JP2928970B2 - Two-wire communication device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a system for transmitting a signal from a transmitter located at the site of an industrial process to a receiver in an instrument room via two signal lines. Connected to another sensor to enable transmission of multiple signals
The present invention relates to a wire communication device. In particular, the present invention relates to a technique for adding an external sensor to a system having an existing two-wire transmitter alone.

[0002]

2. Description of the Related Art FIG. 2 shows a circuit of a two-wire communication apparatus using a conventional transmitter alone. Reference numeral 21 denotes a transmitter such as a differential pressure transmitter, an electromagnetic flow meter, or the like.
A transmitter for transmitting via 23 '. Reference numeral 24 denotes a power supply which is usually installed on the receiving instrument room side, and a 24 V power supply is generally used. Reference numeral 25 denotes a receiving resistor for receiving a signal transmitted from the transmitter 21. When a signal of 4 to 20 mA is received, the signal is received at a voltage of 1 to 5 V due to a receiving instrument such as a controller connected next. Therefore, a resistance of 250Ω is normally used.

The transmitter 21 transmits a measurement signal, for example, from 0 to 10
When transmitting at a numerical value of 0%, 4 mA and 2
Numerical values are digitized and transmitted according to a predetermined communication protocol in which 0 mA corresponds to 0 and 1, respectively. At this time, the minimum value of 4 mA is the drive current of the transmitter 21.
Another feature of these communication devices is that there is no need to take power.

[0004] The device shown here and 4 to 20 m
A, the numerical value of 1 to 5 V is used because the communication device for transmitting the analog signal used so far is used for the device for transmitting the digital signal as it is, and the value of the signal or the like at that time is used as it is. I have. This makes it possible to use both communication systems together.

[0005] The signal transmitted by the transmitter 21 is converted into a voltage signal of 1 to 5 V by the receiving resistor 25, but since the signal line is routed to the process site, it is the highest value of the voltage drop due to the receiving resistor. A voltage sufficiently higher than 5 V is required, and a power supply with a voltage of 24 V is used.

[0006]

In the communication apparatus described above, the transmitter has a higher function and the signal is transmitted digitally. Therefore, other information besides the physical quantity measured by the transmitter is required. Can also be sent. Accordingly, information such as the state of the transmitter can be transmitted at the same time. In addition, signals from an external sensor near the transmitter can be simultaneously transmitted to the transmitter.

However, the external sensor itself requires a power supply, and thus, it is wasteful to newly draw a power supply wiring. Therefore, this power source can be obtained from the two signal lines of the communication device in the same manner as the transmitter, but the voltage drop at the receiving resistor increases by the increase of the current. For example, when the sensor requires a 40 mA power supply,
Since it becomes 44 to 60 mA when superimposed on mA, a voltage of 11 to 15 V is generated at both ends of the receiving resistor (250 Ω).

However, since the power supply voltage is 24 V if the conventional circuit is used, there is a possibility that shortage of voltage occurs and transmission over a long distance becomes difficult, so that it cannot be used as a process device. Therefore, every time an external sensor is connected,
There was a disadvantage that the power supply had to be replaced with a higher voltage. In addition, since the voltage generated at the receiving resistor becomes high, it is necessary to adjust the voltage for the subsequent equipment.

It is conceivable to use an external sensor that consumes a small amount of current. However, providing an external sensor for that purpose requires more time and effort, which is contrary to the original purpose of saving cost and time. Therefore, one of the objects is that any conventional sensor can be used.

[0010]

SUMMARY OF THE INVENTION The present invention solves these problems by using four to two signal lines via two signal lines.
A transmitter for transmitting a current signal of 0 mA, an external sensor connected to the transmitter, receiving a drive current from the signal line, and transmitting a measurement signal to the transmitter, and a receiving side of the two signal lines And a receiving resistor for receiving a signal from the transmitter as a voltage, and a resistor connected in parallel with the receiving resistor. The device is constituted by a constant current circuit which supplies a current corresponding to the increase when the current increases.

[0011]

When the current consumption of the external sensor is superimposed on the signal current of the transmitter by the above-described device, the current for the current consumption of the external sensor is bypassed by the constant current circuit, and the voltage at both ends of the receiving resistor becomes external. It goes down to the state without a sensor. Accordingly, there is no need to change the power supply, the receiving resistor, and the receiving instrument at the subsequent stage, and the line voltage does not change, so that a sufficient value of the voltage required for transmission can be secured.

[0012]

FIG. 1 shows an embodiment of the present invention. Reference numeral 1 denotes a differential pressure transmitter with a built-in microcomputer, which is capable of digital transmission in addition to conventional analog transmission, and is capable of simultaneously transmitting information other than the internal differential pressure, such as static pressure and temperature information. have. In addition, it has a terminal to which a signal of a temperature sensor or the like can be input to the outside when a flow rate is measured by another sensor, for example, a differential pressure transmitter, and correction is performed by measuring a temperature of a fluid.

Reference numeral 4 denotes a 24 V power supply, which is equivalent to a conventional power supply. This is usually the case when the transmitter is transmitting digitally or analogly.
A power supply having a voltage of about V is used. This is to secure a signal transmission voltage sufficient for long-distance transmission.

Reference numeral 5 denotes a receiving resistor, and a receiving instrument 6 such as a controller or a recorder is connected to both ends, and 4 to 20 mA.
Is connected to a resistance of 250 Ω, and converted into a voltage of 1 to 5 V to give a signal to the receiving instrument 6. In the present invention, the detection signal is given as a digital signal. Therefore,
In the signal, in addition to the process signal from the transmitter, other information in a time sharing manner, for example, when the transmitter 1 is a composite sensor, other process signals, diagnostic information of the transmitter 1, and the like can be transmitted by a predetermined protocol. is there.

Reference numeral 2 denotes an external sensor required by the present invention. Power is received and driven by inserting the two signal lines 3 and 3 'in parallel. The measurement signal is input to a terminal of the differential pressure transmitter 1. In the sensor used here, for example, when the transmitter 1 is a differential pressure transmitter and the flow rate of a fluid is measured, the temperature of the fluid affects the measurement of the flow rate, so that temperature correction may be required. . In that case, it is like a temperature sensor inserted into a pipe to measure the temperature of the fluid. However, conventionally, the measurement signal was transmitted to the instrument room using another signal line, and a power line was separately provided. In addition, the labor and cost of wiring for this purpose were enormous.

In the present invention, since the transmitter 1 can transmit a plurality of signals by digital signals, the signal of the external sensor 2 is input to the external signal input terminal of the transmitter 1, and the transmitter 1
A measurement signal is transmitted together with the signal. In addition, by receiving the power from the signal line of the transmitter 1, the problem of the power is also solved.

However, when the power supply of the external sensor 2 is connected in parallel to the transmitter, the current consumption of the external sensor 2 is superimposed on 4 to 20 mA and the current consumption of the external sensor 2 is 40 mA.
60 mA. By the way, the voltage signal received by the receiving instrument 6 is 1 to 5 V, cannot be received at 250Ω, and the current 40 mA consumed by the external sensor 2 does not change. 40 m of current consumed by the external sensor 2 via the connected constant current circuit 7
If the voltage is adjusted so as to bypass A, only the signal voltage is generated at both ends of the receiving resistor 5, and the voltage generated at both ends of the receiving resistor 5 does not change.

Therefore, since the current consumption of the external sensor connected to the transmitter 1 is known in advance, by flowing the current consumption by the constant current circuit 6, no physical change is required on the receiving instrument 6 side. . That is, the device of the present invention is realized by connecting an existing external sensor and a constant current circuit capable of current adjustment to the configuration of the conventional communication device.

However, since an external sensor has been added, the signal output from the transmitter 1 is superimposed on the measurement signal, so that a circuit or software for separating the signal is required.

[0020]

As described above, according to the present invention,
Since the basic configuration of the conventional two-wire communication can be simply changed by connecting the external sensor 2 and the constant current circuit 7 without any change, the configuration is very simple and advantageous in cost. Also, the power supply voltage does not affect the transmission at all as it is in the conventional case.

Furthermore, even if a plurality of external sensors are used, the above configuration does not change to a basic configuration at all. It is only necessary to adjust the constant current circuit 7 and add an external sensor as long as the allowable current of the signal line permits. Wear.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of an apparatus to which the present invention is applied.

FIG. 2 is a diagram showing a configuration of a conventional communication device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Transmitter 2 External sensor 3 Signal line 4 Power supply 5 Receiving resistance 6 Receiving instrument 7 Constant current circuit

Claims (1)

(57) [Claims] 1. A transmitter for transmitting a current signal of 4 to 20 mA via two signal lines, connected to the transmitter, receiving a drive current from the signal line, and transmitting a measurement signal to the transmitter. An external sensor for transmitting, a power supply connected to a receiving side of the two signal lines, a receiving resistor for receiving a signal from the transmitter as a voltage, and a transmitter connected in parallel with the receiving resistor. From the use of the external sensor,
A two-wire communication device, comprising: a constant current circuit that supplies the increased current when the current consumption increases.