Decoder
of Decade |
Decoding
of a Meter of Johnson |
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Created it, 06/09/09
Update it, 06/09/24
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1. 9. - PUT IN CASCADE OF DECIMAL SCALERS
Let us defer on figure 18 the diagram of the exits of meter modulo 10 examined in the preceding theory.
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If we associate (logical positive) as accustomed figure 1 at the high level and figure 0 the bottom grade, we obtain the table of figure 19 where we can notice that from 0 to 9 the circuit counts in binary code.
19. - The exits of the meter are in binary code.
| States of the meter | Q4 | Q3 | Q2 | Q1 |
| 0 | 0 | 0 | 0 | 0 |
| 1 | 0 | 0 | 0 | 1 |
| 2 | 0 | 0 | 1 | 0 |
| 3 | 0 | 0 | 1 | 1 |
| 4 | 0 | 1 | 0 | 0 |
| 5 | 0 | 1 | 0 | 1 |
| 6 | 0 | 1 | 1 | 0 |
| 7 | 0 | 1 | 1 | 1 |
| 8 | 1 | 0 | 0 | 0 |
| 9 | 1 | 0 | 0 | 1 |
As you saw in theory 9, it is possible to put several meters cascades about it by connecting the exit CARRY of the first to the entry Chip ENABLE T (C.E.T.) of the second and so on.
Three synchronous meters modulos 10 thus connected form the circuit represented figure 20.
Each decade, when it passes by again to 0, increments the following decade of 1. Thus the first decade will count each impulse, the second will count of them all ten and the third all hundred.
The levels of the four exits (Q1, Q2, Q3 and Q4) of each decade form a binary code arranged according to the table of figure 19.
One can find the decimal number, result of counting, by knowing that the four exits of the first meter indicate the figure of the units, the four exits of the second that of tens and those of the third that of the hundreds.
In this manner, the meter uses code B.C.D.
1. 10. - DECODER OF DECADE
As for the meter modulo 16, the decade often needs a decoder for its exits. In the trade, one finds decoders with 4 entries and 10 exits which fill this task precisely (figure 21).
These decoders have 4 entries and 10 exits. They allow when one posts a binary number on the 4 entries to obtain the decimal equivalent while returning validates one of the ten exits. They thus accept in entry only the binary numbers ranging between 0000 and 1001 (i.e. between 010 and 910).
These decoders are called 4 towards 10, in the same way the decoders at hexadecimal exits are called 4 towards 16.
Figure 22 a-b represents a decoder MOS, type 4028 B 4 / 10 whose exits are active with state 1. (Of other decoders active exits with state 0 have).
Figure 22 c-d represents the association of two decoders 4028 B in order to produce a decoder 4 / 16.
The truth table indicates the state of the exits for each combination “d, c, b, a” in entry.
One can use the exits to decode various codes. Here, we use two decoders cascades about it in which only the exits from 0 to 7 are exploited.
Up to 716, the entry “d” being with 0, circuit 1 has its validated exits and circuit 2 (the bit “d” being reversed) cannot have exits used validated.
From 810, the bit “d” being with 0, circuit 2 has its exits 0 to 7 which can be validated according to the combinations of “a, b, c” whereas the exits used of circuit 1 cannot be validated. Exits 0 to 7 of the second circuit are interpreted like 8, 9, 10, 11, 12, 13, 14, 15.
We thus have well a hexadecimal decoder.
1. 11. - DECODING OF A METER OF JOHNSON
As you saw in the preceding theory, the meter of Johnson counts in a quite particular way.
Indeed, in this meter made up of five rockers, the exits can take ten combinations different from 2 exits each one as represented in green figure 23.
The Johnson code is not a fixed-count code.
One allots to each of the 10 combinations a decimal code from 0 to 9 such as described figure 24.
It is thus necessary to decode the state of the exits to individualize each combination in order to have the 10 exits.
A good number of solutions are offered then and we will retain that adopted in the integrated circuit 4017 B.
This integrated circuit gathers in the same case a meter of Johnson on 5 floors and the suitable decoder.
The synoptic diagram of this circuit is represented figure 25 like its stitching.
Entry CP0
makes it possible to start the meter on a rising face whereas entry 
1 allows the release of the meter on a
downward face. The entry MR. allows the
handing-over 0 general. It is active on the
level H (high).
The O0
exits in O9 are the decoded exits. Exit 
5
- 9 allows the setting in cascade of the meters: it is enough
to connect it to entry CP0 of the following
meter. It is an active carryforward on the level L
(low).
The figure 26-b shows the internal diagram of the meter decoder 4017 B.
It comprises a Johnson meter with five rockers followed by a combinative network of decoding. The chronogram (figure 26-a) shows the successive validation of the progressively decoded exits with the arrival of the clock pulses.
In the integrated circuit 4017 B, the various combinations are obtained using NOR-circuits follow-ups each one of a buffer (circuit YES used as amplifier).
As example, the validation
of the Q4 exit is represented in fat in the figure 26-b. This exit,
corresponding to the decimal code 4, passes
on a logical level H for the combination of
exits 4
and Q5 of the meter.
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