39.* DRILL QUESTION
B 9-39.* DRILL QUESTION
For each item, indicate whether it is referring to a decoder, an en- coder, a MUX, or a DEMUX.
(a) Has more inputs than outputs. (b) Uses SELECT inputs.
(c) Can be used in parallel-to-serial conversion. (d) Produces a binary code at its output.
(e) Only one of its outputs can be active at one time. (f) Can be used to route an input signal to one of several possible
outputs. (g) Can be used to generate arbitrary logic functions. 9-40. Show how the 7442 decoder can be used as 1-to-8 demultiplexer.
(Hint: See Problem 9-7.) 9-41.*Apply the waveforms of Figure 9-72 to the inputs of the 74LS138
DEMUX of Figure 9-30(a) as follows:
D :A 2 C:A 1 B:A 0 A:E 1
Draw the waveforms at the DEMUX outputs.
C HAPTER 9/ MSI L OGIC C IRCUITS
9-42. Consider the system of Figure 9-31. Assume that the clock frequency is 10 pps. Describe what the monitoring panel indications will be for each of the following cases.
(a) All doors closed (b) All doors open
(c) Doors 2 and 6 open
C, D
9-43.*Modify the system of Figure 9-31 to handle 16 doors. Use a 74150 16- input MUX and two 74LS138 DEMUXes. How many lines are going to the remote monitoring panel?
9-44. Draw the waveforms at transmit_data, and DEMUX outputs O 0 , O 1 , O 2 , and O 3 in Figure 9-33 for the following register data loaded into the transmit registers in Figure 9-32: [A] = 0011, [B] = 0110, [C] = 1001,
[D] = 0111 . 9-45. Figure 9-78 shows an 8*8 graphic LCD display grid controlled by a
74HC138 configured as a decoder, and a 74HC138 configured as a de- multiplexer. Draw 48 cycles of the clock and the data input necessary to activate the pixels shown on the display.
FIGURE 9-78 Problem 74HC138 9-45.
E 2 74HC138
1 EN MOD-8 TC
EN MOD-8 TC
SECTION 9-9
9-46. Consider the control sequencer of Figure 9-26. Describe how each of
the following faults will affect the operation.
(a)*The I 3 input of the MUX is shorted to ground. (b) The connections from sensors 3 and 4 to the MUX are reversed.
P ROBLEMS
T TABLE 9-10
Actual Count
Displayed Count
9-47.*Consider the circuit of Figure 9-24. A test of the circuit yields the re- sults shown in Table 9-10. What are the possible causes of the mal- function?
9-48.*A test of the security monitoring system of Figure 9-31 produces the results recorded in Table 9-11. What are the possible faults that could cause this operation?
TABLE 9-11 Condition
LEDs
All doors closed
All LEDs off
Door 0 open
LED 0 flashing
Door 1 open
LED 2 flashing
Door 2 open
LED 1 flashing
Door 3 open
LED 3 flashing
Door 4 open
LED 4 flashing
Door 5 open
LED 6 flashing
Door 6 open
LED 5 flashing
Door 7 open
LED 7 flashing
9-49.*A test of the security monitoring system of Figure 9-31 produces the re- sults recorded in Table 9-12. What are the possible faults that could cause this operation? How can this be verified or eliminated as a fault?
TABLE 9-12 Condition
LEDs
All doors closed
All LEDs off
Door 0 open
LED 0 flashing
Door 1 open
LED 1 flashing
Door 2 open
LED 2 flashing
Door 3 open
LED 3 flashing
Door 4 open
LED 4 flashing
Door 5 open
LED 5 flashing
Door 6 open
No LED flashing
Door 7 open
No LED flashing
Doors 6 and 7 open LEDs 6 and 7 flashing
9-50.*The synchronous data transmission system of Figure 9-32 and Figure 9-33 is malfunctioning. An oscilloscope is used to monitor the MUX and
C HAPTER 9/ MSI L OGIC C IRCUITS
Transmit data
O 1 DEMUX
output:
t 0 t 2 t 3 t 4 t 5 t 6 t 7 t 8 t 9 t 10 t 11 t 12 t 13 t 14 t 15 t 16 t 17 t 18 t 19
FIGURE 9-79 Problem 9-50.
DEMUX outputs during the transmission cycle, with the results shown in Figure 9-79. What are the possible causes of the malfunction?
9-51. The synchronous data transmission system of Figures 9-32 and 9-33 is not working properly and the troubleshooting tree diagram of Figure 9-35 has been used to isolate the problem to the timing and control section of the receiver. Draw a troubleshooting tree diagram that will isolate the problem further to one of the four blocks in that section (FF1, Bit counter, Word counter, or FF2). Assume that all wires are connected as shown, with no wiring errors.
SECTION 9-10
C, D
9-52. Redesign the circuit of Problem 9-16 using a 74HC85 magnitude com- parator. Add a “copy overflow” feature that will activate an ALARM output if the OPERATE output fails to stop the machine when the re- quested number of copies is done.
D 9-53.*Show how to connect 74HC85s to compare two 10-bit numbers.
SECTION 9-11
9-54. Assume a BCD input of 69 to the code converter of Figure 9-40. Determine the levels at each © output and at the final binary output.
9-55.*A technician tests the code converter of Figure 9-40 and observes the
following results:
BCD Input
Binary Output
27 0011011 What is the probable circuit fault?
SECTIONS 9-12 TO 9-14