JPH0964796A - Communication system of field bus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the extension in the condition range in which a connection is possible under the condition of an intrinsic safe explosion protection by changing the amplitude of the signal waveform of a transmission signal and current consumption of a field equipment. SOLUTION: A decoding processing is performed for the communication signal from a transmission line in a reception circuit 113, the coded signal is converted, the signal is taken out as the form of a digital signal string and the signal string is inputted in a controller 40. The signal inputted in the controller 104 is taken out as received data by a microprocessor 101. At this stage, the conditions of a constant current circuit 110 and the internal circuit of a driver 111 are simultaneously switched to plural stages via an I/O interface 106 by the instruction of the microprocessor 101. Under the condition of an intrinsic safe explosion protection the number of field equipment 1 which can be connected is made twice by simultaneously reducing the current consumption of the field equipment 1 and the signal amplitude of a transmission current signal to half so as not to flow the output current of the field equipment 1 backward in the relation of the constant current circuit 110 and the driver 111.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fieldbus system for transmitting signals on a pair of transmission lines, and in particular, it enables low power consumption of field devices and connection under intrinsically safe explosion-proof conditions with a simple circuit configuration. The present invention relates to a fieldbus communication method capable of expanding the number of various devices and its device.

[0002]

2. Description of the Related Art A so-called field device detects a physical quantity such as pressure, temperature and flow rate of various plants, converts the value into an electric signal, and transmits the electric signal to a host instrument through a transmission line. On the contrary, it is normal to receive the control signal transmitted from the host instrument and control the valve of the plant.

The transmission of the electric signal is standardized when the signal is an analog signal, and an analog current signal of 4 to 20 mA is transmitted between the field device and the host instrument. . In addition, generally, one-way communication with an analog signal is performed between the field device and the host instrument.

However, in recent years, with the improvement of semiconductor integrated circuit technology, field devices incorporating a microprocessor have been developed and put into practical use. According to this, in addition to unidirectional analog signal communication on the transmission line, bidirectional digital signal communication is performed, and range setting and self-diagnosis of field devices can be remotely commanded. .

Further, recently, a field bus system has been proposed as a system in which a plurality of field devices are connected on the same transmission line by multi-drop and communication is performed only by bidirectional digital signals.

A typical configuration example of the fieldbus system will be described with reference to FIG. The figure shows a device configuration example in which a plurality of field devices and upper-level instruments are connected in a tree shape via a transmission path.

The field devices 1a, 1b, 1c are operated by the electric power supplied from the external power source 4 via the transmission line 5, and are bidirectionally transmitted in sequence with the host instrument 3 via the transmission line 5 by digital signals. Communicate and send detected physical quantity,
Performs processing such as receiving control values.

The host communication device 2 includes field devices 1a,
1b, 1c is connected between the upper instrument 3 and the external power source 4, and bidirectional communication is performed with the field devices 1a, 1b, 1c, etc. by digital signals. The terminator 7 includes a resistor and a capacitor connected in series, and is connected to both ends of the transmission path 5.

When shifting from the existing analog signal system to the fieldbus system, it is necessary to change the host instrument and the field device to those compatible with the fieldbus, but since the transmission line 5 can be used as it is, the system can be easily used. It is said that the system can be easily expanded because the number of field devices connected to the transmission path 5 can be increased.

[0010]

However, in the above-mentioned conventional technique, when the existing system is changed to the fieldbus system, the number of field devices connected to one transmission line becomes plural. In addition, in terms of performing digital communication, it is necessary to consider noise resistance in comparison with the existing system, and if communication failure frequently occurs, it has been performed in a constant cycle until now. It includes the problem that field devices cannot be controlled.

Particularly, in consideration of the characteristics of the transmission line that the transmission distance is long and the attenuation, rounding, and distortion of the communication signal largely change depending on the conditions and the noise environment that is installed in the field, the standard , The transmission signal of each device is as large as 0.75 to 1.0Vp-p, and the range of receivable reception signal is 0.15 to 2.0V.
It has a wide range of pp.

In the case of a field device, since the operating power is received via the transmission line, this transmission signal is 15 to 20 mA.
It becomes a current signal of pp.

Here, the transmission current signal has a uniform amplitude in the vertical direction to prevent current from flowing backward during communication.
The current consumption of the field device had to be set to a value of about 8 mA or more. Therefore, there is a limit to the number of field devices that can be connected under the intrinsically safe explosion-proof condition where energy is limited.

As a method for solving the above problems, Japanese Patent Laid-Open No.
No. 41709 proposes a method of reducing the current consumption during non-communication by removing the condition that the amplitude of the transmission current signal is uniform in the vertical direction. In this method, it is possible to reduce the current consumption, but there are the following problems.

1. Since the average current flowing through the transmission line during communication greatly fluctuates, the reliability of communication decreases.

2. In order to suppress the influence of the fluctuation, the characteristic of the filter of the receiving circuit that separates the fluctuation and the communication signal becomes complicated, and only the communication signal cannot be completely separated. 3. It is necessary to consider in the circuit section so that the output voltage of the fieldbus power supply does not oscillate due to the fluctuation of the average current.

Therefore, in order to make up for the drawbacks of the above method, Japanese Patent Application No. 6-118018 proposes a method of reducing the current consumption of the field device while keeping the average current flowing through the transmission line constant. Although this method solves all of the above-mentioned problems, the power consumption could not be reduced epoch-making because it is a method of changing the pulse width of the signal.

The present invention reduces the current consumption of the field device by changing the amplitude of the signal waveform of the transmission signal and the current consumption of the field device within the range where communication reliability is not a problem, and as a result, intrinsically safe explosion-proof. It is an object of the present invention to provide a communication method capable of expanding the range of conditions under which connection is possible under the above conditions, and an apparatus therefor.

[0019]

In order to achieve the above object, the present invention provides a communication system for a communication device such as a field device and a high-order instrument, in which the average communication current of each device varies depending on whether it is communicating or not. The number of devices that can be connected under intrinsically safe explosion-proof conditions by making the pulse amplitude in the vertical direction of the communication pulse smaller according to the current consumption of the field device in order to reduce the current consumption of the field device. Is an enlargement of.

[0020]

According to the proposed fieldbus communication method, the communication signal transmitted from the communication device such as the field device to the transmission path as described above is pulsed in the positive and negative directions with respect to the current consumption for operating the field device. In the superposed signal, the pulse amplitude in each of the positive and negative directions is set to be the same in each direction, and the pulse amplitude is set to a value smaller than the consumed current so that the current does not flow backward. According to the fieldbus standard, the maximum number of devices that can be connected to the bus is 32, and the maximum transmission distance is 1.
The communication specifications allow communication even at 9 km.

Therefore, under the intrinsically safe explosion-proof condition, when the current consumption of the field device is 8 mA, only about four units can be connected, the degree of attenuation of the signal amplitude of the communication signal is small, and there is a sufficient margin. Communication is possible even if the pulse amplitude is reduced.

For example, when the number of field devices to be connected is 10, even if the pulse amplitude is 1/3,
The pulse amplitude can be reduced to about 1/4 by setting the value within the receivable communication signal range according to the standard and shortening the transmission distance within a practically unproblematic range. Therefore, by setting the current consumption of the field device to a small value in accordance with the pulse amplitude of the communication signal in the range where the current does not flow backward, the current consumption for operating the field device while maintaining the reliability of communication. Can be lowered. Therefore, the number of connectable field devices can be increased within a range that satisfies the intrinsically safe explosion-proof condition where the energy of the entire bus is limited.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 2 shows a device configuration example of the fieldbus system using the present invention.

In the figure, field devices 1a, 1
b and 1c perform bidirectional communication by digital signals, and perform process pressure, temperature,
It detects a physical quantity such as a flow rate and transmits the value, or receives a control quantity of a valve or the like.

The field devices 1a, 1b, 1c are operated by the electric power supplied from the external power source 4 via the transmission line 5, and can be connected to any place of the transmission line 5.

In this embodiment, the field device 1
The following shows an example in which a, 1b, and 1c are connected to the junction box (relay box) on the field side.
For example, there is no problem even from the middle of the transmission line 5.

Under the condition of intrinsically safe explosion-proof, the barrier 8 is connected between the dangerous area and the safe area, and limits the current flowing from the external power source 4 to the field device side through the transmission line 5.

The upper instrument 3 is composed of the field devices 1a, 1
b, 1c, higher-level communication device 2 and other fieldbus-compatible devices through the transmission path 5 to communicate digital signals,
Receives various physical quantities (pressure, temperature, flow rate, etc.) detected by field devices, and as plant control information,
Control signals are sent to field devices such as valves.

The upper communication device 2 can be connected to any place on the transmission path 5, and by operating a display or a keyboard in the upper communication device 2, the field device 1 can be operated.
Processes such as monitoring and adjustment of output values of a, 1b, and 1c
The communication is performed via the transmission path 5 and executed. Under the intrinsically safe condition, the upper communication device 2 is connected to the safe area.

The terminator 7 is composed of a resistor and a capacitor connected in series, and is connected to both ends of the transmission line 5.
By setting this terminator to a value that is considerably smaller than the input impedance in the communication frequency band of the field device connected on the transmission path 5, it becomes possible to change the communication signal depending on the connection location of the field device and the number of connected devices. Suppresses the influence of.

Therefore, when the terminator is disengaged from the transmission line, the impedance of the transmission line as seen from the field device increases, and the transmission signal from each field device changes in the increasing direction. However, even if the magnitude of the signal transmitted by the field device connected to the field bus is constant, each signal on the transmission path depends on conditions such as the type of the driver circuit, the length of the transmission path, and the connection form. The magnitude of the signal at the location is different and not uniform.

The communication system of the present invention is based on the field device 1,
It can be commonly used for field bus compatible devices such as host communication device 2 and host instrument 3.

Next, with reference to FIG. 3, as a representative embodiment thereof, the operation when applied to a field device will be described in detail.

In FIG. 3, the DC-DC converter 10
7 generates a voltage VDD for operating the field device 1 itself from a voltage applied from an external power source via a transmission line, and the constant current circuit 110 controls the current consumption of the entire field device 1 to be constant.

Each output of the composite sensor 108 is input to the multiplexer 109.

An input switching signal from the I / O interface 106 is input to the multiplexer 109,
The signal is input to the A / D converter 105. Further, there is a microprocessor 101, and this microprocessor 101 performs a correction calculation by using the output sequentially sent from the A / D converter 105 and various coefficients stored in the ROM 103 and the RAM 102. A value is calculated and the value is stored in the RAM 102.

When the field device 1 communicates,
The following operation is performed.

In the transmission operation, first, in response to a command from the microprocessor 101, data stored in the RAM 102 or the like is output from the controller 104 as a serial digital signal sequence. This signal becomes a signal encoded by the transmission circuit 112, is input to the driver 111, and is output from the driver 111 to the transmission path as a communication signal. Here, as the coding method, for example, there are a method of converting the baseband signal into Manchester code, a method of modulating different frequency signals corresponding to "1" and "0" of the digital signal, and the like.

As a driver system, there are a system for outputting a voltage signal and a system for outputting a current signal.
FIG. 3 shows a method of outputting a current signal.

In the receiving operation, the communication signal from the transmission line is decoded by the receiving circuit 113, converted from the coded signal and taken out in the form of the serial digital signal string, and the digital signal is sent to the controller 104. It is input as data consisting of 1 "and" 0 ". The signal input to the controller 104 is extracted by the microprocessor 101 as received data. Here, according to an instruction from the microprocessor 101, the conditions of the internal circuit of the constant current circuit 110 and the driver 111 can be simultaneously switched to a plurality of stages via the I / O interface 106.

Under the condition of intrinsically safe explosion-proof, each device connected to the transmission line has a limit to the electric power received through the transmission line. Therefore, the current consumed by the entire field device 1 is suppressed to be the same. The number of field devices that can be connected to the transmission path can be increased. For example, the constant current circuit 110 and the driver 111 can be connected by simultaneously reducing the current consumption of the field device and the signal amplitude of the transmission current signal to 1/2 so that the output current of the field device 1 does not flow backward. The number of field devices can be doubled.

Further, the switching is performed by a command from the other device via the transmission line by the communication data, or a hardware-like DIP switch 114 is provided inside the field device 1 so that the microprocessor 101 is powered on. It is possible to read the setting state of the switch and carry out according to the setting state.

In addition, when the processing of the field device is performed at a low speed, when the operation of each circuit such as the MPU is temporarily stopped as in the sleep mode, the current consumption of the field device can be reduced. Therefore, the switching can be performed according to the current consumption of the field device.

Referring to FIG. 1, the case of using a Manchester code will be described as an example of the communication waveform of the communication system of the present invention.

As shown in FIG. 5, the Manchester code is 1
Since "1" and "0" are determined by changing the information bit in the first half and the second half of the bit time, the time between the information bit "1" and "0" is always the same except for the non-data signal. Becomes

FIG. 6 shows an example of the structure of communication data.

As a communication operation, a synchronization timing for each bit of continuously transmitted data is created by a preamble which is the first received data, and a Manchester code decoding process is performed on the subsequent data. To do. Next, the start delimiter is detected, the synchronization timing for each byte of communication data is created, and the data after that is taken out as valid data until the signal pattern of the end delimiter is detected.

In the present invention, as shown in FIG. 1, the amplitude of the communication signal to be superimposed on the average current value (current consumption of the field device 1) is positive and negative in the positive and negative directions, and the amplitude of the field device during communication is uniform. The value is set so that the output current does not flow backward. Here, for example, the positive and negative directional signal amplitudes during the 1-bit time period are the same, and the time widths of the first half and the second half of the 1-bit time are the same, so that the average current value is not disturbed during communication. Absent.

In the example of FIG. 1, since the transmission signal current is a signal in the range of 15 to 20 mAp-p according to the normal fieldbus standard, it is uniform in the vertical direction and the output current of the field device does not flow backward. Therefore, although the current consumption of the field device needs to be about 8 mA or more, the current consumption of the field device is reduced from the standard mode to the power down mode to 4 mA by using the embodiment.

In the example of FIG. 1, the switching command is
In this example, communication is performed from another device via the transmission path, and the mode is changed from the standard mode to the power down mode in the online state. In this example, an example of switching to two stages of the standard mode and the power-down mode is shown, but there is also an example of switching to multiple stages.

When the power down mode is set, as shown in FIG.
, The transmission signal current is 15 mAp-p to 7 mAp-p
However, under the intrinsically safe explosion-proof condition, only about 4 field devices can be connected in the standard mode, and up to about 8 field devices can be connected in the power-down mode example of Fig. 1. Compared with the maximum 32 units of the fieldbus standard, it is about 1/4 and has a sufficient margin, and communication reliability can be maintained. The contents will be described in detail with reference to FIG.

FIG. 4 shows the maximum value and the minimum value of the reception signal measured in the instrument room side where the host instrument is installed and the field side where the field device is installed in the state where the transmission current signal is 20 mAp-p. The number of loaded devices (3 to 50) and the transmission path length (200 m, 1800 m) are shown as parameters. Here, when the transmission current signal is halved of 20 mAp-p, the reception signal level is also halved, but if the maximum number of connected loads is 10, the minimum value of the reception signal is the fieldbus standard. It is 0.28 Vp-p with a sufficient margin as compared with the specified minimum receiving sensitivity of 0.15 Vp-p.

From this, if the maximum number of connected loads is 10, the transmission current signal can be communicated between the devices even if it is reduced to 6 mAp-p, and the length of the transmission path is 1800 m. Without any practical problem (eg 2
The transmission current signal can be reduced to about 4 mAp-p, if it is limited to the range of 100 m or less). Therefore, it is possible to reduce the current consumption of the field device accordingly, and it is possible to increase the number of devices that can be connected under the intrinsically safe explosion-proof condition where there is a power limitation for the entire transmission path. There is.

Since the standard mode and the power down mode can be switched and used, there is another effect that the optimum condition can be used according to the usage of the fieldbus system.

As described above, according to the present invention,
There is an effect that a highly reliable communication system can be easily realized with a simple circuit configuration.

[0057]

As is apparent from the above description, by configuring a fieldbus system such as a field device and a host instrument using the communication system according to the present invention, the intrinsically safe explosion-proof condition in which energy is limited is constituted. Also in the above, there is an effect that the number of connectable devices can be increased and a highly reliable communication system can be easily configured with an inexpensive configuration.

Further, there is another effect that the communicable range concerning external conditions such as the transmission path length and the number of connected devices can be expanded.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a communication waveform according to the present invention.

FIG. 2 is a device configuration diagram of a fieldbus system.

FIG. 3 is an internal block diagram showing an embodiment of a communication method field device according to the present invention.

FIG. 4 is a diagram showing an example of attenuation characteristics of a communication signal.

FIG. 5 is an explanatory diagram of a Manchester code.

FIG. 6 is a data configuration diagram of a communication signal.

[Explanation of symbols]

1, 1a, 1b, 1c ... Field device, 2 ... Host communication device, 3 ... Host instrument, 4 ... External power supply, 5 ... Transmission line, 7 ...
terminator.

Claims (4)

[Claims] Claim: What is claimed is: 1. At least one field device connected on a transmission path of a field bus composed of a pair of transmission lines communicates with a host instrument, and the field device is used as a communication signal between the respective devices. In the communication method of the fieldbus system, which uses a communication signal (hereinafter, a superposition signal) in which a signal changed in positive and negative directions in a pulse shape with respect to the current consumption for operating the Means for reducing the pulse amplitude in accordance with the current consumption of each of the field devices, and means for switching the current consumption and the amplitude of the superimposed signal in at least two stages, the average of each of the field devices and the host instrument A fieldbus communication method in which the communication current is the same during communication and during non-communication. 2. The communication system according to claim 1, wherein the field device determines the consumption current and the amplitude of the superimposed signal in response to a command from communication data from another field device connected to a transmission path and a host instrument. A fieldbus communication method, characterized in that it comprises means for switching between the current consumption and the amplitude of the superimposed signal at the same time. 3. The communication system according to claim 1, wherein the field device has a built-in hard switch, and comprises means for switching between the consumption current and the amplitude of the superimposed signal in accordance with a setting state of the switch. A field bus communication method, wherein the switching is performed when the power supply to the field device and the host instrument is started. 4. The communication system according to claim 1, wherein the field device has a function of switching superposed signal amplitude in at least two stages, and a state in which a current flowing from the device is always a unidirectional current is maintained. The fieldbus communication method is provided with a function of selecting the superimposed signal amplitude according to the current consumption of the device.