Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/14—Two-way operation using the same type of signal, i.e. duplex
  • H04L5/1423—Two-way operation using the same type of signal, i.e. duplex for simultaneous baseband signals

Definitions

• the present invention relates to a method and a system for two-way transmission of data via a two-wire network between a central unit and a plurality of centrally fed terminal units.
• the central unit and terminal units are connected in parallel to the two-wire network.
• the signal transmission in one direction takes place by generation of voltage signals and in the opposite direction by generation of current signals.
• Background Art A communication system is known from the U.S.
• Patent Specification No. 4,477,896 where information may be simultaneously transmitted in both directions over a single common wire between a base unit and a plurality of remote units.
• the information from the remote units is sent in the form of time divided multiplexed current pulses and the information from the base unit is sent in the form of time divided voltage pulses.
• the object of the present invention is to provide a method and a system where data is transmit table in two-way directions on a two-wire network between a central unit and a large number of terminal units, so-called multipoint connections, where the number of terminal units can be several hundreds.
• the data transmission shall have great immunity to noise and shall be able to take place over relatively long distances (at least three kilometers ⁇ with high transmission speed (approximately 19.2 kbaud).
• Further objects are to achieve an uncomplicated line layout, and free branchability where no demands are made on the type of line.
• an essential object is that the system can be built at low cost.
• the method in accordance with the invention which is characterized by using balanced signals in the branches of the two-wire network, in one direction taking place by generating balanced voltage variations by simultaneous reversal of the voltage supply to each one of the branches, while signal transmission in the opposite direction takes place by closing and opening a current circuit connected with the two branches of the two-wire network for generation of current variations.
• the balanced signals are differentially detected by detection circuits having a built-in hysteresis function.
• this method can be carried out in a system where the central unit has a means comprising a two-pole, two-way switch causing polarity reversal of a balanced voltage fed from the central unit to two-wire network, and where each terminal unit includes current regulating means comprising a switch for closing and interrupting a balanced current signal from a current generator supplied through a rectifier to the two-wire network.
• the detection circuits for detecting the balanced voltage and current signals have a built- in hysteresis function.
• Figure 1A is a graph showing the signals appearing at the branches of the network
• Figure 2 is a wiring diagram for an embodiment of the central unit
• Figure 3 is a wiring diagram for an embodiment of a terminal unit in accordance with the invention
• Figure 4 is a wiring diagram for a modified embodiment of a terminal unit in accordance with the invention.
• Data communication over a two-wire network N takes place in one direction with the aid of a balanced voltage feed and in the opposite direction with a current feed.
• This is illustrated by the principle layout according to Figure 1, which to the right shows a central unit C and to the left one of a plurality of like terminal units T, connected in parallel to the two-wire network N.
• an unlimited number of terminal units T can be connected to the two-wire network.
• the central unit C has a voltage feed +U, -U to the line side of the terminal units T.
• the voltage feed takes place via a switch having its contacts K1, K2 actuated by a relay coil Rel.
• the relay coil Rel is activated by transmission data S1, which is to transmsitted from the central unit C to one or more of the terminal units T.
• a reversal of the voltage feed +U, -U thus takes place on branches N1, N2 of the two-wire network N in the transmission of data from the central unit C.
• Each terminal unit T is formed with a voltage detector for sensing the balanced voltage signal +U , -U departing from the central unit C.
• the voltage detector is illustrated by a differential amplifier D1 with built-in hysteresis function, the output of which transmits received data M1.
• the received data signal M1 has an amplitude which is the difference between +U and -U measured between the branches N1 and N2, as illustrated in Figure 1A. Since the voltage signal +U, -U is balanced (differential mode), possible noise (common mode) B will not affect the amplitude of the received data signal M1, since the noise B occurs with a direction which is the same in both branches in the two- wire network N, and the original difference signal between +U and -U is maintained.
• a current regulating means is used for data communication from a terminal unit 1, this means being illustrated in Figure 1 by a switch K3 which is controlled by a relay coil Re2.
• the relay coil Re2 is activated by transmission data S2 which are to be transmitted from the terminal unit T to the central unit C.
• the switch K3 closes and interrupts a current circuit which, via rectifier L, is supplied with voltage from the central voltage supply +U , -U and which includes a current generator G. With the current circuit closed, there is thus generated a current in the two-wire network N.
• the differential amplifier D1 included in the terminal unit T will not be acted on by the current signal, the amplifier is balanced, as indicated by the denotations V+ and V-.
• the balanced current signal generated by transmission data S2 is thus fed into both branches N1, N2 of the two-wire network N.
• the current signal is sensed at the central unit C via two resistors R1, R2 and the signal is sensed as two voltage drop signals having a wave form as illustrated in Figure 1A.
• These signals are differential, as opposed to noise, and are fed to a differential amplifier D2 having a hysteresis function, via the derivation circuit RC2.
• the received data signal M2 occurring on the output of the differential amplifier D2 is insensitive to noise, for the same reason as described above with regard to the received data signal M1.
• FIG. 2 An embodiment of the central unit C is illustrated in the form of a wiring diagram in Figure 2.
• the same reference characters have been used as in Figure 1 for means corresponding to each other in both figures.
• the transmission data S1 of the central unit C are fed in via a phototransistor, and received data M2 detected in the transistor are taken out via a photodiode.
• a Schmidt trigger ST is connected to the output of the differential amplifier D2 for further suppression of any noise.
• the connection of the central unit C to the two-wire net- work N is carried out with the aid of a protective circuit including a high frequency-attenuating transformer T1 and PTC resistors which give protection against short-circuit- mg.
• terminal unit T An embodiment of the terminal unit T is illustrated in the form of a wiring diagram in Figure 3.
• the same reference characters as in Figure 1 are used for means corresponding to each other in both figures.
• This emb o d i men t is not a "genuine" balanced terminal unit, but it can be used to advantage for many purposes.
• the terminal unit T is galvanically insulated from the rest of the equipment which is connected to the unit. Accordingly, transmission data S2 is fed in via a photo-transistor, and a detected, balanced voltage signal is taken out via two photodiodes P1, P2 before feeding to the differential amplifier D1, on the output of which there is received data M1.
• the connection of the terminal unit 1 to the two-wire network N takes place via a PCT resistor for protection against short-circuiting.
• a modified embodiment of the terminal unit T, which gives a "genuine" balancing of signals received by the unit from the central unit C is illustrated in Figure 4
• the voltage detector here comprises a double flip-flop device D1' having a built-in hystoresis function. Balancing is achieved by the detector being supplied with voltage symmetrically in relation to the potential of the two-wire network N.
• the received data signal M1 cannot be changed other than by both branches N1, N2 of the two-wire line simultaneously changing polarity in opposite directions.
• the circuit G operates as a constant current generator, by the base voltage on the transistor included in the circuit being kept within a narrow range with the aid of a Zener diode Z, which is included in the current regulating circuit Re2, K3. Constant base voltage gives constant emitter voltage, which gives constant current through the emitter resistor Rg and thereby through the transistor in the circuit G.

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• Engineering & Computer Science (AREA)
• Signal Processing (AREA)
• Computer Networks & Wireless Communication (AREA)
• Small-Scale Networks (AREA)
• Dc Digital Transmission (AREA)
• Arrangements For Transmission Of Measured Signals (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

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Family Applications (1)

Country Status (2)

Cited By (1)

Citations (3)

• 1987
  • 1987-11-23 SE SE8704629A patent/SE459948B/sv not_active IP Right Cessation
• 1988
  • 1988-11-23 WO PCT/SE1988/000638 patent/WO1989005074A1/en unknown

Patent Citations (3)

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