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1. Experiment preview requirements
1. Review how data selectors work.
2. Use the data selector to pre-design each logic function in the experiment content.
2. The purpose of the experiment
1. Master the logical function and usage of the medium-scale integrated data selector.
2. Learn to use medium-scale integrated data selectors to form combinational logic circuits.
3. Experimental principle
In the process of multi-channel data transmission, the circuit that can selectively transmit a certain channel from the multi-channel signal to the unique public data output terminal is called a data selector, and its selection function is determined by the input terminal encoding of the address code (selection control). The function of the data selector is similar to a single-pole, multi-throw switch, so it is also called a "multiplex switch". Its schematic diagram is shown in Figure 1.
图1 4选1数据选择器示意图
There are four channels of data D0~D3 in the figure. When the strobe terminal (enable terminal) is valid (low level is active), select one channel of data from the four channels of data and send it to the public data by selecting the control signal (address code) A1 and A0. output terminal Y.
The data selector is a very widely used combinational logic component in the current logic design. Its application can not only realize the multiplexing of signals, but also constitute a function generator, a waveform generator, a serial-to-parallel converter and so on. Using it to realize the combinational logic function has the characteristics of simpler circuit and more convenient use. Its integrated circuit products mainly include 2-to-1, 4-to-1, 8-to-1, 16-to-1 and other data selector types.
1. Dual 4-to-1 data selector 74LS153
The so-called dual 4-to-1 data selector is to integrate two completely independent 4-to-1 data selectors on an integrated chip. The pin arrangement is shown in Figure 2, and the function is shown in Table 1.
图2双4选1数据选择器74LS153引脚排列图 表1 数据选择器74LS153功能表
The logical expression of the 4-to-1 data selector can be written as
2. 8-to-1 data selector 74LS151
74LS151 is an 8-to-1 data selector with complementary output, its pin arrangement is shown in Figure 3, and its function is shown in Table 2.
图3 8选1数据选择器74LS151的引脚排列图 表2 数据选择器74LS151功能表
The logical expression of the 8-to-1 data selector can be written as
3. The specific application of data selector
For a data selector with n address inputs, its expression can also be written as
In the formula, mi is the minimum item corresponding to the address variables An-1, ..., A1, A0, which is called the minimum item of the address. The above formula can also be expressed in matrix form as
where 
is the row matrix composed of the minimum address item, 
is the row matrix composed of the minimum address item, ... is the transpose of the array composed of D0～Dn-1.
And any logistic function with l input variables can be represented by the sum of the minimum terms:
Here mi is the smallest term consisting of the input variables A, B, C, . . . of the function.
By comparing the expressions of Y and F, it can be seen that as long as the input variables A, B, C, ... of the logic function are added to the address input of the data selector, and the value of Di is appropriately selected so that F=Y, the The data selector implements the function F. Therefore, the key to implementing a function with a data selector is how to determine the corresponding value of Di. In the case of
(1) , l is the number of input variables of the function, and n is the number of address input terminals of the selected data selector. When l=n, as long as the input variables A, B, C, ... of the logic function are connected to the address input terminals of the data selector in turn, the value of Di in the data selector is determined according to the minimum item required by the function F (0 Or 1); when l<n, the high-order address input of the data selector is not used (connected to 0 or 1), and the rest are the same as above. Example 1. Try 8-to-1 data selector 74LS151 to implement logic function
Solution : First, find the minimum term expression of the logical function formula: 
When using the 8-to-1 data selector, there are
Let A2=A, A1=B, A0=C, to make F=Y, input data D0=D3=D5=D7=1, D1=D2= D4=D6=0, at this time Y=(ABC)m (10010101)T=m(0,3,5,7), so F=Y. The logic diagram of realizing logic function F with 74LS151 is shown in Figure 4.
图4例1逻辑图
It should be noted that because the labels of the minimum items in the function F are written according to the weights of A, B, and C as 4, 2, and 1, A, B, and C must be added to A2, A1, and A0 in turn. Case
(2) When the number of variables of the logic function is larger than the number n of address input terminals of the data selector, the simple method described above cannot be used. If n input variables are selected directly as the address input of the data selector, the redundant variables will be reflected to the data input Di of the data selector, that is, Di is the function of the redundant input variables, referred to as the residual function. Therefore, the key to the design is how to obtain the function Di. The algebraic method or the dimensionality reduction K-graph method can be used to determine the co-function Di. Example 2. Use the 4-to-1 data selector 74LS153 to implement the logic function Solution: First select the address input, let A1A0=AB, then the redundant input variable is C, and the remainder function Di=f©. In this experiment, the algebraic method is used to determine the co-function Di, that is, the algebraic method is used to transform the expression of F into the corresponding form of Y:
Comparing F and Y can be obtained 
, so the logic diagram of implementing logic function F with 74LS153 is shown in Figure 5.
图5例2逻辑图
4. Experimental equipment and devices
1. TH-SZ type digital system design experiment box;
2. 74LS04 six-inverter, 74LS151 8-to-1 data selector;
3. Several wires/patch wires.
5. Experiment content and steps
1. Test the logic function
of 74LS151 According to the pinout diagram of 74LS151, the 8th pin GND of 74LS151 is connected to the ground on the experimental box, and the 16th pin VCC is connected to the +5V power supply on the experimental box. Connect the output terminal Y to the logic level display socket, the address input terminal A2A1A0, the data input terminals D0~D7, and the strobe terminal are respectively connected to the logic level switch output socket, test its logic function item by item according to the function table of 74LS151, and Record test results. When testing the circuit, special attention should be paid. In the tested chip A2A1A0, the former should be high.
2. Implement a one-bit full adder with 74LS151. Requirements: Write the design process, draw the logic circuit diagram, and verify the logic function.
仿真图
3. Use 74LS151 to realize a "transfusion rule" circuit in which the blood type of the transfuser matches the blood type of the recipient. The known blood transfusion rule diagram is shown in Figure 6. Requirements: Write the design process, draw the logic circuit diagram, and verify the logic function.
Design hints: The blood transfusion rules and codes are shown in Figure 6. When the blood type of the transfuser and the recipient conform to the "transfusion rules", the output of the circuit is 1, otherwise the output is 0.
图6 输血规则图
仿真图
6. Experiment report requirements
1. Write out the detailed process of designing the experimental content with the data selector, draw the wiring diagram, and test the logic function.
2. Summarize the experimental results, experience and circuit structure characteristics of the data selector, and explain what the address variable does to the data selector.
3. Organize the experimental records and analyze the results.