Question

12. This problem is a simple example of the first step in the design process for one type of digital system. The following is a simplified description of when you are allowed to move through an intersection that is controlled by a traffic light with only a red and a green light (no yellow light). One light is always on, but they are never both on at the same time. That is, if the light is red, it is not green. You may move through such an intersection if the light is green and you are not turning left, or if the light is red and you are turning right and you have verified that there is no interfering cross-traffic. Translate this situation into a 4-input, 1-output truth table. Define each binary variable that you use and give the meaning of a 0 or 1 value for each variable.

13. In the last problem, you found a truth table describing a set of simplified rules for moving through an intersection with a red/green traffic signal. For this problem, you are to draw a combinational logic network that describes that same set of rules. Your network should mimic the way the problem was stated in words in the last problem. In particular, you should have one AND gate for each "and" in the problem statement, one OR gate for each "or", and one NOT gate for each "not".

Answer 1

Let there be four input variables S, L, R and I where S is 1 if the signal is green, 0 if it is red, L is 1 if one wishes to turn left, 0 if he does not wish to turn left, R is 1 if one wishes to turn right, 0 if he does not wish to turn right and I is 1 if there is intersecting traffic and 0 otherwise. Let there be an output variable M, which is 1 if the person is supposed to move and 0 otherwise. The required truth table is:

S	L	R	I	M
0	0	0	0	0
0	0	0	1	0
0	0	1	0	1
0	0	1	1	0
0	1	0	0	0
0	1	0	1	0
0	1	1	0	1
0	1	1	1	0
1	0	0	0	1
1	0	0	1	0
1	0	1	0	1
1	0	1	1	0
1	1	0	0	0
1	1	0	1	0
1	1	1	0	0
1	1	1	1	0

The following is the combinational logic network required.