JPH02309835A - 2-wire type bi-directional interface circuit - Google Patents

Abstract

PURPOSE:To eliminate the need for a transistor(TR) and a relay and to simplify the circuit constitution by using a logic element to switch transmission and reception. CONSTITUTION:When a loop direction switching signal SEL is brought into a logical L, an output of a NAND gate iC1 goes to H and an output of a buffer iC2 goes to L to form a current loop in the reception direction, a device A enters the reception state and the signal is sent from a transmission photocoupler PC 4 of a device B to a reception photocoupler PC 2. When a loop direction switching signal SEL goes to H, an output of the buffer iC2 goes to H, the device A enters the transmission state and an output of the NAND gate iC1 goes to L or H depending whether the transmission data input Data goes to H or L, the current loop in the transmission direction is intermitted via the NAND gate iC1 and the signal is delivered to the reception photocoupler PC 3 of the device B. Thus, the circuit constitution is simplified by using the NAND gate iC1 and the buffer iC2 being logic elements so as to apply switching.

Description

[Detailed description of the invention]

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[Industrial application field]

The present invention relates to an interface circuit of a communication device for a two-wire bidirectional transmission line that performs two-way communication by switching the current direction of a current loop formed by reciprocating a two-wire data transmission line. The present invention relates to a two-wire bidirectional interface circuit that is capable of switching between transmission and reception and detecting disconnection of a transmission line. Note that in the following figures, the same reference numerals indicate the same or corresponding parts.

[Conventional technology]

Conventional current loop type two-wire bidirectional interface circuits use relays or transistors to form a current loop in the transmission line and switch the current direction, and the direction of the current loop determines the transmission or reception status, and this loop current The presence or absence of this corresponds to the logical state of the communication data. The transmitting circuit and receiving circuit at this time are independent. That is, FIG. 4 is a conceptual diagram of this type of conventional current loop type two-wire interface circuit. In the same figure, A,
B is a device that communicates via two lines (transmission lines), El
is a power source for transmission provided in device A, E2 is a power source for reception as well, and S is a loop direction switch that determines the direction of the current loop, and therefore the transmission and reception status of devices A and B, by the loop direction switching signal SEL. It's a switch. Tri is a transmission transistor also provided in the device A, and is turned on and off by the transmission data input Data. PO2 is a photocoupler for receiving equipment A, and R is a resistor for limiting the loop current of the line. Also, PO3 and PO2 are a receiving photocoupler and a transmitting photocoupler provided in device B, respectively, and DI and D2 are photocouplers P C3 and P.
This is a diode that bypasses C4 in the reverse direction. In this system, the presence or absence of a loop current corresponds to the logical state of data, and the current loop is interrupted and communicated by the transistor Tri or the photocoupler PC4. For example, when device A sends data to device B,
The switch S is switched to the side of the transmitter aE1, and by opening and closing the transmitter transistor Tr1, the transmitter type BE1-
Transistor Tri → diode D2 → photocoupler P
A transmission current as an intermittent loop current flows in the path C3→resistance R−sinch S−transmission power source E1, and device B receives this transmission data via the reception photocoupler PC3. On the other hand, when device A receives data from device B,
The switch S is switched to the receiving power source E2 side, and by opening and closing the transmitting photocoupler PC4 in the device B, the receiving power source E2 - switch S - resistor R - diode D1 → transmitting photocoupler PC4 = receiving photocoupler PC2 - A reception current as an intermittent loop current flows in the path of the reception power source E2, and the device A receives this reception data via the reception photocoupler PC2. Next, Figure 5 shows the loop direction changeover switch S in Figure 4.
This is a conventional circuit in which the portion corresponding to the above is configured with relay contacts SL and S2, and the transmitting power source E1 and the receiving power source E2 are replaced with a common power source E. FIG. 6 also shows the relay contact 31 of FIG. This is a conventional circuit in which S2 is replaced with transistors Tr2 to Tr4. Next, FIG. 7 shows a conventional circuit in which an anti-parallel circuit of a disconnection detection photocoupler PC5 and a diode 3 is added to the circuit of device A in FIG. It is. That is, when the switch S is switched to the transmittable state of the device A, the loop current in the transmission direction of the device A due to turning on the transistor Tri flows through the photocoupler PCB. PC
5, it is possible to check for disconnection of the line.

[Problem to be solved by the invention]

However, in conventional circuits such as those shown in FIGS. 4 to 6, the switches constituting the circuit. Since the degree of integration of relays and transistors is low, the interface circuit becomes complicated and the device becomes large. In addition, with the conventional current loop type two-wire bidirectional interface circuit, it is necessary to add a separate circuit as shown in Figure 7 in order to check for disconnection of the communication line and self-diagnose the transmitting circuit. However, there were disadvantages in that the device became larger and the cost increased. Therefore, the present invention switches the direction of the current loop using a logic element, or uses an AC input photocoupler as the receiving photocoupler, and uses two wires for both transmission (disconnection detection, self-diagnosis) and reception. An object of the present invention is to solve the above-mentioned problems by providing a two-way interface circuit.

[Means to solve the problem]

In order to solve the above problems, the interface circuit of the present invention is a two-wire bidirectional interface circuit that switches between transmission and reception by switching the current direction of a current loop that reciprocates on a two-wire transmission line. Switching is performed using a logic element (NAN
D gate + CI. Detection of the received signal from the transmission path and detection of the received signal from the transmission path via a photocoupler (transmission/reception photocoupler PCIO, etc.) of the I'AC input. It is assumed that line diagnosis is performed by detecting current in the transmission direction.

[For use]

By connecting the outputs of two logic elements (e.g., NAND gates and buffers) with a common input and opposing outputs to both ends of the line, it is possible to change the polarity of both ends of the line depending on the logic state of the inputs. It can be used to switch the direction of the current loop in the line. Furthermore, from the AC input photocoupler, it is possible to detect not only the received signal but also the state of the current loop during transmission.

【Example】

The present invention will be described in detail below with reference to FIGS. 1 to 3. FIG. 1 is a circuit diagram showing the configuration of a first embodiment of the present invention, in which switches S in FIGS. 4 to 6,
Relay contact 5LS2 or transistor Tri~Tr
NAND gate ic1.4 as two logic elements. and buffer iC2. Naori O is a diode that bypasses the receiving photocoupler PC2 in antiparallel. In FIG. 1, when the loop direction switching signal SEL is set to the logic state II L ++, the output of the NAND gate iC1 is "H" and the output of the buffer iC2 is "L", forming a current loop in the receiving direction as shown in the figure. Device A enters the receiving state, and the signal is transmitted to the transmitting photocoupler PC of device B.
4 to the receiving photocoupler PC2. Next, when the loop direction switching signal SEL is set to "H++", the output of the buffer iC2 becomes "H", and the device A enters the transmission state, and the output of the NAND gate ic1 becomes "L" due to the transmission data input Data being "H" or HL++. or “H”
'', and the current loop in the transmission direction shown in the figure is NAND
The signal is transmitted to the receiving photocoupler PC3 of the device B through the gate 1-icl. FIG. 2 is a circuit diagram showing a configuration as a second embodiment of the present invention, and corresponds to FIG. 7. Further, FIG. 3 is a conceptual diagram of FIG. 2. In FIGS. 2 and 3, the reception photocoupler PC2 and disconnection detection photocoupler PC5 in FIG. 7 are used for transmission and reception (
AC input) has been replaced with a photocoupler PCIO,
In addition, in FIG. 2, the switch S, transistor Tri, etc. in FIG.
1 and buffer iC2. In FIG. 2, set the loop direction switching signal SEL and the transmission data manually to "H" degrees as described in FIG. 1, or in FIG. 3, set the switch S to the transmitting side and make the transistor Tri conductive. By causing a loop current in the transmitting direction to flow and detecting this current with the transmitting/receiving photocoupler PCIO, it is possible to check for disconnection of the communication line and perform self-diagnosis of the transmitting line.

【Effect of the invention】

According to the present invention, in a two-wire bidirectional interface circuit that switches between transmission and reception by switching the current direction of a current loop that reciprocates in a two-wire transmission path, the switching is performed using a NAND gate as a logic element. ic1. Since this is done via the buffer iC2, transistors and relays can be eliminated, the circuit configuration becomes simpler, and costs and space can be reduced. In addition, lower power consumption can be expected compared to conventional methods. In addition, the circuit diagnosis is performed by detecting the received signal from the transmission line and detecting the current in the transmission direction to the transmission line via the transmitting/receiving photocoupler PCIO as an AC input photocoupler. By simply changing the circuit by replacing the receiving photocoupler with the transmitting/receiving photocoupler, it becomes possible to detect communication line disconnections and self-diagnose the transmitting circuit.
The reliability of a two-wire bidirectional interface circuit can be improved at low cost.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a configuration as a first embodiment of the present invention, Fig. 2 is a circuit diagram showing a configuration as a second embodiment of the invention, and Fig. 3 is a conceptual diagram of Fig. 2. , FIGS. 4 to 6 are conventional circuit diagrams corresponding to FIG. 1, and FIG. 7 is a conventional circuit diagram corresponding to FIG. 2. A, B: Device, PC2, PC3: Receiving photocoupler,
PC4: Photocoupler for transmission, pcto: Photocoupler for transmission and reception, i C1: NAND gate, iC2: Buffer, DO~D2: Diode, Data: Transmission data input, SEL: Loop direction term Fig. 1 Me→IA--- Reiko Hiromi IB-- Figure 3, Figure 4, Figure 5, Figure 6

Claims (1)

[Claims] 1) In a two-wire bidirectional interface circuit that switches between transmission and reception by switching the current direction of a current loop that reciprocates in a two-wire transmission line, the switching is performed via a logic element. A two-wire bidirectional interface circuit characterized by: 2) In a two-wire bidirectional interface circuit that switches between transmission and reception by switching the current direction of a current loop that reciprocates on a two-wire transmission line, the reception signal from the transmission line is input via an AC input photocoupler. A two-wire bidirectional interface circuit characterized in that it performs detection and line diagnosis by detecting a current in a transmission direction to the transmission path.