GB1206639A - Data-terminal control equipment - Google Patents

Abstract

1,206,639. Automatic exchange systems; digital transmission systems. STANDARD TELEPHONES & CABLES Ltd. 26 Sept., 1968 [28 Sept., 1967], No. 44221/67. Headings H4K and H4P. In a data transmission system in which a connection between two terminals e.g. a computer and an outstation is established over a telephone switching network, the receiving terminal responds to a calling signal to connect a data modem to line and to transmit an indication to the calling terminal of the mode viz. send or receive of said data modem and means at the calling terminal responds to the indication to connect a second data modem to line in a mode suitable for communicating with the first data modem, the transmitting modem sending synchronization signals before any actual transmission of data takes place. The data terminal shown comprises a transmitter 2 and a receiver 3 which are selectively connectible to an exchange line 11 that normally extends over line 10 to a local subscribers set. The connection of the data modems to line is controlled by a control circuit 4. The main data processing equipment is represented at 91. Assuming that this terminal is set for fully automatic working viz the local subset is unattended, then in response to an incoming call from another manual or automatic data terminal a ringing current detector 33 operates to transmit a Π6V D.C. signal over line 31 via gate 41 to set flip-flop 42. Thereupon the "1" output from the latter maintains (or returns) flip-flop 44 in its reset state and the "0" output is effective via amplifier 421 and inverter 422 to bring up relay 303 in the receive modem. Supervisory carrier from a supervisory channel transmitter in this modem is sent to line as an indication that the called terminal is in its receiving mode. The calling terminal may then, after transmission of synchronization signals, transmit data in V.F. format which is applied in Π6V D.C. format via receiver 36 of receive modem 3 and wire 14 to the data processing equipment 91. If the caller wishes the called terminal to transmit, he sends a 20 second "0" signal which is effective, after delay in circuit 45 to check that it is not a false signal, to set flip-flop 44 and thereby reset flipflop 42 and connect the transmitter modem 2 to line in place of the receiver. The resultant cessation of supervisory carrier indicates to the calling terminal that it should switch to the receive mode. Synchronizing signals followed by data are then sent to the calling terminal. At the end of data sending a signal over wire 46 causes the receive modem to be switched into circuit in place of the transmit modem. The caller may then send synch signals and data or may disconnect. After a time determined by delay circuit 47 and in the absence of any signals from the caller, both flip-flops 42, 44 are reset in order to release the connection. If the data processor contains faulty equipment, the operation of flip-flops 42, 44 is inhibited by signals on wires 911, 912. Echo suppressors in the connection path can be switched out by means of tone signals from the currently transmitting data terminal. In order to allow for noise on the path, the 20 second "0" signal detector circuit 45 may include a counter which accepts such a signal as being genuine if it is not interrupted more than say 8 times in any 2 second period. The necessary switches for converting a calling terminal from voice to data operation are indicated in Fig. 4 (not shown), this conversion being required after the initial setting up of the connection irrespective of whether manual or auto-dialling is used.