JPH0883389A - Field communication device - Google Patents

Abstract

(57) [Summary] [Object] An object of the present invention is to provide a field communication device capable of maintaining a communication function even if a bus cable is disconnected. [Structure] Stations 2-1 and 2 on bus cable 1a
-N is connected, and a backup bus cable 1b is provided alongside the bus cable 1a in a field communication device that transmits signals between stations via the bus cable 1a, the bus cable 1a, and the backup bus cable. A switching unit 11 provided between each one end of 1b and the power source 3 and normally connecting the power source 3 to the bus cable 1a, and an electric quantity and a reference value of the bus cable 1a to which the power source 3 is connected. And a comparator 1 for switching the bus cable for supplying the power supply 3 to the backup bus cable 1b according to the comparison result.
3 is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control signal for transmitting a process signal output from a field device installed in each place of a plant facility to a control and monitoring device, or for controlling a process amount from the control and monitoring device to the field device. The present invention relates to a field communication device used for transmitting data, and more particularly, to improvement of a field bus type transmission line.

[0002]

2. Description of the Related Art In general plant equipment, various sensors are installed at various places in the field to control various process amounts such as pressure and flow rate, and signals output from these sensors are sent to a central control and monitoring device. To transmit. Then, the control monitoring device monitors the process based on the sensor signal, and transmits a signal for controlling the process amount to the field device.

Conventionally, the signal used to transmit the process amount from each sensor to the control and monitoring device is a direct current of 4 to 2
Although it was an analog signal of 0 mA, a so-called field bus system has recently been considered in which all transmission between a control and monitoring device and a field device such as a sensor is a digital signal.

FIG. 2 is a block diagram of a field communication device adopting the fieldbus system. In the conventional field communication device, a plurality of stations 2-1 to 2-n are connected on one bus cable 1, a power supply 3 is connected to one end of the bus cable 1 through a filter 4, and the bus cable is further connected. A terminator 5 is connected to the other end of 1. The filter 4 is for preventing the communication signal from flowing into the power supply 3, and has a cutoff band of about 32 kHz. The terminator 5 is provided to prevent the communication signal from being reflected at the end.

The bus cable 1 mainly uses a pair of electric wires such as twisted pair wires, and the stations 2-1 to 2-
n is connected to each of the twisted pair lines. The stations 2-1 to 2-n connected on the bus cable are
A sensor that measures the process amount, a recorder that records the output signal of the sensor, an actuator such as a valve that adjusts the process amount, a controller that controls the actuator, and the like.

In the above field communication device, when a digital signal is output to the bus from the stations 2-1 to 2-n connected to the bus, the required station 2 takes the digital signal into itself. The data transmission is performed by.

By the way, most of the accidents occurring in the bus cable are caused by the disconnection. In the field communication device shown in FIG. 2, since transmission between stations depends on one bus cable 1, if the bus cable 1 is disconnected, communication becomes impossible, and depending on the disconnection point, the process cannot be controlled. Occurs.

[0008]

As described above, the field communication device of the field bus system has a problem that when the bus cable is disconnected, the communication becomes impossible and the process cannot be controlled depending on the disconnection point.

The present invention has been made in view of the above circumstances, and it is possible to always check the soundness of the bus cable and to maintain the communication function even if the bus cable is broken. An object of the present invention is to provide a highly reliable field communication device capable of preventing the influence on the process.

[0010]

The present invention has taken the following means in order to achieve the above object. In the present invention corresponding to claim 1, a power source is connected to one end of a bus cable through a low-pass filter, and a terminator for preventing reflection of a communication signal is connected to the other end of the bus cable,
In a field communication device in which a plurality of stations connected on the bus cable transmit a process signal or a signal for controlling a process amount through the bus cable, a resistor is inserted between the bus cable and the power supply. , The soundness of the bus cable is judged based on the signal on the low-pass filter side of the resistor.

According to a second aspect of the present invention, a signal from the low-pass filter side of the resistor is used as a first input, and the low range of the resistor when the voltage drop generated in the resistor is zero. The comparator is provided with the reference signal corresponding to the signal on the pass filter side as the second input.

According to a third aspect of the present invention, when a signal from the low-pass filter side of the resistor is used as a first input and a voltage drop occurs in the resistor due to the sum of consumption currents of all stations. The comparator is provided with a reference signal corresponding to the signal on the low pass filter side of the resistor as a second input.

According to a fourth aspect of the present invention, a field device and a control / monitor device are connected on a bus cable having a power source connected to one end thereof, and a process amount measured by the field device is transmitted via the bus cable. In a field communication device for transmitting a control signal for controlling a process amount to a control / monitoring device and transmitting the control signal from the control / monitoring device to a field device via a bus cable, a backup bus cable provided adjacent to the bus cable And a switching unit that is provided between each end of the bus cable and the backup bus cable and the power supply, and normally connects the power supply to the bus cable, and between the power supply and the bus cable. When the voltage drop in the inserted resistor is compared with the reference voltage, and the abnormality of the bus cable is detected, the switching unit is turned on. It has a configuration comprising a bus switching control means for connecting the bus cable for backup to a power supply and control.

[0014]

The present invention has the following effects by taking the above measures. According to the present invention corresponding to claim 1, a plurality of stations perform data transmission on the bus cable to which the power source is connected via the low-pass filter, so that the total sum of the consumption currents at the respective stations is calculated from the power source. It flows to the bus cable side through the resistor, and the resistor causes a voltage drop corresponding to the sum of the current consumption. On the other hand, when the bus cable is disconnected, the consumption current does not flow, and the voltage drop in the resistor is eliminated. Therefore, the soundness of the bus cable can be determined by monitoring the signal appearing on the low pass filter side of the resistor.

According to the present invention corresponding to claim 2, the signal appearing on the low-pass filter side of the resistor and the reference value are compared by the comparator. Since the reference signal corresponding to the signal appearing on the low-pass filter side of the resistor when the voltage drop occurring in the resistor is zero is used as the second input of the comparator, so if the output of the comparator is inverted, the bus cable It can be determined that an abnormality has occurred in the.

According to the present invention corresponding to claim 3, a reference signal corresponding to a signal appearing on the low-pass filter side of the resistor when a voltage drop occurs in the resistor due to the sum of consumption currents of all stations is used. Since it is used as the second input of the comparator, it can be determined that an abnormality has occurred in the bus cable if the output of the comparator is similarly inverted.

According to the present invention corresponding to claim 4, the power source is normally connected to the bus cable by the switching unit, and the amount of electricity supplied from the power source to the bus cable is input to the comparator and compared with the reference value. It Since the amount of electricity supplied from the power supply to the bus cable changes depending on the current consumption of the station connected to the bus cable, if the bus cable is disconnected, the current consumption by the station becomes zero. The comparator detects disconnection by comparing the amount of electricity supplied to the bus cable with a reference value. Then, the comparator controls the switching unit to switch the bus cable to be supplied with power to the backup bus cable. As a result, even if the bus cable on the regular side is broken, it can be immediately switched to the backup bus cable, so that each station can continue communication.

[0018]

Embodiments of the present invention will be described below. FIG. 1 shows the configuration of a field communication device according to an embodiment of the present invention. The field communication device of this embodiment is
Bus cable 1a made of twisted pair wire and the like, and backup bus cable 1b having the same structure as this.
And are installed in the field. Each of the two bus cables 1a and 1b is connected to each station 2-1 to 2-
n are connected. In addition, each bus cable 1a, 1b
One end of each of the bus cables 1a and 1b is connected to a terminator 5 that prevents reflection of communication signals, and the other end of each of the bus cables 1a and 1b is connected to the dual power supply device 10 through filters 4a and 4b that prevent the communication signals from being mixed into the power supply side. It is connected.

The dual power supply unit 10 includes one filter 4
a terminal T1 to which the positive terminal of a is connected, and its filter 4a
Terminal T2 to which the negative terminal of is connected, and the other filter 4b
Is provided with a terminal T3 to which the positive side terminal is connected and a terminal T4 to which the negative side terminal of the filter 4b is connected. The terminals T1 to T4 are connected to the power supply 3 via the switch 11. A resistor 12 is inserted between the terminal T1 to which the bus cable 1a on the regular side is connected and the switcher 11. The point A between the terminal T1 and the resistor 12 is connected to one input terminal of the comparator 13. The reference voltage Va is set from the reference voltage source 14 to the other input terminal of the comparator 13. The output signal of the comparator 13 is given to the switch 11, and the switch 11 operates to switch from the bus cable 1a on the regular side to the bus cable 1b on the backup side in accordance with the output state of the comparator 13.

The filters 4a and 4b are constituted by low-pass filters in order to prevent communication signals between stations from flowing into the power source 3 side. In addition, the stations 2-1 to 2-n have two bus cables 1a, 1
Connected to each of b is as described above,
Bus cable 1 for each station 2-1 to 2-n
A diode 15 for preventing backflow is provided between a and 1b.

Next, the operation of this embodiment configured as described above will be described. Normally (when the bus cable 1a is not broken), the switching device 11 connects the power supply 3 and the bus cable 1a as shown by the solid line in FIG. 1, and in this state, the stations 2-1 to 2- n is the bus cable 1a
It communicates via.

At this time, the sum of currents consumed by the stations 2-1 to 2-n flows into the bus cable 1a from the power source 3 through the resistor 12, so that the resistor 12 responds to the sum of consumed currents. A voltage drop occurs, and a voltage obtained by subtracting the voltage drop from the power supply voltage is generated at point A which is one end of the resistor 12.

When the bus cable 1a is not broken and is operating normally, a voltage Vb, which is the power supply voltage of the power supply 3 minus the voltage drop due to the resistor 12, is generated at the point A. Vb is not fixed, but station 2-1
It fluctuates within a predetermined range according to the consumed current of ~ 2-n.

Here, the resistor 12 is caused by a cable break or the like.
Voltage V appearing at point A when the voltage drop at
A value slightly smaller than b'is set in the comparator 13 as the reference voltage Va. And 2 of the comparator 13
While one of the inputs satisfies the input condition of the reference voltage Va> the point A voltage Vb, the first level signal for instructing the selection of the bus cable 1a is sent to the switch 11, and when the input condition is not satisfied. To switch 11 to bus cable 1b
The comparator 13 is adjusted in advance so as to send out the second level signal instructing the selection.

Therefore, when the bus cable 1a is not broken and is operating normally, the input condition of the reference voltage Va> the point A voltage Vb is satisfied in the comparator 13, and the bus cable is transferred from the comparator 13 to the switcher 11. A first level signal for selecting 1a is transmitted. Station 2
The point A voltage Vb varies depending on the current consumption of -1 to 2-n, but the above input condition is satisfied unless the operation is stopped in all the stations 2-1 to 2-n, so the connection state shown in FIG. Is maintained.

On the other hand, when the bus cable 1a is broken, the voltage drop at the resistor 12 disappears, so the voltage at point A Vb =
Vb 'and the reference voltage Va in the comparator 13 is reached.
> The input condition of the point A voltage Vb is not satisfied. Then, the second level signal is sent from the comparator 13 to the switch 13. When the second level signal is input to the switch 11, the cable to be connected to the power supply 3 is switched to the bus cable 1b as shown by the broken line in FIG. As a result,
Power is supplied to each of the stations 2-1 to 2-n from the bus cable 1b, and communication can be continued.

As described above, according to this embodiment, the resistor 12 is interposed between the bus cable 1a on the regular side and the power source 3,
Since the magnitude of the voltage drop in the resistor 12 is compared by the comparator 13, the soundness of the bus cable 1a can be confirmed, and when an abnormality occurs in the bus cable 1a, it is possible to take prompt action. .

According to this embodiment, stations 2-1 to 2-1
2-n is connected to the two bus cables 1a and 1b, and the resistor 1 is provided between the bus cable 1a on the regular side and the power supply 3.
Since the disconnection is detected based on the voltage drop at 2 and the power supply 3 is connected to the backup-side bus cable 1b, the communication function can be maintained even if the bus cable 1a is disconnected.

By measuring the voltage drop across the resistor 12, it is possible to detect the number of stations currently connected on the bus cable. On the contrary, if the number of stations currently connected to the bus cable is known, it is possible to judge whether or not each station is operating normally by monitoring the voltage drop in the resistor 12. .

Further, when the bus cable is short-circuited, the voltage drop of the resistor 12 becomes large. Therefore, the resistor 12
You may make it detect the cable short circuit by monitoring the voltage drop of.

Further, the input (voltage Vb at point A) of the comparator 13 may be provided with hysteresis to prevent malfunction due to noise. The present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

[0032]

As described in detail above, according to the present invention, it is possible to provide a highly reliable field communication device capable of maintaining the communication function even if the bus cable is broken and preventing the influence on the process. .

[Brief description of drawings]

FIG. 1 is a configuration diagram of a field communication device according to an embodiment of the present invention.

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

[Explanation of symbols]

1a, 1b ... Bus cable, 2-1 to 2-n ... Station, 3 ... Power supply, 4a, 4b ... Filter, 5 ... Terminator, 10 ... Dual power supply device, 11 ... Switching device, 12 ... Resistor, 13 ... Comparator , 14 ... Reference voltage source, 15 ...
diode.

Claims (4)

[Claims] 1. A power supply is connected to one end of a bus cable through a low-pass filter, and a terminator for preventing reflection of communication signals is connected to the other end of the bus cable, and the bus cable is connected to the bus cable. In a field communication device in which a plurality of stations transmits a process signal or a signal for controlling a process amount through the bus cable, a resistor is inserted between the bus cable and the power supply,
A field communication device, characterized in that the soundness of the bus cable is judged based on a signal from the low-pass filter side of the resistor. 2. A signal from the low-pass filter side of the resistor is used as a first input, and corresponds to a signal on the low-pass filter side of the resistor when the voltage drop occurring in the resistor is zero. 2. The field communication device according to claim 1, further comprising a comparator having a reference signal as a second input. 3. A low-pass filter of the resistor when a signal from the low-pass filter side of the resistor is used as a first input and a voltage drop occurs in the resistor due to a sum of consumption currents of all stations. 2. The field communication device according to claim 1, further comprising a comparator having a second input that is a reference signal corresponding to a signal on the filter side. 4. A field device and a control and monitoring device are connected on a bus cable having a power source connected to one end, and a process amount measured by the field device is transmitted to the control and monitoring device through the bus cable. A field communication device for transmitting a control signal for controlling a quantity from a control / monitoring device to a field device through a bus cable, comprising: a backup bus cable provided adjacent to the bus cable; the bus cable and the backup A switching unit that is provided between each end of the bus cable for use and the power supply and normally connects the power supply to the bus cable; and a resistor that is interposed between the power supply and the bus cable. When the voltage drop is compared with the reference voltage, and when an abnormality of the bus cable is detected, the switching unit is controlled to backup the backup voltage. And a bus switching control means for connecting a cable to a power source.