned to computer state ASC and state (8), awaiting (ATS) the first character from the computer, is assumed. If EOT is received the terminal assumes state (1) and if a character other than STX is received the terminal assumes state 13 but if STX (start of test) character is received state (9) is assumed wherein it carries out reception RITE of the message which continues until ETX or ETB is received. If ETX or ETB is received a verification signal is transmitted VERI 1 in state 10. If this is made within time TO2 the controller assumes state 11 and sends ACK or NACK comprising or denying coincidence between check data sent by the computer and generated by the terminal. If vertification takes place after the elapse of time TO2 an error lamp SETER is illuminated and state (11) assumed. Similarly if ETX or ETB are not recognized the error lamp is illuminated. State (12) is a waiting state which if EDT is received causes return to state (1) and if STX is received causes return to state (0) and if neither are received causes the store to be blocked and returns to state (9).