Control of the PROM performing the Memory Decoding :
5. - THIRD EXPERIENCE : CONTROL OF THE PROM PERFORMING THE MEMORY DECODING
If you can not do this experience because it involves buying a PROM programmer and also requires knowledge of the technical programming, nothing prevents us afterwards to study the theories dealing with assembly codes and to this end, to return to this experience.
The memory of a Computer System is made up of a certain number of boxes of different types (ROM, RAM, EPROM, PROM), of different capacities, in general multiple of 1 kilobyte (1024 x 8 bits).
The microprocessor progressively selects the memory unit to which it wishes to access to read or write. This is possible thanks to a decoding circuit realized with a PROM memory (remember that a PROM is a programmable memory whose content is permanent).
The purpose of the experiment you are going to carry out is to check the content of this memory after having programmed it (see the listing in Figure 14), that is to say to check that it is correctly programmed. It is therefore a question of applying to the address inputs all the possible combinations and of checking that at the output of the PROM, the correct voltage levels are found.
Each group is located by means of an address coded in binary on 9 bits.
The PROM in question (which is identified by the acronym of your choice, CPU - MZ1 for example, opposite mark after programming) is of the 74S472 type and has a capacity of 512 x 8 = 4096 bits. It is also said to have a capacity of 4 k bits or 512 Bytes (remembering that a Byte is equivalent to 8 bits or one byte). The bits are arranged in groups of 8 bits according to the diagram in Figure 9.
The PROM therefore has nine address inputs and eight outputs as shown in Figure 10. Each combination of the 9 address bits corresponds to an output of a group of 8 bits.
Figure 11 shows the pins of the component : A0, A1, A2, A3, A4, A5, A6, A7 and A8 are the address bits ; O1, O2, O3, O4, O5, O6, O7 and O8 are the outputs (O stands for Output).
5. 1. - REALIZATION OF THE CIRCUIT
a) Remove the components and connections used in the previous experiment from the contact plate.
b) Figure 12 shows the block diagram of the circuit you are going to wire ; the addresses are displayed by displays DIS0 and DIS1 of type TIL 311, while the output data are represented by LEDs L0 to L7.
c)Insert the following integrated circuits : CPU MZ1 for example, (this indication shows that the PROM 74S472 memory that you programmed using a programmer before starting this experiment bears the name CPU MZ1 or others of your choice after having programmed this memory, to see the listing Figure 14), 74LS393 (two counters of 4 bits), 74LS74 (double rocker D) and the eight resistances of 1 kΩ, by placing each component in the way indicated in the Figure 13-a.
d) Carry out the connections by following the diagram of the Figure 13-a. You have just carried out the assembly of the electric circuit represented in the Figure 13-b.
5. 2. - OPERATION TEST
a) Switch on the digilab.
b) Press the P0 button ; in this way, the module counter 9 composed of the flip-flop and the two 4 bits counters is reset to zero.
c) Observe the DIS0 and DIS1 displays : both must display 0 and their decimal points must be off, which shows that the first address sent to the PROM is made up of nine zeros. The entry A0 must therefore be at the low level, corresponding to 0, as well as the other entries, up to A8.
The address is therefore :
A0, A1, A2, A3, A4, A5, A6, A7, A8 = 000 000 000
To facilitate writing, the address can be abbreviated, using the hexadecimal code ; we therefore group the nine binary digits into two groups of four, then a number that remains unchanging.
So we will write :
0 0000 0000binary
= 000hexadecimal
For example, the address
0 1101 0011, abbreviated in hexadecimal gives :
0 1101 0011binary
=
0D3hexadecimal
To read the address, the displays DIS0 and DIS1 are used ; however, these can only represent two hexadecimal digits, while the address is made up of three digits.
Fortunately, the first digit can only have two values : 0 and 1, so it can be viewed using the dot on the DIS1 display which will indicate 0 when it is off and 1 when it is on.
d) Now observe the LEDs : they indicate the data, or better, the 8 bits which constitute the information stored in memory at the address read on the display.
An unlit LED shows that the corresponding bit is 0 while the lit LED indicates 1. The address 000, for example, will be indicated as follows :
L7
L6
L5
L4
L3
L2
L1
L0
0
1
0
1
1
1
1
1
e) Press P1 ; the counter thus advances by one step and the displays now indicate :
0
0
1
Point of DIS1
DIS1
DIS0
The first digit of the address is read on the dot of the DIS1 display, the second is read on the DIS1 display,the third on DIS0.
f) Read the data on the LEDs and check if they correspond to those shown in the second line of Figure 14, that is to say : 0101 1111.
g) Press P1 again ; the counter then indicates address 002 and the LEDs display the eight data bits : 1111 1111.
h) By pressing P1 each time, advance the counter and check the content of the PROM at each address by referring to the table and this up to the hexadecimal address 1FF which corresponds to the binary address 1 1111 1111.
i) If the test is positive, you can be sure that the PROM is working properly ; otherwise, it must be replaced.
j) Switch off the digilab.
5. 3. - CONCLUSIONS
This exercise allowed us to examine the content of the PROM memory or as we generally say of "reading" it.
The write operation was done previously and that you had already programmed for those or for those having a programmer. In the following practices, you will discover what is the practical use of this microcomputer memory ; for now, keep it carefully.
By observing the diagram of the Figure 13-b, you will notice that the outputs of the PROM are connected to the positive pole of the supply voltage by resistances of 1 kΩ ; these are necessary because the PROM outputs are of a particular type.
If the PROM is of the 74S472 type that you bought, its outputs are TRI-STATE (three states) ; they therefore have the possibility of being in the high impedance state.
On the other hand, if you had bought a PROM of the type 74S473, its outputs are outputs "open collector", that is to say with open collector.
The Figure 15 represents the block diagram of the output stage of an integrated circuit with open collector, produced with a Schottky transistor.
As you can see, the collector of the transistor is directly connected to a pin of the integrated circuit and this therefore does not have an internal load ; this is why it is called "open collector circuit".
This type of circuit allows the load to be placed outside the integrated circuit, which thus offers the possibility of varying the fan-out of the circuit.
When using an open collector circuit, you must be very careful not to forget to connect the outputs to the power supply via suitable external resistors.
On some CPU cards, the PROM outputs are linked to the memory validation inputs. Its inputs must always be connected to a positive voltage or to earth and cannot be left free (that is to say in the air).
This could happen if the PROM outputs were TRI-STATE or, in the case of the open collector, were not connected to the + through a resistor. For these reasons, these resistances are important and should therefore be used.
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