Patent GB-1383037-A (GB)
Title: Two-wire/four-wire converter circuits
Inventor: FIEBIG H-H
IPC: H04L5/16
Filed: 1972-09-11
Abstract: 1383037 Digital data transmission; timing circuits SIEMENS AG 11 Sept 1972 [13 Sept 1971] 42053/72 Headings H4P and H3T A two wire single current telegraph subscriber T n  is connected to a four wire exchange line SD-ED by an electronic converter using logic gates which operate, when a "ready-to-print" condition has been established, to recognize and hold the first "start" polarity to occur thereafter on either one of the transmit and receive lines SD, ED. The logic gates simultaneously cause the other one of the lines either to be blocked (receive line ED) or to acquire stop polarity (transmit line SD). Before the printing condition has been set up (time t 1 -t 4 , Fig. 4) the converter responds to signals from the transmit contact S k , Fig. 3, and also from the receive line ED, the high levels being "stop" and the low levels "start" polarity. Thus at t 1 , S k  is closed to indicate intention of transmitting, and negative current J m  flows in the subscriber loop, by-passing R2 and raising the potential at R1-R2 junction enough to cause the trigger Tr to send a high (stop) level. At t 2  a stop level at E D  from the exchange instructs "proceed to select" and is detected by relay EM which drops out due to T1 turning off in response to E D . At t 3 , dialling from S k  occurs, and is followed by a stop signal from the exchange at E D  (t 4 ) indicating connection to distant subscriber. As long as this persists for 60 mS, the delay L output becomes the same as E D  and the exclusive OR G4 output goes low. This causes NAND G1 output to stay high regardless of SD, and T1 therefore stays on unaffected by received signals at E D  (unless a start signal is received for at least 600 mS, as at t12). The delay L output also operates a relay U to reverse current flow in the subscriber loop (U1, U2) and this starts the teleprinter motor. Between t 5  and t 6  S k  transmits. A start polarity is received E D  at t 6  indicating the remote subscriber's desire to transmit, and as this makes G4 inputs different, a high level (stop) goes to G1. The next S k  closure (t 7 ) raises S D  to stop polarity; and G1, in response to two high inputs, turns off T1 to lock S D  in the stop polarity condition. The received message t 8 -t 9  turns on and off T1 to operate EM. At t 9 , SK is opened to indicate desire to transmit ("start" polarity), and the next received stop polarity E D  at t 10  causes G4 output to go low, G1 output high, T1 to turn on and (as SK is open) Tr to produce a low (start) polarity at SD. The message is sent between t 10  and t 11 , at which point a start polarity from SK (opened) which lasts for 350 mS indicates "clear". A "start" polarity received at E D  from the exchange at t 12  agrees to clear, and provided it persists for 600 mS the delay L output changes to de-energize relay U. This reverses the subscriber loop current JM again, and stops the motor. Between times t 14 -t 19  reception is initiated by the remote subscriber, and a "clear" signal ends its transmission, in generally similar fashion. A transistorized embodiment (Fig. 5, not shown) of Fig. 3 uses an operational amplifier with positive feedback for the trigger Tr; a third contact (U 3 ) on relay U and an inverter transistor (T 3 ) for the exclusive OR G4, and diodes (D1, D2) for the NAND gate G1. The time delay circuit includes capacitors (C4, C5) and a fourth contact (U 4 ) on the relay U. Supposing the contact to be connecting C4 through a resistor (R29) to +12 V (dotted position), then when the input signal ED goes high an inverter (T3) reverse biases a diode (D4) to allow C4 to charge negatively. After 60 mS, C4 potential is low enough to turn on three transistors (T4, T5, T6) which turn off three more (T7, T8, T9), and relay U is de-energized. The contact U4 therefore switches the +12 V from C4 to C5 which is thus discharged (through T6). When ED goes low, the inverter (T3) turns on the diode (D4) and turns off the first three transistors (T4, T5, T6). Thereupon C5 charges negatively, and after 600 mS its potential turns on the last three transistors (T7, T8, T9) and relay U is energized again. The circuit is now reset.