Bill-posters
with 7 Segments and Decoders corresponding |
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Created it, 06/09/09
Update it, 06/09/24
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2. - BILL - POSTERS
2. 1. - TUBES METERS
2. 1. 1. - THE DÉCATRON
It was used as tube meter in certain electronic computers.
It comprises a central anode surrounded by ten cathodes.
At a given moment, the discharge occurs only between only one cathode and the anode. Between adjacent cathodes electrodes of transfer are to which are applied the impulses to count. With each impulse, the discharge is deviated of one cathode to the following one. When the discharge made a full rotation of the tube, an impulse of counting is sent to a second similar tube which will mark for example tens. It is enough to add other tubes to record the hundreds, the thousands, etc…
These tubes are given up, they indeed required a discharge of 200 volts and the use of transformers.
2. 1. 2. - TUBES DECIMAL SCALERS “TROCHOTRON”
The principle of the tube decimal scaler is given on figure 27.
The electronic beam is emitted by a hot cathode (thermoelectronic effect).
This beam is concentrated, accelerated then deviated by means of deflection plates. It passes through the anode perforated after having crossed a system of grids, then comes to strike the fluorescent screen which are registered the figures from 0 to 9 or of the alphabetical characters.
They are usable up to maximum 1 MHz.
These tubes require a control circuit including/understanding a digital / analogical conversion in order to supply the deflecting electrodes.
2. 1. 3. - TUBE OF POSTING “NIXIE” OR NUMERICAL INDICATOR
It includes understands ten cathodes in the shape of figures (or signs) superimposed. It is represented figure 28.
The posting of a luminous figure is obtained by application of the suitable tension (approximately 160 V) on corresponding cathode.
Although it was launched in the Sixties by BURROUGHS this technology, though old, still meets.
2. 2. - BILL-POSTERS
A SEVEN CORRESPONDING SEGMENTS AND DECODERS
With decoders 4 towards 10 and 4 towards 16, it is possible to know the state of a meter by connecting for example a LED to each exit and by registering beside each one of it the decimal digit or hexadecimal which it represents.
Figure 29 shows a decoder 4 towards 10 at active exits with state 0 with its network of associated LED.
This method is however not very practical because it requires the use of 10 or 16 LED and that the numerical value of the state of the meter is not really posted in light.
This is why, it was created more complex circuits making it possible to post the corresponding signs directly.
These circuits are called bill-posters or in English “display”.
The bill-poster term or “display” indicates any circuit making it possible to post in light a numerical or alphanumeric value. Diode LED is the simplest element of the bill-posters, it indeed makes it possible to visualize only one bit.
There are also bill-posters specialized such as the numerical faces of watches. The video screens also make it possible to post information.
However, there is a type of circuit making it possible to post information in decimal or hexadecimal code by means of a matrix of 7 segments made up of 7 diodes LED, this is why we will call these bill-posters, bill-posters 7 segments.
The bill-poster with 7 segments allows to reproduce signs 0 to 9 and A with F, it also comprises a point called DP (of “Decimal English Point”) which is not other than the equivalent of our decimal point.
Each segment is indicated by a letter a, b, c, d, e, f, g and the point by D.P.
Figure 30 shows the provision of these segments.
Figure 31 represents a complete bill-poster 7 segments in his case.
By suitably ordering the lighting of certain segments, one visualizes the desired numbers.
To visualize one zero, one will light the segments a, b, c, d, e, f. To visualize one 1, one will light the segments b, c and for one 2, the segments a, b, g, e, d for example.
The useful combinations are represented for a circuit decimal decoder on figure 32.
The internal circuit of a bill-poster 7 segments is very simple.
Figure 33 illustrates the two existing cases.
Appear 33-a, the 8 LED are connected between them by their anode, it acts of a bill-poster with common anode, whereas the figure 33-b, the 8 LED are connected by their cathode, the bill-poster is known as with common cathode.
In the bill-posters with common cathode to light a segment, it is necessary to apply to the anodes corresponding to the selected sign a positive tension so that the corresponding diodes ignite.
As example, figure 34 gives the stitching of a bill-poster to common anodes of Monsato mark, as are equivalent diagram.
It is clear that one cannot order a bill-poster of this type directly with the exits of a binary counter; indeed, this one requires a special order to reveal the selected decimal digit. It is thus advisable to intercalate between these two circuits a decoder 4 towards 7 as you can see it in synoptic figure 35.
We have to make with a decoder 4 towards 7 for which we can draw up the truth table.
Knowing that the entries corresponding to the segments a, b, c, d, e, f, g will be for example, to 0 (case of a bill-poster with common anode) for an illuminated segment and 1 for an extinguished segment, we can establish a correspondence between the binary code representative of the selected decimal digit and the combination necessary to visualize it.
Thus, for example to postal code 410 is 01002, it is necessary to clarify the segments f, g, b, c.
We can say that in this case, the entries d, e, a are equal to 0 and the entries f, g, b, c are equal to 1.
Figure 36 represents the truth table of a decoder BCD towards 7 segments necessary to order a bill-poster with common anodes (active exit with state 0).
The exits here are noted ON and OFF because they are exits with open collector (ON lights the segment, OFF extinguishes it) from where the use of external resistances.
There are also decoders (7448 TTL for example) whose exits are active to 1 to order the bill-posters with common cathode.
Generally, the 4 bits of code BCD are indicated by the letters A, B, C, D where D is the most significant bit of weight 23, while that the least significant A a weight of 20.
We can see figure 37 a decoder 7447 commander a bill-poster 7 segments.
This decoder has exits with collectors open authorizing a direct connection with bill-posters to common anode. Additional entries are also envisaged :
LT or “lamp
test” which makes it possible to check the operation of the
bill-poster by lighting all the segments if BI
is with state 1.
BI / RBO or “blanking
input” which allows the obliteration of the segments of the
bill-poster some are the state of the other entries.
RBI or “ripple
blanking input” which allows the obliteration of the 0
on the left if A, B, C, D are to 0.
The truth table of decoder 7447 is given figure 38 as its logigramme intern which is a combinative network, and his stitching.
Figure 39 shows how one can use the
entries 
and the exits 
to remove 0 the nonsignificant ones on a
whole of decoding to 3 digits.
of the decoder of the hundreds is to 0
permanently. With each time one 0 is
presented on the decoder of the hundreds, this 0
are erased and the exit 
passes to 0 validating
following decoder.
If one 0
is presented on the decoder of tens, it is in its unobtrusive turn and one will
validate
decoder of the units.
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