U.S. patent application number 10/265316 was filed with the patent office on 2003-02-13 for field bus system and virtual field apparatus.
Invention is credited to Kogure, Makoto.
Application Number | 20030030537 10/265316 |
Document ID | / |
Family ID | 18148286 |
Filed Date | 2003-02-13 |
United States Patent
Application |
20030030537 |
Kind Code |
A1 |
Kogure, Makoto |
February 13, 2003 |
Field bus system and virtual field apparatus
Abstract
In a system for performing signal transmission, through a field
bus, between field devices and higher-rank apparatuses connected on
a transmission line, the field devices are of the analog signal
type, and a substitution means is provided between the analog type
field devices and the field bus so that the substitution means acts
to perform communication on the field bus as a substitute for the
field devices. By the substitution means, the field devices can be
gradually changed to those adapted to the field bus as occasion
demands, so that an existing system can be transformed step by step
into a field bus system.
Inventors: |
Kogure, Makoto;
(Katsuta-shi, JP) |
Correspondence
Address: |
ANTONELLI TERRY STOUT AND KRAUS
SUITE 1800
1300 NORTH SEVENTEENTH STREET
ARLINGTON
VA
22209
|
Family ID: |
18148286 |
Appl. No.: |
10/265316 |
Filed: |
October 7, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10265316 |
Oct 7, 2002 |
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08932917 |
Sep 18, 1997 |
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08932917 |
Sep 18, 1997 |
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08369299 |
Jan 6, 1995 |
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5717385 |
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08369299 |
Jan 6, 1995 |
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07798941 |
Nov 27, 1991 |
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Current U.S.
Class: |
340/3.5 |
Current CPC
Class: |
Y02P 90/18 20151101;
Y02P 90/02 20151101; G05B 19/4185 20130101 |
Class at
Publication: |
340/3.5 |
International
Class: |
G05B 023/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 28, 1990 |
JP |
02-322849 |
Claims
What is claimed is:
1. A field bus system for connecting to a pair of transmission
lines a plurality of field devices which detect physical amounts
and/or output operation amounts at a field-side of said
transmission lines, a higher-rank apparatus at a higher-rank side
of said transmission lines, said higher-rank apparatus monitors
and/or controls said field devices, and an external power supply at
thee higher-rank side of said transmission lines, said external
power supply supplies electric power to said transmission lines,
said field bus system further conducting communication by using
digital signals between said field devices and said higher-rank
apparatus through said transmission lines, said field devices
include at least one analog signal type field device whose
input/output signals are analog signals and at least one digital
signal type field device whose input/output signals are digital
signals, said at least one digital signal type field device being
connected to said transmission lines, said field bus system
comprising: a processing device at the field-side of said
transmission lines, said processing device performs processing
including signal conversion between communication signals including
an analog signal input and/or output to and/or from said at least
one analog signal type field device and communication signals
including a digital signal used on said transmission lines, wherein
said processing device converts the electric power on said
transmission lines to another electric power which is applied to
said at least one analog signal type field device to drive said at
least one analog signal type field device, wherein said processing
device is connected to said transmission lines, wherein said at
least one analog signal type field device is connected to said
processing device, and wherein said digital signal type field
device communicates with said higher-rank apparatus without using
said processing device.
2. A virtual field apparatus for connecting to a pair of
transmission lines of a field bus system at least one analog signal
type field device, said field bus system conducts communication by
digital signals between field devices at a field-side of said
transmission lines, said field devices including said at least one
analog signal type field device which detects physical amounts
and/or output operation amounts in at least a plant and whose
input/output signals includes analog signals, and at least one
digital signal type field device which detects physical amounts
and/or output operation amounts in at least the plant and whose
input/output signals include digital signals, said field bus system
further connecting to said transmission lines a higher-rank
apparatus at a higher-rank side of said transmission lines, said
higher-rank apparatus monitors and/or controls said field devices
via said transmission lines, said at least one digital signal type
field device being connected to said transmission lines, and an
external power supply at the higher-rank side of said transmission
lines, said external power supply applies electric power to said
transmission lines, said virtual field apparatus comprising: a
digital signal connection terminal connected to said transmission
lines at the field-side of said transmission lines; a plurality of
analog signal connection terminals, wherein at least one analog
signal connection terminal is connected to said at least one analog
signal type field device; a signal converter for performing signal
conversion between a communication signal including an analog
signal which is inputted or outputted at said at least one analog
signal connection terminal and a communication signal including a
digital signal used on said transmission lines; and an electric
power converter, connected to said digital signal connection
terminal, for converting the electric power on said transmission
lines to another electric power and applying said another electric
power to said at least one analog signal type field device to drive
said at least one analog signal type field device via said at least
one analog signal connection terminal, wherein said digital signal
type field device communicates with said higher-rank apparatus
without using said virtual field apparatus .
3. A field bus system for connecting to a pair of transmission
lines a plurality of field devices which detect physical amounts
and/or output operation amounts at a field-side of said
transmission lines, a higher-rank apparatus at a higher-rank side
of said transmission lines, said higher-rank apparatus monitors
and/or controls said field devices, and an external power supply at
the higher-rank side of said transmission lines, said external
power supply supplies electric power to which drives said field
devices at a high-rank side of said transmission lines, said field
bus system further conducting communication by using digital
signals between said field devices and said higher-rank apparatus
through said transmission lines, said field devices include a
plurality of analog signal type field devices whose input/output
signals are analog signals and at least one digital signal type
field device whose input/output signals are digital signals, said
at least one digital signal type field device being connected to
said transmission lines, said field bus system comprising: a
processing device at the field-side of said transmission lines,
said processing device performs signal conversion between
communication signals including an analog signal input and/or
output to and/or from an analog signal type field device and
communication signals including a digital signal used on said
transmission lines, wherein said processing device converts the
electric power on said transmission lines to another electric power
which is applied to said analog signal type field devices to drive
said analog signal type field devices, wherein said processing
device is connected to said transmission lines, wherein each of
said analog signal type field devices is connected to said
processing device, and wherein said digital signal type field
device communicates with said higher-rank apparatus without using
said processing device.
4. A virtual field apparatus for connecting to a pair of
transmission lines of a field bus system a plurality of analog
signal type field devices, said field bus system conducts
communication by digital signals between said field devices at a
field-side of said transmission lines, including said analog signal
type field devices which detects physical amounts and/or output
operation amounts in at least a plant and whose input/output
signals includes analog signals, and at least one digital signal
type field device which detects physical amounts and/or output
operation amounts in at least the plant and whose input/output
signals include digital signals, said field bus system further
connecting to said transmission lines a higher-rank apparatus at a
higher-rank of said transmission lines, said higher-rank apparatus
monitors and/or controls said field devices via said transmission
lines, said at least one digital signal type field device being
connected to said transmission lines, and an external power supply
at the higher-rank side of said transmission lines, said external
power supply applies electric power to said transmission lines,
said virtual field apparatus comprising: a digital signal
connection terminal connected to said transmission lines at the
field-side of said transmission lines; a plurality of analog signal
connection terminals, wherein at least one analog signal connection
terminal is connected to each of said analog signal type field
devices; a signal converter for performing signal conversion
between a communication signal including an analog signal which is
inputted or outputted at said at least analog signal connection
terminal and a communication signal including a digital signal used
on said transmission lines; and an electric power converter,
connected to said digital signal connection terminal, for
converting the electric power on said transmission lines to another
electric power and applying said another electric power to said at
least one analog signal type field device to drive said at least
one analog signal type field device via said at least one analog
signal connection terminal, wherein said digital signal type field
device communicates with said higher-rank apparatus without using
said virtual field apparatus.
5. A field bus system for connecting to a pair of transmission
lines a plurality of field devices which detect physical amounts
and/or output operation amounts at a field-side of said
transmission lines, a higher-rank apparatus at a higher-rank side
of said transmission lines, said higher-rank apparatus monitors
and/or controls said field devices, and an external power supply at
the higher-rank side of said transmission lines, said external
power supply supplies electric power to said transmission lines,
said field bus system further conducting communication by using
digital signals between said field devices and said higher-rank
apparatus through said transmission lines, said field devices
include at least one analog signal type field device whose
input/output signals are analog signals and at least one digital
signal type field device whose input/output signals are digital
signals, said at least one digital signal type field device being
connected to said transmission lines, said field bus system
comprising: a processing device at the field-side of said
transmission lines, said processing device performs processing
including signal conversion between communication signals including
an analog signal input and/or output to and/or from said at least
one analog signal type field device and communication signals
including a digital signal used on said transmission lines, wherein
said processing device includes a power supply which converts the
electric power on said transmission lines to another electric power
and applies the another electric power to said at least one analog
signal type field device to drive said at least one analog signal
type field device, wherein said processing device is connected to
said transmission lines, wherein said at least one analog signal
type field device is connected to said processing device, and
wherein said digital signal type field device communicates with
said higher-rank apparatus without using said processing
device.
6. A virtual field apparatus for connecting to a pair of
transmission lines of a field bus system at least one analog signal
type field device at a field-side of said field bus system, said
field bus system conducts communication by digital signals between
field devices at the field-side of said transmission lines,
including said at least one analog signal type field device which
detects various data regarding at least a plant and whose
input/output signals includes analog signals, and at least one
digital signal type field device which detects various data
regarding said at least a plant and whose input/output signals
include digital signals, said field bus system further connecting
to said transmission lines a higher-rank apparatus at a higher-rank
side of said transmission lines, said higher-rank apparatus
monitors and/or controls said field devices via said transmission
lines, said at least one digital signal type field device being
connected to said transmission lines, and an external power supply
at the higher-rank side of said transmission lines, said external
power supply applies electric power to said transmission lines,
said virtual field apparatus comprising: a digital signal
connection terminal connected to said transmission lines at the
field-side of said transmission lines; a plurality of analog signal
connection terminals, wherein at least one analog signal connection
terminal is connected to said at least one analog signal type field
device; a signal converter for performing signal conversion between
a communication signal including an analog signal which is inputted
or outputted at said at least one analog signal connection terminal
and a communication signal including a digital signal used on said
transmission lines; and an electric power converter, connected to
said digital signal connection terminal, for converting the
electric power on said transmission lines to another electric power
and applying said another electric power to said at least one
analog signal type field device to drive said at least one analog
signal type field device, wherein said digital signal type field
device communicates with said higher-rank apparatus without using
said vertical field apparatus.
7. A virtual field apparatus according to claim 6, wherein said
various data regarding said at least a plant detected by said at
least one analog signal type field device includes physical amounts
and/or output operation output amounts of said at least a plant,
and wherein said various data regarding said at least a plant
detected by at least one digital signal type field device includes
physical amounts and/or output operation amounts of said at least a
plant.
8. A field bus system for connecting to a pair of transmission
lines a plurality of field devices which detect various data at a
field-side of said transmission lines, a higher-rank apparatus at a
higher-rank side of said transmission lines, said higher-rank
apparatus monitors and/or controls said field devices, and an
external power supply at the higher-rank side of said transmission
lines, said external power supply supplies electric power to said
transmission lines, said field bus system further conducting
communication by using digital signals between said field devices
and said higher-rank apparatus through said transmission lines,
said field devices include at least one analog signal type field
device whose input/output signals are analog signals and at least
one digital signal type field device whose input/output signals are
digital signals, said at least one digital signal type field device
being connected to said transmission lines, said field bus system
comprising: a processing device at the field-side of said
transmission lines, said processing device which performs signal
conversion between communication signals including an analog signal
input and/or output to and/or from said at least one analog signal
type field device and communication signals including a digital
signal used on said transmission lines, wherein said processing
device includes a power supply which converts the electric power on
said transmission lines to another electric power and applies the
another electric power to said at least one analog signal type
field device to drive said at least one analog signal type field
device, wherein said processing device is connected to said
transmission lines, wherein said at least one analog signal type
field device is connected to said transmission lines through said
processing device, and wherein said digital signal type field
device communicates with said higher-rank apparatus without using
said processing device.
9. A virtual field apparatus according to claim 8, wherein said
various data regarding said at least plant detected by said at
least one analog signal type field device includes physical amounts
and/or output operation output amounts of said at least a plant,
and wherein said various data regarding said at least a plant
detected by at least one digital signal type field device includes
physical amounts and/or output operation amounts of said at least a
plant.
Description
[0001] The present application is a continuation of application
Ser. No. 08/932,917, filed Sep. 18, 1997; which is a continuation
of application Ser. No. 08/369,299, filed Jan. 6, 1995, now U.S.
Pat. No. 5,717,385; which is a continuation of application Ser. No.
07/798,941, filed Nov. 27, 1991, now abandoned, the contents of
which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to field apparatuses for
performing signal transmission on a two-wire transmission line,
and, particularly, it relates to a field bus system in which system
transformation and system extension into such a field bus can be
made easily.
[0003] So-called field apparatuses are generally used so that the
field devices measure physical quantities, such as a pressure, a
temperature, a flow rate, and the like, in various kinds of plants,
convert the measured values of such physical quantities into
electric signals, and transmit the electric signals to higher-rank
apparatuses. Further, the field devices reversely used so that the
field devices receive control signals transmitted from the
higher-rank apparatuses to thereby control valves or the like in
the plants.
[0004] In the case where the electric signals are analog signals,
the transmission of those electric signals is standardized so that
analog current signals of 4 to 20 mA are transmitted between the
field devices and the higher-rank apparatuses. In general,
communication between the field devices and the higher-rank
apparatuses is one-way communication with analog signals.
[0005] In recent years, however, field devices with built-in
microprocessors have been developed and put into practice with the
advance of the semiconductor integration technique (IC technique).
According to the developed field devices, not only one-way
communication with analog signals can be performed on the
transmission line but also two-way communication with digital
signals can be performed on the transmission line, so that range
setting, self diagnosis, etc., of the field devices can be remotely
instructed. Such apparatuses of the type as mentioned above are
disclosed in Japanese Patent Unexamined Publication Nos.
Sho-58-48198 and Sho-59-201535.
[0006] A typical example of this type devices will be described
hereunder with reference to FIG. 4. FIG. 4 shows an example of the
configuration of analog current output type field devices which
require an external power supply. Field devices Ia, Ib and Ic are
operated by electric power supplied from an external power supply 4
and respectively output analog current signals so that the field
devices Ia, Ib and Ic serve as constant current sources for sending
currents corresponding to measured physical quantities onto the
transmission line. A higher-rank receiver apparatus 3 detects a
potential difference across each of resistors inserted in series in
the transmission line to thereby receive the analog current signals
(hereinafter simply called "analog signals") flowing in the
resistors. The analog signals thus received are used as instruction
values of the respective field device 1a, 1b and 1c. A higher-rank
communication apparatus 2 is connected between the set of the field
device 1a, 1b and 1c and the use of the higher-rank apparatus 3 and
the external power supply 4 so as to perform two-way communication
between the field devices 1a, 1b and 1c and the higher-rank
apparatus 3 with digital signals.
[0007] As a system for signal transmission on such a transmission
line, known are a signal transmission system in which a digital
signal is superposed on an analog signal for the purpose of
performing communication with the digital signal without influence
on the analog signal, a signal transmission system in which an
analog signal and a digital signal are switched over each other, a
signal transmission system in which only a digital signal is
used.
[0008] Recently, a field bus system has been proposed as a system
in which a plurality of field devices are connected on one
transmission line to perform two-way communication only with
digital signals. A typical example of the field bus system will be
described hereunder with reference to FIG. 5. FIG. 5 shows an
example of the configuration of a system in which a plurality of
field devices are connected, in the form of a tree, to higher-rank
apparatuses through a transmission line. Field device 1a', 1b' and
1c' are operated by electric power supplied from an external power
supply 4', so that two-way communication between the field devices
and the higher-rank apparatuses are successively performed with
digital signals on a transmission line 5' for the double purpose of
transmitting measured physical quantities and receiving control
quantities. A higher-rank communication apparatus 2' is connected
between a set of the field device 1a', 1b' and 1c' and a set of the
higher-rank apparatus 3' and the external power supply 4' and
performs two-way communication between the field device 1a', 1b'
and 1c' and the higher-rank apparatus 3' with digital signals.
Terminators 7, each of which is composed of a resistor and a
capacitor connected in series, are arranged across the ends of the
transmission line 5'.
[0009] In the case where the system of FIG. 4 is transformed into
the system of FIG. 5, it is necessary to replace the higher-rank
apparatuses and the field devices by new ones adapted to the field
bus. It is however said that system transformation can be made
easily because the transmission line 5 can be used with no change,
and also system expansion can be made easily because the number of
field devices connected on the transmission line can be
increased.
[0010] In the aforementioned conventional techniques, all that
except the transmission line must be once changed when an existing
system is transformed into a field bus system.
[0011] In the case where such a field bus system is to be used,
therefore, a problem arises in that equipment at the beginning is
extremely expensive. In particular, it is impossible to gradually
transform an existing system into a field bus system while field
devices connected in the existing system are used as they are
temporarily, and therefore the high cost of equipment at the
beginning cannot be avoided. As a result, from the point of view of
cost performance, it is difficult to apply the conventional
techniques to such an existing system.
SUMMARY OF THE INVENTION
[0012] A first object of the present invention is therefore to
provide a field bus system in which it is possible to transform a
system of field devices not adapted to a field bus gradually into
the field bus system.
[0013] A second object of the present invention is to provide a
virtual field apparatus by which a field bus system can be used
while an existing system not adapted to a field bus is kept to be
used.
[0014] To attain the foregoing objects, according to the present
invention, a substitution means is provided between a field bus and
a plurality of field devices not adapted to the field bus so that
the substitution means acts as a substitute for the field devices
to perform communication on the field bus. By the substitution
means, the field devices can be gradually changed to those adapted
to the field bus as occasion demands, so that transformation from
an existing system into a field bus system can be attained step by
step. Further, the present invention, newly provides a virtual
field apparatus. The virtual field apparatus is provided with a
connection terminal for the field bus and a plurality of analog
signal connection terminals for the existing field devices, and has
a function so that the virtual field apparatus acts to perform
signal processing, between the connection terminal and the analog
signal connection terminals, in place of the existing field devices
on the field bus. Accordingly, system transformation so as to be
adapted to the field bus can be made easily by connecting the
virtual field apparatus to a known system.
[0015] The field bus system according to the present invention is
directly connected to the transmission line, but the existing field
devices not adapted to the field bus is indirectly connected to the
transmission line through the newly provided field device acting as
a substitute for the existing field devices. As a result,
information including electric power for operating the existing
field device not adapted to the field bus is supplied from the
transmission line to the existing field device through the newly
provided field device according to the present invention. When, for
example, the existing field devices are of the analog signal
current output type, analog current signals of 4 to 20 mA as the
output signals thereof are converted into digital signals in the
newly provided field device according to the present invention and
transmitted virtually. When, on the contrary, the existing field
devices are of the analog current input type, analog current
signals of 4 to 20 mA as the input signals thereof are received in
the newly provided field device according to the present invention
virtually so that digital signals are converted into analog signals
to be outputted from the newly provided field device.
[0016] By using the newly provided field apparatus, the existing
system can be transformed into a field bus system while the
existing analog signal type field devices are kept used. In short,
a field bus system can be attained simply by changing the
higher-rank apparatus and the external power supply into new ones
adapted to the field bus.
[0017] Further, the existing field devices can be exchanged to new
ones adapted to the field bus simply by changing the connection
thereof into direct connection from the transmission line.
Accordingly, system transformation from the existing system into
the field bus system can be made easily step by step as occasion
demands.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a view showing the configuration of an embodiment
of the field bus system according to the present invention;
[0019] FIG. 2 is a block diagram showing an embodiment of the
virtual field apparatus according to the invention;
[0020] FIG. 3 is a block diagram showing another embodiment of the
virtual field apparatus according to the invention;
[0021] FIG. 4 is view showing an example of the system
configuration of a conventional field device; and
[0022] FIG. 5 is a view showing an example of a field bus system
which has been proposed recently.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] An embodiment of the present invention will be described
hereunder with reference to the drawings.
[0024] FIG. 1 is a view showing the configuration of a field bus
system according to the present invention.
[0025] In FIG. 1, in field devices 1a, 1b and 1c, the output
signals (or input signals) thereof are analog current signals of 4
to 20 mA or analog current signals of 4 to 20 mA on which digital
signals are superimposed so that two-way communication can be
performed, whereby the field devices 1a, 1b and 1c serve to
transmit detection values obtained by measuring physical
quantities, such as a pressure, a temperature, a flow rate, etc.,
in a process in each of various kinds of plants or serve to receive
control quantities for controlling valves or the in the plant. A
virtual field apparatus 6 is operated with electric power supplied
from an external power supply 4 through a transmission line 5 and
is disposed between the transmission line 5 and the field devices
1a, 1b and 1c. Although this embodiment shows the case where the
virtual field apparatus 6 is disposed in a field-side junction box,
the invention can be applied to the case where the virtual field
apparatus is disposed in a higher-rank apparatus side.
[0026] The virtual field apparatus 6 supplies electric power to the
field devices 1a, 1b and 1c through the transmission lines 5a, 5b
and 5c respectively, and the virtual field apparatus receives
analog output signals from the field devices 1a, 1b and 1c also
through the transmission lines 5a, 5b and 5c respectively. The
virtual field apparatus 6 converts the analog output signals into
digital signals and virtually uses the digital signals as output
signals from the respective field devices to perform communication
on the field bus system. When, for example, the field devices 1a,
1b and 1c are of the type of two-way communication by use of the
analog current signals and the digital signals superimposed one on
the other, the virtual field apparatus 6 has a function of
communication protocol conversion between the field bus and the
field devices 1a, 1b and 1c so that communication between the
virtual field apparatus 6 and each of the field devices 1a, 1b and
1c can be performed not only by use of the analog signals but by
use of the digital signals, through the transmission lines 5a, 5b
and 5c, respectively. When, for example, the field devices 1a, 1b
and 1c are of the type of performing communication only with the
analog current signals, the virtual field apparatus 6 acts as a
substitute for the field devices 1a, 1b and 1c virtually with
respect to the field bus. When, for example, system
alteration/expansion is made by modifying the field device into one
adapted to the field bus or newly providing a field device adapted
to the field bus, a field device Id adapted to the field bus is
connected on the field bus transmission line 5 so that electric
power for operating the field device Id can be directly supplied
from the external power supply 4 to the field device Id through the
transmission line 5, and so that the field device id can perform
communication with apparatuses (a higher-rank apparatus 3, a
higher-rank communication apparatus 2, etc.) connected on the field
bus. Here, the electric power consumed by the virtual field
apparatus 6 includes electric power consumed by the internal
circuits of the virtual field apparatus 6 and electric power
consumed by the field devices 1a, 1b and 1c. Accordingly, the value
of electric power supplied from the external electric source 4
through the transmission line 5 is little influenced by addition of
the field device 1d adapted to the field bus, if the electric power
consumed by the virtual field apparatus 6 can be controlled in the
case where, for example, the field device 1c or the like is
disconnected from the system.
[0027] The higher-rank apparatus 3 performs communication with the
apparatuses (the virtual field apparatus 6, the field device 1d,
the higher-rank communication apparatus 2, etc.) adapted to the
field bus by use of digital signals through the transmission line
5, so that the higher-rank apparatus 3 receives various kinds of
physical quantities (such as a pressure, a temperature, a flow
rate, etc.) measured by the field devices or transmits control
signals to the field devices such as valves or the like, as plant
control information.
[0028] The higher-rank communication apparatus 2 can be connected
in a desired position on the transmission line. The higher-rank
communication apparatus 2 performs communication through the
transmission line 5 while operating a display or a keyboard to
thereby execute monitoring, adjusting and the like, of the output
values from the virtual field apparatus 6 and the field device
1d.
[0029] The operation of the field apparatus 6 will be described
hereunder more in detail with reference to FIGS. 2 and 3. FIG. 2'
shows an example in which existing field devices 8a to 8n to be
connected are of the type of analog signal outputs and of the type
of two-way communication by using digital signals. FIG. 3 shows an
example in which existing field devices 9a to 9n to be connected
are of the type of analog signal inputs and of the type of two-way
communication by using digital signals.
[0030] Although this embodiment shows the case where existing field
devices to be connected are of the analog signal type and of the
type of two-way communication by using digital signals, the
operation of the virtual field apparatus 6 is considered to be the
same in the viewpoint from the field bus side because the field
apparatus 6 merely acts as a substitute for the existing field
devices in the case where the existing field devices are of the
analog current signal type.
[0031] In FIG. 2, a DC/DC converter 609 is supplied with a voltage
from an external power supply through a transmission line so as to
produces a voltage V.sub.DD for operating the field apparatus 6 and
voltages Ea, Eb, . . . , En for operating analog signal receivers
612a, 612b, . . . , 612n so that the analog signal receivers 612a,
612b, . . . , 612n supply electric power to the field devices 8a to
8n which are not adapted to the field bus. Here, the DC/DC
converter 609 controls the electric power supplied through the
field bus 5 to keep the value thereof constant, so that the value
of the electric power changes according to the number of the analog
signal type field devices 8a to 8n to be connected.
[0032] The analog signal receivers 612a, 612b, . . . , are composed
of power supply for supplying electric power to the respective
field devices 8a to 8n not adapted to the field bus, and receivers
for detecting analog current signals of 4 to 20 mA as the output
signals from the respective field devices 8a to 8n. The output
signals from the analog signal receivers are inputted, as analog
signals of 1 to 5 V.sub.1 into a multiplexer 610. The multiplexer
610 successively supplies those signals into the A/D converter 604
while successively switching the signals on the basis of the
instruction supplied thereto from an I/O interface 605. The A/D
converter 604 converts the signals into digital signals and an RAM
602 stores those digital signals. A microprocessor 601 controls the
series of procedures described above on the basis of data
programmed in an ROM 603.
[0033] Modems 613a, 613b, . . . , 613n are interface circuits for
performing two-way communication by use digital signals in a manner
that the digital signals are superimposed on the analog signals of
the existing field devices connected or that the analog signals are
switched over into the digital signals. The digital signals are
outputted as signals which are demodulated by the modems 613a,
613b, . . . , 613n selected by a multiplexer 611 on the basis of
the instruction from the I/O interface 605 in order to send the
output from a transmitter/receiver circuit 607 to the corresponding
demodulators 613a, 613b, . . . , 613n. As the output signals from
the modems 613a, 613b, . . . , 613n, for example, used are two
kinds of frequency signals corresponding to digital signals "1" and
"0" like frequency modulation, two kinds of phase signals
corresponding to the two phases "1" and "0" like phase modulation,
or the like. On the contrary, digital signals outputted from the
existing field devices 8a to 8n are demodulated by the respective
modems 613a, 613b, . . . , 613n and received through the
transmitter/receiver circuit 607.
[0034] If modulated signals used in communication between the
existing analog signal type field devices 8a to 8n and the field
apparatus 6 are square wave signals having equal amplitude with
respect to the positive and negative directions or sinusoidal wave
signals having small amplitude, the influence on the analog signal
receivers 612a, 612b, . . . , 612n can be avoided by performing
communication while superposing the output digital signals on the
analog signals.
[0035] The communication between the existing analog signal type
field devices 8a to 8n and the field apparatus 6 is performed only
when the necessity of performing communication with the analog
signal type field devices 8a to 8n occurs, on the basis of
communication with the field bus 5. The communication between the
existing analog signal type field devices 8a to 8n and the field
apparatus 6 is not used usually. When the communication is
performed, the field apparatus 6 fulfills a role of protocol
conversion between the field bus 5 and the existing analog signal
type field devices 8a to 8n.
[0036] Also in the case of communication with the field bus 5, a
modem 608 serves as an interface circuit for performing two-way
communication with digital signals. The modem 608 sends a
transmission signal as the output from the transmitter/receiver
circuit 606 onto the field bus after coding treatment into
Manchester code or the like. The reception signal from the field
bus is decoded by the modem 608 and then the decoded signal is
inputted into the transmitter/receiver circuit 606.
[0037] In the aforementioned configuration, the virtual field
apparatus 6 has no decided address on the field bus, but addresses
of the analog signal type field devices 8a to 8n connected are
stored in the RAM 602. When the address coincides with the address
in communication data from the field bus 5, the virtual field
apparatus 6 performs processing of the analog signal type field
devices 8a to 8n virtually. Because not only the output values from
the analog signal type field devices 8a to 8n are periodically
stored in the RAM 602 after A/D conversion by means of the A/D
converter 604 but also the set values of the analog signal type
field devices 8a to 8n are preliminarily stored in the RAM 602 by
communication with the field bus 5, the field apparatus 6 performs
substitution without communication with the analog signal type
field devices 8a to 8n when transmission of the output values from
the field bus 5 is requested. In the case where communication with
the analog signal type field devices 8a to 8n is needed by the
request from the field bus 5, the virtual field apparatus 6
fulfills a role of protocol conversion between the field bus 5 and
the existing analog signal type field devices 8a to 8n as described
above.
[0038] An embodiment in which field devices 9a to 9n not adapted to
the field bus and connected to the virtual field apparatus 6 are of
the analog signal input type will be described hereunder with
reference to FIG. 3. In the same manner as in the embodiment of
FIG. 2, the DC/DC converter 609 is supplied with a voltage from an
external electric source through a transmission line so as to
produces a voltage V.sub.DD for operating the field apparatus 6 and
voltages Ea', Eb', . . . , En' for operating analog signal
receivers 615a, 615b, . . . , 615n so that the analog signal
receivers 615a, 615b, . . . , 615n supply electric power to the
field devices 9a to 9n which are not adapted to the field bus. The
analog signal transmitters 615a, 615b, . . . , 615n are constituted
by electric source for supplying electric power to the field
devices 9a to 9n not adapted to the field bus, and transmitters for
sending analog circuit signals of 4 to 20 mA as the input signals
to the field devices 9a to 9n. The input signals are inputted, as
analog signals of 1 to 5 V, from the D/A converter 604 through the
multiplexer 610. The switching signal of the multiplexer 610 is
inputted from the I/O interface 605 on the basis of the instruction
from the microprocessor 601. The analog signal transmitters 615a,
615b, . . . , 615n include sample-hold circuits and damping
circuits respectively, so that the input value from the D/A
converter 614 is updated when updating is requested from the field
bus.
[0039] Communication with the analog signal input type field
devices 9a to 9n and communication with the field bus 5 in this
embodiment are quite the same as those described in the embodiment
of FIG. 2.
[0040] Although FIGS. 2 and 3 respectively show the case where the
existing field devices connected are of the analog signal output
type and the case where the field devices are of the analog signal
input type, the present invention can be applied to the case where
the two examples of configuration are mixed with each other.
[0041] According to this embodiment, an effect arises in that
existing analog signal type field devices and field devices adapted
to a field bus can coexist on one control system. Another effect
arises in that a field bus system can be attained simply by
changing in minimum requirements.
[0042] As is obvious from the above description, an effect in that
system transformation into a field bus system can be made while
field devices used in an existing system and not adapted to a field
bus are used continuously is attained by providing means for acting
a substitute for the existing field devices on the field bus.
[0043] Further, not only the devices not adapted to the field bus
can be exchanged to new ones adapted to the field bus easily by
changing the connection thereof, but also the exchange to devices
adapted to the field bus can be made without changing the settings
of the other devices on the field bus. There arises an effect in
that the control system can be transformed gradually.
* * * * *