JPH04287535A - Field bus system - Google Patents

Abstract

PURPOSE:To improve the reliability of communication in the use of a parallel wire and to extend the length of a usable transmission line by reducing crosstalk between transmission lines. CONSTITUTION:Field devices 1a, 1b, 1c are operated with power supplied from an external power supply 4 via a transmission line 5 and the communication of a digital signal is implemented in bi-direction by a host device. Through the constitution above, a parallel wire is employed for the transmission line 5 and a terminator 7 to reduce the transmission signal is connected to the field devices 1a, 1b, 1c and the host device 3. Thus, the communication current flowing to the transmission line 5 is reduced to reduce crosstalk between the transmission lines. In addition, a method of reducing the impedance of terminators 6a, 6b, a method of switching the communication current of each device or a method employing a special wiring for the transmission line is adopted to reduce the crosstalk.

Description

[Detailed description of the invention]

0001

[Industrial Field of Application] The present invention relates to a fieldbus system that transmits signals on a two-wire transmission line, and in particular, uses parallel cables for the transmission line and is reliable over long distances. The present invention relates to a fieldbus system suitable for highly flexible communication.

0002

[Prior Art] Equipment called field equipment detects physical quantities such as pressure, temperature, and flow rate in various plants, converts the values into electrical signals, and transmits them to higher-level equipment via transmission lines. Conversely, it usually receives control signals transmitted from higher-level equipment to control plant valves and the like.

[0003] Transmission of the electric signal is standardized when the transmission signal is an analog signal, and an analog current signal of 4 to 20 mA is transmitted between the field device and the host device. There is. Furthermore, one-way communication using analog signals has generally been performed between field devices and host devices.

However, in recent years, with improvements in semiconductor integrated circuit technology, field devices with built-in microprocessors have been developed and put into practical use. According to this, in addition to unidirectional analog signal communication on the transmission path, bidirectional digital signal communication has become possible, making it possible to remotely command range settings, self-diagnosis, etc. of field devices. .

[0005] Recently, a fieldbus system has been proposed as a system that connects multiple field devices on the same transmission path and communicates using only bidirectional digital signals, and standardization work is underway to unify its specifications. is in progress.

A typical configuration example of a fieldbus system will be explained using FIG. 1. This figure shows an example of a device configuration in which a plurality of field devices and host devices are connected in a tree shape via transmission paths. Field equipment 1a, 1
b, 1c are operated by power supplied from an external power supply 4 via a transmission line 5, and communicate bidirectionally with the host device 3 via the transmission line 5 using digital signals in order to obtain detected physical quantities. Processes such as sending and receiving control values. The host communication device 2 is connected between the field devices 1a, 1b, 1c, the host device 3, and the external power source 4, and performs bidirectional communication with the field devices 1a, 1b, 1c, etc. using digital signals. Furthermore, the terminators 6a and 6b are composed of a resistor and a capacitor connected in series, and are connected to both ends of the transmission line 5.

[0007] By setting this terminator to a value that is considerably smaller than the input impedance in the communication frequency band of the field devices connected to the transmission path 5, it is possible to The effect on communication signals is kept to a minimum.

[0008] Furthermore, there are two types of transmission circuits (hereinafter referred to as drivers) that perform communication: voltage drivers and current drivers, but their transmission signals are defined by the transmission voltage value with a terminator connected. ing. Specifications regarding the transmission and reception of this signal were determined by standards for only twisted pair wires as transmission lines.

[0009]

[Problems to be Solved by the Invention] However, in the above-mentioned conventional technology, the range of transmitting and receiving signal levels of each field device is specified when twisted pair wires are used for the transmission line, so it is difficult to use the existing wiring. No consideration is given to the use of parallel lines. For this reason, when parallel lines are used, crosstalk between transmission lines is greater than when twisted pair lines are used for the transmission line, and this is the main factor that reduces communication reliability. It was necessary to limit the length of the road.

[0010] The present invention reduces the crosstalk between transmission lines by reducing the signal level at which each device connected to the field bus communicates while satisfying the communication standard. It is an object of the present invention to provide a fieldbus system and a field device that can improve the reliability of communication during use and, as a result, increase the length of usable parallel line transmission paths.

[0011]

[Means for Solving the Problems] In order to achieve the above object, the present invention reduces the signal level that each device connected to a field bus outputs to a transmission line. In addition, as another method, the parallel wire wiring method was designed to be less susceptible to crosstalk. As the means, there are the following means 1 to 4.

1. A terminator for absorbing communication signals is connected to the connection port of each device to the transmission line to reduce the transmitted signal output to the transmission line.

2. Reduce the impedance of the terminators connected to both ends of the transmission line below the specified value.

3. Each device has a function to switch the driver output, and the output transmission signal is reduced by an external command.

4. The pairs of parallel lines used are two adjacent lines, and at least one unused line is provided between adjacent transmission lines.

[0016] In this way, when parallel lines are used as transmission lines, crosstalk between transmission lines is reduced and communication reliability is improved.

[0017]

[Operation] In the field bus system proposed by the present invention, field devices are directly connected to arbitrary locations on the transmission path for communication. Therefore, the size of the signal transmitted by the field device connected to the fieldbus is
Even if it is constant, the signal magnitude at each location on the transmission path differs and is not uniform depending on conditions such as the driver circuit system, the length of the transmission path, and the connection form. Furthermore, when parallel lines are used as transmission lines, the influence of crosstalk between the transmission lines is so large that it does not pose a problem as a factor that reduces reliability, rather than the attenuation of communication signals in the transmission lines.

[0018] The value of this crosstalk is determined by the transmitted signal (
The communication current flowing through the transmission line can be reduced by connecting a terminator to the connection port of each device that lowers the impedance below the specified value in the communication frequency band. . Alternatively, crosstalk can be reduced by increasing the distance between transmission lines running in parallel. Therefore, each device connected to the transmission line can reduce crosstalk between the transmission lines while satisfying the specifications of the standard, so even if parallel lines are used, communication reliability can be maintained and the device can be used. The length of the transmission path also becomes longer.

[0019]

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

FIG. 1 shows a fieldbus system using the present invention.
This is an example of the device configuration of the system.

In the figure, field devices 1a and 1b
, 1c performs two-way communication using digital signals, and is used to detect physical quantities such as pressure, temperature, and flow rate in processes in various plants and transmit the values, or to receive control quantities such as valves. It is something to do. The field devices 1a, 1b, 1c are operated by power supplied from an external power source 4 via a transmission line 5, and can be connected to any location on the transmission line 5. In this embodiment, an example is shown in which field devices 1a, 1b, and 1c are connected to a junction box on the field side. do not have.

[0022] The host device 3 includes field devices 1a and 1b.
, 1c, communicates digital signals with fieldbus compatible devices such as the upper communication device 2 via the transmission line 5,
Receives various physical quantities (pressure, temperature, flow rate, etc.) detected by field devices, and also receives them as plant control information.
It sends control signals to field devices such as valves.

The upper communication device 2 can be connected to any location on the transmission line 5, and can be connected to the field device 1a by operating the display or keyboard in the upper communication device 2.
, 1b, 1c, monitor and adjust the output values, etc.
Communication is carried out via the transmission path 5.

The terminators 6a and 6b are composed of a resistor and a capacitor connected in series, and are 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 field devices etc. connected to the transmission line 5, communication signals can be The impact on the market is kept small. Note that the specifications of the terminators 6a and 6b are determined by standards.

Here, even if the magnitude of the signal transmitted by the field device connected to the field bus is constant, it varies depending on conditions such as the driver circuit system, the length of the transmission path, and the connection form. The magnitude of the signal at each location on the transmission path is different and not uniform. Therefore, in the standard, as an example, the transmission signal is 0.75 to 1.0V.
p-p, each device has a margin of 0.15 to 2
．． It is specified that a signal of 0Vp-p can be received. In addition, in this case, the noise level is set to 7, for example.
There is a condition to keep it below 5mVp-p.

These values are determined based on the case where twisted pair wires are used for the transmission line.

For this reason, when parallel lines are used, as in the example of transmission characteristics shown in FIG. I needed to. Furthermore, since the value of this crosstalk is proportional to the magnitude of the communication current flowing through the transmission path that affects it, the influence value can be reduced by reducing the communication current flowing through the transmission path.

In the embodiment shown in FIG. 1, each device is a current driver, and a terminator 7 is connected to the connection port of the transmission line. Therefore, since the communication current loops between the terminal 7 and the terminator 7, less current flows through the transmission path, and crosstalk can be reduced. Therefore, in this embodiment, crosstalk can be reduced by simply connecting terminators to each device individually while all devices satisfy the fieldbus standards.

As another embodiment, there is an example in which the terminators 6a and 6b connected to both ends of the transmission line are of non-standard impedance. In this example,
The transmission current looped between each device and the closer terminator 6a (or 6b) is increased, and the communication current flowing through the transmission path is decreased. Therefore, in this case, simply changing the terminators at both ends of the transmission line to those for parallel lines has the effect of reducing crosstalk.

As another embodiment, there is an example shown in FIG. 3 in which communication current values output by each device are switched through communication.

The operation of a field device will be explained in detail as a typical example using FIG. 3. In FIG. 3, a DC-DC converter 107 generates a voltage VDD for operating the field device 1 itself from a voltage applied from an external power supply via a transmission line, and a 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 a multiplexer 109. The input switching signal from the I/O interface 106 is input to the multiplexer 109, and the signal is sent to the A/D converter 1.
05. Furthermore, there is a microprocessor 101, and this microprocessor 101
performs correction calculations using the output sequentially sent from the A/D converter 105 and various coefficients stored in the ROM 103 and RAM 102, thereby obtaining the true value, and storing the value in the RAM 102. Ru.

[0032] When the field device 1 communicates,
Perform the following actions. In the transmission operation, first, under the command of the microprocessor 101, data stored in the RAM 102 is output as a serial digital signal string from the transmission/reception circuit 104. This signal becomes a signal modulated by the modulation 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 a modulation method, for example, "1" of a digital signal is used.
There is a frequency modulation method that modulates with two types of frequencies corresponding to "," and "0", and a method that converts the baseband signal to Manchester code.Also, as a driver method, there is a method that outputs a voltage signal and a method that outputs a voltage signal. Although there is a method of outputting as a current signal, this embodiment uses a method of outputting as a current signal, and the output current of the driver 111 is switched by an instruction from the I/O interface 106.

In the reception operation, the demodulation circuit 113 extracts the communication signal from the transmission path as the serial digital signal string, and sends the digital signal "1" and "0" to the transmission/reception circuit 104.
” is input as data consisting of ”.Transmission/reception circuit 104
The signal input to the microprocessor 101 is extracted as received data.

Therefore, in this embodiment, by reducing the communication current itself output from each device without changing the device configuration, the communication current flowing through the transmission path is reduced, and crosstalk to other transmission paths is reduced. This has the effect of reducing the impact of

Next, another embodiment regarding the method of using the transmission path will be explained using FIG. 4.

When the existing wiring of a conventional system is used as it is as a fieldbus transmission path, parallel lines are used in most cases. These parallel wires are mainly used in the range of 2 cores to about 50 cores, and the numbering is sequential from the center layer of the wire bundle as shown in Figure 4, so the wiring is Use the one next to it on the same layer. In this embodiment, for example, as shown in FIG.
By using 7 continuous wires and leaving the outer numbers 5 and 8 unused as backup lines to create a distance between the transmission lines, other transmission lines in the same layer (wires numbered 9 and 14) This makes it difficult to receive crosstalk from other sources. Furthermore, since the directions of currents flowing through a pair of transmission lines are opposite, and adjacent lines are used as a pair, crosstalk from lines in adjacent layers cancels each other out in total. In this case, there is no problem whether the unused line is grounded or left open.

Therefore, according to this embodiment, there is an effect that crosstalk can be reduced simply by using the method of using the transmission path.

[0038]

[Effects of the Invention] As is clear from the above explanation, according to the present invention, crosstalk between transmission lines can be reduced, so communication reliability can be improved when parallel lines are used, and furthermore, it can be used more easily. This has the effect of increasing the length of the transmission path.

[Brief explanation of the drawing]

FIG. 1 is a device configuration diagram showing one embodiment of a fieldbus system according to the present invention, FIG. 2 is a diagram showing an example of parallel line transmission characteristics, and FIG. 3 is a field device for explaining another embodiment according to the present invention. FIG. 4 is a block diagram for explaining how to use parallel lines.

[Explanation of symbols]

1, 1a, 1b, 1c...Field device, 2...Upper communication device, 3...Upper device, 4...External power supply, 5...Transmission line, 6a
, 6b, 7...Terminator.

Claims (5)

[Claims] Claim 1: In a system for communicating between field devices and host devices connected on a fieldbus transmission path, means for using electric wires consisting of a plurality of parallel wires as a transmission path, and each device outputting to the transmission path. 1. A fieldbus system comprising: means for reducing the magnitude of a transmitted signal to be smaller than a specified value. 2. In the fieldbus system according to claim 1, the drivers of each device are unified to current drivers, and a terminator is provided at the connection port of each device to the transmission path to reduce impedance in the communication frequency band of the transmission path. A fieldbus system that is characterized by reducing the size of the transmission signal output to the transmission line by connecting the fieldbus. 3. In the fieldbus system according to claim 1, the drivers of each device are unified to current drivers, and the impedance of the terminators connected to both ends of the transmission path is lowered from a specified value in the communication frequency band. A fieldbus system characterized by a reduced transmission signal output to the transmission line. 4. The fieldbus system according to claim 1, wherein each device has a function of setting the magnitude of the communication signal output by itself, and the transmission signal level is switched by an external command. fieldbus system. 5. A system for communicating between field devices and host devices connected on a field bus transmission path, including means for using electric wires consisting of a plurality of parallel wires as transmission paths, and means for connecting each transmission path to two adjacent transmission paths. 1. A fieldbus system comprising: a means for wiring with a single line and at least one unused line between each transmission line.