4. Different types of components, possible data which should be requested from them, their controllability, their
3.6 Manipulating System. Generalities
In Reference C74D we have analyzed the alternatives in
connection with interactive graphical displays when they are used in process control applications.
The analysis establishes that when a large number of dis
plays are used in a Supervision and Control system /which is the case in a process control application/, the best alter
native fcr the category selection is when this task is performed by the computer. Additionally, it recommends the refresh memory to be included in tie display’s hardware
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-either when computers of limited memory capacity are used, or in case of multi-display systems, or when long communication paths are used. It is observed also that in case a magnetic disk is used as back-up memory of the computer system, and data transfer between the disk memory and each display unit
can be carried out without the interruption of the CPXJ opera
tion of the computer, then refreshing of displays could be accomplished by the disk.
Basing some arguments on these criteria, we can point out the following:
- The display’s refresh memory capacity is limited to a
maximal number of mr-bit words given mainly by the number of cells in which the rectangular useful surface of the screen has been hypothetically subdivided.
The total number of cells "Z" is given by:
Z » M X N where:
M is the number of cells per row and N is the number of cells per column.
The refresh memory is considered as an object-oriented memory having a "cell map" with a total number "Z" of cells c1543.
Analyses of the values for "M" and "N" and of the total number of bits per word "m", have been carried out in another part of this study /See Sect. 2.6.3/.
- The information stored in any display’s refresh memory must be only that previously requested from the computer.
It belongs to only one drawing of the "MQ" possible
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-drawings previously stored and continuously updated by the computer. The selection of the corresponding drawing
/category selection/ must be performed by software by the computer. Updating of information must also be performed only by the computer which continuously receives the actual information through the Interface from the process being supervised. The updating information related to a parti
cular drawing "i" must only be transmitted to the parti
cular display unit showing the drawing "i" at that moment, but only in the case when it is different to the informa
tion previously stored in the computer memory, and there
fore, to that currently stored in the corresponding refresh memory of the display. Otherwise, this transmission must not be performed. The decision about the occasion when the updating information should be transmitted to the display units, might be implemented either by hardware or by soft
ware. If it is done by software, it must be performed by program in the computer; if it is done by hardware, this decision must be taken by the Controller.
It is not advisable for state information to arrive at the Display Controller directly from the process /but rather at the computer Interface/ because of the following
reasons:
- If it is sent directly from the process, /considering the best case when it can be interpreted directly by the display units/, then it would be necessary to get also together with the state information the position informa
tion of the component in the corresponding drawing /or drawings/ and also the code of the drawing /or drawings/
to which it belongs. This is particularly complicated when the system deals with a great number of components and many sections, areas, etc. as in this application.
Thus, from a given information provided by the components together with their "state" information, the computer should determine which type of component it is dealing
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-with, it selects which drawing /or drawings/ it belongs to, it finds out the particular position of the component inside the selected drawing, etc. Only after this, will the computer could update the previously stored informa
tion with the new one recently received from the process and effectuate the prescribed calculations.
After receiving all the information about the component /identification code and state information/, the computer would perform some calculations in order to detect ab
normalities, to find out overall values, etc. As a
product of these calculations also, the computer should decide, among other things, the change in color /if any/
to have in the symbol of the previously analyzed
component, data, etc. Obviously, if the updating informa
tion is sent to the Controller directly from the process, these calculations can not be performed before they are shown onto the display’s screen. This does not exclude the possibility of a display with processing facilities, but it would make more expensive the cost per unit and would impose new requirements for the whole system. The use of firmware, microprocessors, and other technologies in this field, is beginning just now promising to be the solution for the future, although at present they do not substitute all the facilities of a minicomputer yet.
If we consider that the only task of each display unit consists in showing to operators as a "robot" the in
formation previously handled and formatted by the com
puter for every cell, then we have that:
- auxiliary memory circuits are less complex because of the sequential order imposed
- it needs less capacity, because of it is not necessary to store the corresponding position in
formation belonging to every character, symbol, etc.
to be shown on the screen. This makes possible a
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-considerable saving of memory in the display
- Display refresh memory can be refreshed always in the same form, i.e. following always the same sequence of cells from the first upto the last one - the overall cost of display units is considerably
reduced.
Concluding, the information arising from the components of the process being supervised, must arrive first to the
computer through the Interface and there, after the analysis of the "state” information, the determination of the parti
cular drawing /or drawings/ to which it belongs together with the exact positioning address inside it /or them/, and after other tasks are carried out, it must be sent /through the Controller/ to the corresponding display unit showing in that moment the drawing including the component which has been supervised. How all these tasks can be carried out is dealt with later in this paper /See Sect. 3-6.3.3/.
Starting from the fact that the display’s memory capacity is limited to a given number of m-bit words corresponding each one to each cell, there are two very different forms to store in the computer memory the information related to a drawing and to transmit it from the computer to the display units, namely: in prefixed order and without order.
If the information /updated or not/ must be sent to the dis
play units in order, it implies that in the computer memory there must be stored an exact /updated or virgin/ replica from all points of view of the information to be trans
mitted to a given display, i.e. of the picture file. In ge
neral, this is required in some form by the system, but in this case, the graphical coded information related to a
particular drawing must each time be transmitted sequentially cell by cell and in a predefined order to the display unit which previously requested it. With this alternative, the
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-display does not need to receive position information /because it can be internally generated/, but the system presents the following disadvantages:
- the computer requires, each time an updating takes place in a given drawing being shown, to transmit cell by cell all the stored graphical information related to it;
- the computer must leave free locations for those cells having no information;
those advantages offered by data compression in the com
puter are lost.
If the information can be sent to the display units without order /at random/, it implies that a position instruction must always precede the graphical instructions. This alter
native offers the advantage that the computer can save memory locations /because it does not need to leave free locations when some cells have no information/, but it could have three main disadvantages, namely:
- updating is more complicated, because of the difficulty met in searching for the exact location in the picture
file wherein the information related to a particular com
ponent has been stored; this can be overcome by means of computer tables created for this purpose and adequate algorithms to carry out this task;
- it requires /at least/ a number of computer words equal to the double of the total number of graphical instructions belonging to each drawing in order to store, preceding each one, the respective position information related to it; this can be overcome if the position information is calculated each time an updating takes place, it then
being unnecessary to store all of them, which would require in complex pictures a relatively high additional number of
154 -locations in the computer memory;
- it could also present serious difficulties when the system requires enlargements or modifications and as a product of this, the total number of locations destined to each
drawing in the computer /size of the picture files/ is less than the maximal possible capacity of information required to show a drawing on the display screen; this can be over
come either making the picture files sufficiently large /which is not so economical from the point of view of memory capacity required/ or creating conditions for
getting a considerable data-compression for storing the in
formation related to every drawing.
Prom the analysis carried out above, it is seen that the al
ternative of storing the graphical information at random in the computer memory and transmission at random of the visual information to the display units, would present more
advantages than if these task are performed in sequential order. All that is necessary is to create the conditions which will permit us to make the whole system realizable.
Firstly for this, some further design criteria are proposed for the display units.
3.6.1 Display Unit
3.6.1.1 Three design criteria for the display units
The following three criteria are established and discussed for the display units:
1. Each display unit must be able to interpret directly,