Patent GB-1457929-A (GB)
Title: Data transmissions system
IPC: H04L7/00
Filed: 1974-06-18
Abstract: 1457929 Data transmission systems INTERNATIONAL STANDARD ELECTRIC CORP 18 June 1974 [26 June 1973] 26932/74 Heading G4A [Also in Division H4] In a data transmission system which interconnects a pair of data handling units U, D, Fig. 1, having a message output register REGU of unit U connected to a message input register REGD of unit D by a connection BD over which characters of the message are transmitted in bitserial mode, each unit includes a signalling transmitter apparatus receiver TRU, TRD having a connection SBU, SBD to the advance input for the register of that unit, the two transmitter/  receiver apparatuses TRU, TRD are interconnected by a first single connection SY reserved for the transmission of clock pulses from one apparatus to the other and a second single connection SG reserved for the transmission of signals between apparatuses. Only one of the signalling apparatuses is connected to a clock CL generating the clock pulses. The apparatuses each have inputs to which are applied conditions CANU/D, RELU/D, BLOU/D and PREU/D for controlling the operations of the units and means under control of those inputs for generating signals for application to the second one of the connections SG. Some at least of the controlling conditions cause changes in the electrical condition of the signalling connection between the apparatuses. Each such change is produced at a time during the serial transmission of one of the characters which is appropriate to the condition to be signalled. The controls exercised by the apparatuses TRU, TRD control, via SBU and SBD, the transmission of characters from the output register REGU and the reception of characters in the input register REGD. The transmitter/receivers TRU and TRD are disclosed in detail (Figs. 2 and 3, not shown) respectively and the states of the signalling condition in the signal transmission connection SG are defined at each predetermined moment of the serial transmission of a character by the passage or absence of a current. Each transmitter/receiver TRU, TRD has a switch (9, 17) for applying during each of these moments a potential to the corresponding end of the signalling transmission connection SGU, SGD and control gate circuits (23-34 and 35-46) for controlling the switch (9 or 17) as a function of the signalling to be transmitted or received at each of the predetermined moments. The predetermined moments are determined by counters (2 and 5) and differentiators (21 and 5<SP>1</SP>) in the respective units TRU and TRD providing timing outputs (t 0 -t 7 , t<SP>1</SP> 0 -t<SP>1</SP> 7 ) to gates and bistables in the transmitter receiver units under control of the clock pulse generator CL. A detector in the form of a differential amplifier (10, 18) in each transmitter receiver unit detects at each moment the state of the signalling transmission connection SG for deducing the signalling and controls gating and bi-stable circuits for transmitting the clock pulses to the advance input SBU, SBD of the corresponding message register REGU, REGD. The unit U can be used for storage and the unit D can be an arithmetic circuit in a computer. The input CANU is activated when cancellation of the content of the registers REGU and REGD is required such as when unit D detects an error in a character. Input PREU is activated when unit U is ready to begin transmission of the start of a message. Input BLOU is activated when it is required to stop the transmission for the duration of one character and input RELU is activated when unit U is to release unit D at the end of a transmission. The inputs of the unit D function in a similar manner.