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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a programmable controller (or programmable
ladder controller, hereinunder referred to as "PC") for automatically
controlling various equipments in accordance with a sequence program.
2. Description of the Related Art
PC's are used for controlling various pieces of equipment such as a
transfer machine and a PC having more functions and easy to use has
recently been demanded for realizing factory automation.
As shown in FIG. 1, a conventional PC 10 includes a memory 12 for storing a
sequence program and a control processor 14 for controlling a piece of
equipment in accordance with the sequence program. The PC 10 further
includes I/O portion (not shown) connected to an external piece of
equipment, data memory (not shown), system memory (not shown), etc. FIG. 2
shows what is called a relay ladder for showing the order and the
conditions of control with respect to the equipment being controlled.
Equipment being controlled includes a virtual equipment (dummy equipment)
which exists only in a program.
A relay symbol 16 in the relay ladder diagram represents a piece of
equipment being controlled. An address 18 shown in the vicinity of the
relay symbol 16 is a local I/O address which is only effective in the PC
10. The I/O address is expressed by an octal digit, a hexadecimal digit or
the like and individually allotted to each piece of equipment.
The order and the conditions of control shown in the relay ladder diagram
are programmed by a programming language such as a logic symbol language
and a relay symbol language and stored in the memory 12 as a sequence
program. To state this more concretely, in the sequence program, a command
and an I/O address to each piece of equipment are entered, as shown in
FIG. 1. The sequence program memory 12 therefore includes a region 12a for
storing a command and a region 12b for storing an address.
Such a conventional PC, however, is disadvantageous in that it is very
difficult to check or correct a program.
When a program is checked or corrected, a programming tool is connected to
the PC. In this case, the I/O address in the program which is displayed on
the programming tool is expressed merely by a numeral (numeral such as an
octal or hexadecimal digit). It is therefore difficult for the operator to
judge which equipment the I/O address indicates.
Accordingly, at the time of creating, checking or correcting a program, the
operator must constantly confirm the I/O address with reference to the
relay ladder diagram, which is the basis of a program. According to this
programming method, the operability is bad and an inputting error is
easily caused.
Especially, in a system in which a plurality of PC's are dispersively
arranged, the above-described problems are more prominent. Since the same
I/O address exists in each PC, it is difficult for the operator to
identify the I/O address allotted to the equipment at the time of creating
a program not to mention at the time of checking and correcting the
program.
Japanese Patent Publication No. 46683/1976 discloses a system in which a
plurality of PC's are dispersively arranged for a parallel processing. In
this system, addresses different from each other are allotted to the
respective pieces of equipment. This system, however, has a complicated
rule for allotting the addresses, so that it is not easy to create a
program and correct it. It is also not easy to judge to which equipment an
address corresponds.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a PC which
facilitates checking, correction and other processings of a sequence
program.
It is another object of the present invention to provide a PC which enables
the operator easily to judge which equipment and which PC a certain I/O
address indicates when the operator sees a program in a system having a
plurality of PC's.
To achieve these objects, the present invention provides a PC comprising an
equipment information storing means for storing equipment information on
the equipment to which an I/O address is allotted, and an equipment
information interpreting means for judging which I/O address corresponds
to the input equipment information.
The equipment information is an equipment identification code which is
composed of a PC number code, an equipment type code, an equipment number
code, etc.
When the sequence program is displayed, the equipment information is
displayed together with the sequence program. The operator can easily
judge to which equipment a certain I/O address corresponds by seeing the
equipment information.
In addition, it is possible for the operator to correct a program by
inputting equipment information which is easy to understand in place of
inputting an I/O address written in a machine language which is difficult
to understand, thereby improving the operability of a program and reducing
an inputting error.
Furthermore, it is possible to communicate between PC's while using the
global equipment information as a transfer unit.
The above and other objects, features and advantages of the present
invention will become clear from the following description of a preferred
embodiment thereof, taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory view of the structure of a conventional PC and an
example of a program thereof;
FIG. 2 is an explanatory view of an example of a relay ladder diagram;
FIG. 3 is a block diagram of the structure of a PC according to the present
invention;
FIG. 4 is a block diagram of the structure of an embodiment of a PC
according to the present invention;
FIG. 5 is a block diagram of PC's which are dispersively arranged for
controlling a transfer machine;
FIG. 6 is a perspective view of a machining device controlled by PC's;
FIG. 7 schematically shows an image of a link data file which is circulated
to each PC;
FIG. 8 is an explanatory view of the data structure of a link data file;
FIG. 9 is an explanatory view of a plurality of PC's which are connected to
a link line and a link data file which is circulated to each PC;
FIG. 10 is a flowchart of the operation of creating a PC link control
table;
FIG. 11 shows a relay ladder explaining a program with certain control
conditions set therein;
FIG. 12 is a relay ladder explaining a bypass processing;
FIG. 13 is an explanatory view of a programming tool which is connected to
a PC;
FIGS. 14A and 14B are flowcharts of a program display processing;
FIG. 15 is a flowchart of a display format selecting processing; and
FIG. 16 is a flowchart of a processing based on a designated display format
.
DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of the present invention will be explained hereinunder with
reference to the accompanying drawings.
FIG. 3 schematically shows the structure of a programmable controller (PC)
according to the present invention. The characteristic structure is mainly
composed of an equipment information storing means 20 and an equipment
information interpreting means 22. The details of these means will be made
clear in the following embodiment.
FIG. 4 shows the detailed structure of an embodiment of a PC 24 according
to the present invention. FIG. 5 shows the entire structure of a transfer
machine 26 which is controlled by a plurality of PC's 24 in parallel. This
embodiment will first be explained with reference to FIG. 5.
In FIG. 5, a plurality of machining devices 30 are arranged on both sides
of a transfer bar 28 for carrying a work W. This transfer machine 26
machines the work W while carrying it. A first station ST1 is composed of
a pair of machining devices 30L and 30R.
FIG. 6 is a perspective view of the first station ST1. The operation of the
first station ST1 will be explained. The work W carried by the transfer
bar 28 is temporarily clamped by a clamping device 32. In this state, the
work W is machined by the machining device 30L, 30R. The machining device
30L is controlled by a PC 11, while the machining device 30R is controlled
by a PC 12. That is, the machining devices 30 L and 30R are controlled by
the PC 11 or PC 12. When machining is finished, the clamping of the work W
is released and the work W is fed to the next station ST2 by the transfer
bar 28.
In this way, each PC controls the machining device which is allotted
thereto. However, all the PC's transmit and receive link data between each
other through a link line 34 for accurately carrying and machining the
work W. An example of link data is interlock information. A station ST0 in
FIG. 5 is provided so as to drive the transfer bar 28 and controlled by a
PC 01.
The concrete structure of a PC will now be explained with reference to FIG.
4.
A control processor 35 controls equipment (including virtual equipment)
such as a switch and a coil. The pieces of equipment are connected to an
I/O portion 36 through which signals are supplied and detection signals
are fetched.
A program memory 38 in this embodiment has three storage regions, namely, a
sequence program storage region 38a, information code storage region 38b
and PC link control table storage region 38c.
In this embodiment, a sequence program contains a command and an I/O
address which are set for each step. For this purpose, the sequence
program storage region 38a is divided into two regions, namely, a command
storage region 38a1 and an I/O address storage region 38a2. Strictly
speaking, the regions 38a1 and 38a2 are mixed with each other.
In this embodiment, a command is input in the form of a logic symbol (e.g.,
LD). It is naturally possible to adopt a system in which a command is
input in the form of a relay symbol. An I/O address is an octal or
hexadecimal digit. The I/O address is a local address which is effective
only in the corresponding PC, and a duplicate address exists in each PC.
It is therefore difficult for the operator to judge to which equipment the
address is allotted merely by seeing the I/O address.
As a countermeasure, an equipment code for each I/O address is stored in
the equipment information storage region 38b. Each of the pieces of
equipment which are controlled by a plurality of PC's is allotted its own
individual global equipment code. The equipment code is composed of a code
which enables the operator to easily understand the contents thereof.
This will be explained more concretely with reference to an equipment code
"31LS1" as an example. The first letter 3 means a third station (ST3). The
second letter 1 means the PC (PC 31) on the left side, and the third and
fourth letters LS mean a limit switch. The last letter 1 means a limit
switch No. 1. The equipment code "31LS1" thus collectively indicates a
limit switch No. 1 which is controlled by the PC on the left side of the
third station.
Another example "11F1D1P" will be explained. Similarly, the letters 11 mean
the PC (PC 11) on the left side of the first station (ST1). The letters F1
means a fitting equipment No. 1, and the letters D2P means dummy
information that the fitting equipment is at the right-hand end.
It is preferable to display an equipment code together with a program when
the sequence program is displayed on a display. This facilitates the
identification of the equipment to which the I/O address is allotted.
Alternatively, only a command and an equipment code may be displayed in
displaying the sequence program.
The PC link control table storage region 38c stores the number of another
PC to which the link data is to be supplied. To state this more
concretely, if "01, 12" is set at a certain step, the link data (e.g.,
interlock information) is supplied from the current PC to the PC 01 and
the PC 12 when executing the step. The PC 01 and PC 12 which have received
the link data execute the next control in accordance with the received
link data. The PC link control table is automatically created, as will be
described later.
An equipment code interpreter 40 will now be explained.
The equipment code interpreter 40 identifies the I/O address which
corresponds to the equipment code input by the operator or received from
another PC. The operator therefore can create or correct a program by
using an equipment code which is easy to understand without the need for
inputting the I/O address, which is a machine language. This is effective
for not only improving the operability at the time of creating a program
but also reducing an inputting error. The writing and reading to and from
the program memory 38 are actually controlled by the control processor 35.
In this embodiment, the control processor 35 and the equipment code
interpreter 40 are constituted by a CPU. The system memory, data memory
and the like are not shown.
As a modification of this embodiment, a table memory 40a for storing an
equipment code/address conversion table may be provided separately from
the program memory 38 in place of providing the equipment code storage
region 38b in the program memory 38. The equipment code interpreter 40
converts an equipment code into an I/O address and an I/O address into an
equipment code in accordance with the table.
The PC 24 of this embodiment has a program linkage condition memory 42. The
program linkage condition memory 42 stores the conditions for linkage
between the sequence programs which are stored in the respective PC's. The
linkage conditions will be described later.
In the parallel control by a plurality of PC's, the transmission and
reception of link data are important. In other words, in the control of
the current PC, the states of the contacts of the relays which are
controlled by other PC's are necessary as controlling conditions. I/O
addresses are also set for such contacts.
A PC link control table is provided in order to control the PC to which
such link data is supplied. In this embodiment, the creation of such a PC
link control table is automated. The processing of the creation of a PC
link control table will be described in the following.
FIG. 7 shows an image of a link data file 44. A control table 38c is
created by using the link data file 44. The file 44 is obtained by listing
up the link data which are necessary for the current PC and is circulated
to the PC's other than the current PC. The file 44 is composed of PC
number data 44a on the PC which supplies the file 44, PC number data 44b
on the PC from which the link data are demanded, equipment code data 44c
on the equipment which produces the demanded data and reception
confirmation data (Yes/No) 44d written by the PC which receives this file.
By circulating the file 44 having the above-described data structure, the
PC which has received this file 44 can judge what kind of link data the PC
(PC 21) which supplied the file 44 demands therefrom. The PC which has
received this file 44 writes the PC number 44a of the PC which supplied
the file 44 in the corresponding position in its own PC link control table
storage region 38c on the basis of the contents of the file 44.
Simultaneously, the PC which has received the file 44 writes the reception
confirmation data (Yes) 44d in the file.
Actually, the file 44 is transmitted serially as a data train 46 shown in
FIG. 8. The data train 46 is composed of the PC number data 44a of the
PC's which supplied the file 44 and data blocks 48 on the respective PC's.
The data block 48 is composed of PC number data 44b of the PC from which
the link data are demanded, equipment code data 44c of the equipment which
produces the demanded data and reception confirmation data 44d. Another
data arrangement is naturally possible.
FIG. 9 shows the circulation of the file 44 between a plurality of PC's
which are connected to each other by the link line 34. As shown in FIG. 9,
the file 44 supplied from the PC 21 is circulated to the PC 32, PC 31, . .
. and finally restored to the PC 21. Thereafter, the file 44 is supplied
from and restored to the PC 32. This process is repeated for each PC and
the creation of the PC link control table 38c for each PC is finished. The
control table creating processing will be explained in detail with
reference to FIG. 10.
The following processings are executed in the state in which the sequence
program of each PC has been completed. At step 101, the file 44 is first
created. The equipment code storage region 38b of the program memory 38 is
retrieved to pick up the pieces of equipment which are not controlled by
the current PC. Since the equipment code includes a PC number code, it is
easy to pick up the piece of equipment on the basis of the PC number code.
The picked-up equipment codes are listed in the file 44. In this
embodiment, the linkage conditions which are stored in the program linkage
condition memory 42 (see FIG. 4) are referred to at the time of creating
the list of the equipment codes.
The linkage conditions will now be explained. Each PC controls in
accordance with its own sequence program. Each PC therefore cannot know
what program another PC has. In this embodiment, the program linkage
condition memory 42 stores the conditions under which the programs of the
PC's in the system are read out in linkage. In other words, the conditions
for the order in which the programs held by the respective PC's are read
out, as will be described later, are the linkage conditions. For example,
a condition that "a certain step in a certain PC should be followed by a
certain step in another PC", and a condition that "the final step in a
certain PC should be followed by the first step in another PC" are
determined. These are only examples and the operator can naturally
determine the conditions as desired in accordance with the contents of the
programs in the total system.
In this manner, each PC can easily judge how many PC's exist in the total
system by confirming the contents of the program linkage condition memory
42. At the time of creating the file 44 (step 101), the lists of the
equipment codes are created for all the PC's which have been judged to
exist in the system. However, if a PC is not required to supply link data,
"NONE" is written as the data 44c on the equipment code.
At step 102, judgment is made as to whether there is a right to supply the
file 44. The order of supplying the file 44 can be set as desired. If the
file 44 is supplied and restored from and to all the PC's, the order of
supply does not matter. For example, the order is determined on the basis
of the program linkage conditions.
If the answer is Yes at step 102, the process proceeds to step 103. At step
103, judgement is made as to whether the file 44 has been supplied to all
the PC's. If the answer is Yes, this routine is finished.
On the other hand, if the answer is No at step 103, the file 44 is supplied
to the link line 34 at step 104. At step 105, the file 44 circulated to
the PC's other than the current PC is restored.
At step 106, the reception confirmation data 44d of the restored file 44
are read and whether the reception has been confirmed is judged with
respect to each PC. If the result is No, the process proceeds to step 107
to execute bypass processing. The bypass processing will be described
later. If the answer is Yes at step 106, step 108 is executed to transfer
the right to supply the file 44 to the next PC.
On the other hand, if it is judged that there is no right to supply the
file 44 at step 102, step 110 is executed and the PC is in the state of
waiting for the reception of the file 44 which is supplied from another
PC.
If the reception is confirmed, the contents of the received file are read
at step 111. That is, the list 44c of the equipment codes which correspond
to the PC number of the current PC are read out of the file 44.
At step 112, data are written into its own PC link control table 38c in
accordance with thus-read equipment code list. At the step which
corresponds to the listed equipment code, the PC number (21) which has
supplied the file 44 is written in the PC link control table 38c. When
this processing is finished, step 113 is executed so as to write the
reception confirmation data (Yes) 44d in the file 44. Even when no
equipment code which corresponds to the current PC exists, the reception
confirmation data (No) 44d is written in the file 44.
At step 114, after the writing processing is completed, the file 44 is
released and transferred to the next PC.
By executing the above routine in each PC, the creation of the PC link
control table 38c is finally completed by each PC. The operator himself
can therefore easily plan the system without the need for creating a
control table in accordance with complicated contents of a program.
The routine shown in FIG. 10 can be executed even in the middle of system
planning. For this purpose, the step of bypass processing is provided as
step 107.
The bypass processing will now be explained. A program concerning the PC
which has not imparted the reception confirmation data 44d is subjected to
bypass processing. That is, the next proceeding is executed on the
assumption that the conditions for that PC are satisfied.
The bypass processing (step 107) will be explained concretely with
reference to FIGS. 11 and 12.
In FIG. 11, a relay ladder of a certain program block is shown. The symbol
X corresponds to an input signal from another PC. It is now assumed that
the PC which is to supply the input signal X cannot supply it because the
program is incomplete or for another reason.
In this circumstance, if no measure is taken, the execution of the program
is stopped here. This is because the relay R is not started unless the
input signal X is obtained even if all the other conditions K1, K2, K3, K4
and K5 are satisfied.
Therefore, the processing is executed on the assumption that the condition
of inputting the signal X is also satisfied or this condition does not
exist. This processing is the bypass processing. That is, the program is
executed on the assumption that there is an OR circuit Y with respect to
the condition X.
According to such bypass processing, when a certain program is incomplete
or has a defect, it does not influence the execution of the programs of
the other PC's.
A trial run of a part of the system is thus enabled.
In the bypass processing at step 107 shown in FIG. 10, the bypass condition
Y is stored in a bypass condition memory (not shown). The control
processor 35 executes the program while considering such a bypass
condition during the PC control.
Another modification of this embodiment will be explained. At step 112
shown in FIG. 10, the PC number is directly written into the control
table. Alternatively, the PC number may be written into an "output signal
table". The output signal table is a list of output signals which are to
be supplied from the current PC to other PC's. When the routine shown in
FIG. 9 is finished, the PC link control table is created on the basis of
the output signal table. In the routine shown in FIG. 10, it is also
possible to create an "input signal table" at the time of restoring the
file 44. The input signal table is a list of signals which are input to
the current PC from the other PC's. In the bypass processing (at step
107), necessary input signals are written into the input signal table.
Such output signal table and input signal table are created, for example,
in a data memory (not shown). The control processor 35 executes the bypass
processing on the basis of the input signal table.
The successive reading of the sequence program which is executed for the
maintenance or inspection of the system will now be explained.
FIG. 13 shows a plurality of PC's which are connected to each other through
the link line 34. In inspecting the programs of the respective PC's, a
programming tool 50 having a display 52 is connected to one of the PC's.
Conventionally, the programming tool 50 is connected to the PC having the
program which needs to be read out, but in this embodiment, it is possible
to successively read out the programs which are stored in all the PC's by
connecting the programming tool 50 to any given PC. The concrete
processing thereof will be explained in the following.
FIGS. 14A and 14B are flowcharts of the program reading processings in the
PC to which the programming tool 50 is connected and in another PC,
respectively.
When the operator inputs the command of reading a program into the tool 50,
the following steps are executed. The case of displaying on the display 52
of the tool 50 the program stored in a PC to which the tool 50 is not
connected will here be explained.
For example, when the equipment code "21F1D2P" and the command of
"retrieval and reading" are input with a keyboard 54 of the tool 50, the
next processings are executed.
At step 201 in FIG. 14A, the display format is selected. The concrete
processing is shown in FIG. 15. In FIG. 15, the prompt "Program will be
displayed with a comment added?" is first displayed on the display 52 of
the tool 50 at step 401. If the operator selects No, this routine is
finished. On the other hand, if Yes is selected, "1" is substituted into a
flag CMT.
At step 403, it is determined whether a comment in the form of a message is
required or not (namely, a comment in the form of an equipment code is
required). The comment in the form of a message is, for example, a comment
explaining the details such as "Forward end of the transfer" and "Limit
switch ON". If Yes is selected at step 403, "1" is substituted into a flag
DOC at step 404, while if No is selected, "0" is substituted into the flag
DOC at step 405. This routine is thereby finished.
In FIG. 14A, after the step 201 is finished, step 202 is executed. At step
202, the PC number code "21" of the PC at the top is read out of the input
equipment code "21F1D2P". In this case, since the PC (PC 11) to which the
tool 50 is connected can judge that this PC number is not its own code,
the PC 11 requires the PC 21 to supply the program block (consisting of 1
to several steps) which corresponds to the input equipment code by
transferring the input equipment code "21F1D2P" and the command "retrieval
and reading" to the PC 21.
In FIG. 14B, another PC which has received the requirement executes step
301. At step 301, the contents of the input equipment code are judged by
the equipment code interpreter 40. If the input equipment code is judged
to indicate the equipment which the current PC controls by the equipment
code interpreter 40, the current PC supplies the data on the program block
concerning the control of the equipment to the PC 11 which has required
the data. Together with the program data, the data on the equipment code
which is stored in the equipment code storage region 38b is also supplied.
In FIG. 14A, the supplied data on the program block and data on the
equipment code supplied from the PC are received and stored in a data
memory (not shown) at step 203.
At step 204, the display format is processed. FIG. 16 shows the concrete
processing. At step 501 in FIG. 16, judgment is made as to whether the
comment is displayed or not in accordance with the flag CMT. If the
comment is not displayed (CMT=0), this routine is finished.
On the other hand, if the comment is displayed (CMT=1), whether the flag
DOC is "1" or not is judged at step 502. If DOC is "1", step 503 is
executed, and the comment in the form of a message which corresponds to
the input equipment code is read out of a floppy disk 56 of the tool 50.
The data on the program block stored in the data memory of the PC 11 is
also supplied to the tool 50.
If the DOC is judged not to be "1" at step 502, the data on the program
block and the data on the equipment code stored in the data memory are
supplied to the tool 50 at step 504.
In FIG. 14A, the program is displayed on the display 52 at step 205. In
this case, the comment in the form of a sentence or an equipment code is
displayed together with the program.
The operator can therefore judge easily which equipment is allotted to the
I/O address from the displayed I/O address and comment.
The operations of scrolling the displayed program and moving to the next
page will be explained.
At step 206, judgement is made as to whether or not a scroll key 60 or a
move to next page key 58 has been operated. If it is judged that the
scroll key 60 has been pressed, the process goes to step 207 and a scroll
command is issued to the PC (PC 21) which holds the displayed program. On
the other hand, if it is judged that the move to next page key 58 has been
pressed, the process goes to step 208 and a command of moving to the next
page is issued to the PC 21 in the same way as at step 207.
In FIG. 14B, the PC which has received either of these commands judges as
to the presence of the command at step 302, and when the command is
confirmed, step 303 is executed.
At step 303, judgement is made as to whether the program block supplied at
step 301 (or step 304) is situated at the top or end of the whole program.
If it is judged that the program block is not situated either at the top
or at the end, step 304 is executed and the next program block is supplied
at step 304 in the same way as step 301.
On the other hand, if the program block is judged to be situated at the end
of the whole program at step 303, judgement is made as to whether or not
the program block is to be scrolled or moved toward the end of the program
block at step 305. If the answer is No, since the next program block can
be supplied to the PC 11, step 304 is executed. On the other hand, if the
answer is Yes at step 305, step 306 is executed.
At step 306, judgement is made as to whether or not there is a PC
succeeding the current PC in the reading of the programs in accordance
with the linkage conditions stored in the program linkage condition memory
42.
The program linkage condition memory 42 stores the linkage conditions
concerning the reading of the programs stored in all the PC's.
In order to successively display the programs on the display 52 of the tool
50, whether or not there is a succeeding program is judged at step 306.
If the answer is Yes at step 306, the processing is turned over to the next
PC at step 307.
On the other hand, if the answer is No at step 306, the processing is
turned over to the PC at the top in the linkage of the programs at step
307. That is, since that the answer is No at step 306 means that the
current PC is the last PC in the program linkage conditions, the
processing of supplying the program is turned over to the PC at the top.
Since the processings at steps 309 to 312 are almost the same as those at
steps 305 to 308 except the direction of scrolling or moving to the next
page is different, explanation thereof will be omitted.
In this way, the operator can successively see the programs which are
stored in the respective PC's by the operation of the scroll key 60 or the
move to next page key 58 on the display 52. On the display 52, when the
display of one program is finished, another program is displayed, as shown
in FIG. 13 as an image S. It is therefore possible to display all the
programs as a series of programs. When the display of the last program in
the program linking conditions is finished, the program at the top can be
displayed subsequently. The operator thus feels as if all the programs
were connected to each other in the form of a ring.
Therefore, the conventional operation of connecting the tool 50 to each PC
for inspecting the respective contents is obviated and it is possible to
get the contents of the programs in the whole system by connecting the
tool 50 to any given PC.
The program linkage condition memory 42 may not necessarily be provided in
each PC. For example, the program linkage condition memory 42 may be
provided in the host PC (PC 01) and the other PC's may refer to the
program linkage condition memory 42 of the host PC. Alternatively, each PC
may be provided with a memory 42 which stores only the conditions
concerning its own PC and the linkage with the precedent PC and the
following PC.
While there has been described what is at present considered to be a
preferred embodiment of the invention, it will be understood that various
modifications may be made thereto, and it is intended that the appended
claims cover all such modifications as fall within the true spirit and
scope of the invention.
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