GB1360605A - Process control arrangements - Google Patents

Abstract

1360605 Data transmission systems; semiconductor switching circuits PLESSEY CO Ltd 6 Aug 1971 [6 Aug 1970] 37912/70 Headings H4P and H3T A double-current digital transmission system employs equal currents in both wires to avoid return earth currents and therefore cross-talk from them. A so-called marker module 1, say a computer processor, transmits to one of a number of slave modules 2, e.g. stores. A standing line current of i/4 flows in each wire 3, 4 in opposite directions from sources +V, -V to earth, or, and over resistances R5, R6 and R4, R3, but which cancel in terminating resistances R7, R8. On sending a mark signal from module 1 switches S1 and S2 close to send current i to each wire from constant current sources CS1, CS2 again, resulting in, a current of i/4 in each of resistors R3, R4, R5, R6, but in the opposite directions, producing a change (reversal) in the potential difference between wires 3 and 4 which is detected in differential amplifiers 2 in the slave modules. Transmission from a slave to the master module over a separate pair of wires employs similar components (Fig. 3, not shown). The current sources CS1, CS2 and switches S1, S2 may be provided by the transistor circuit shown in Fig. 4. Antiphase signals applied simultaneously to T1, T2 switches on T3, T4 to supply the double current signals.