Activity on Data signals

In this activity you will look at the signals a keyboard sends to a computer using a simulated oscilloscope. Each group of students needs a computer with a web browser that has the Macromedia Shockwave plug-in. (If you don't have it, your browser will begin downloading it when you try to view the simulation.)

Before You Start:
Answer the following questions to the best of your ability before doing the experiment.
What do you think a keyboard signal will look like?
Which format (NRZ, RZ, Manchester, Bipolar) would you expect to be the best choice for a keyboard? Why or why not?
Do you think a keyboard will send letters using ASCII or some other format?
Do you think the signal for e will be different from E? Why or why not?
Give reasons for each of your predictions.
After you have thought about your answers, compare notes with your group members. Does everyone have the same predictions, or are there differing opinions?

Open up the simulation.

What type of signal is it?

Look at the signal for the letter j, then answer the following questions.

1. Is the signal sent from the keyboard decimal, binary, or something else? Justify your answer, using a sketch if helpful.

2. Does the signal use NRZ, RZ, Manchester, or Bipolar coding? Justify your answer, using a sketch if helpful.

STOP. The class will discuss these questions before continuing.

What are the properties of the signal?

Still looking at the signal for the letter j, answer the following questions.

3. The keyboard signal should look like a series of short pulses, either representing 1 (lower voltage) or 0 (higher equilibrium voltage). Using these definitions of 1 and 0, write down the binary code for the letter j.

4. How many bits does the code for j appear to require?

5. The horizontal scale of the simulated oscilloscope represents time. The time per centimeter division appears at the top of the screen. It will probably be 250ms/div, meaning that one centimeter division represents 250 x 10^-6 seconds. How much time does one letter take to send?

6. How many letters could be sent from the keyboard to the computer in one minute? Compare this to typical typing speeds.

7. The number you calculated in the previous question is the bitrate of your signal. Based on the coding format used by the keyboard (NRZ, RZ, . . . ) and this bitrate, approximately what bandwidth will you need to send this signal?

8. Can you determine yet WITHOUT LOOKING AT THE ASCII TABLE whether the keyboard uses ASCII?

How fast does your signal rise or fall?

With the time/division setting of 250ms, the edges of each bit probably look sharp, but no change in a physical quantity is truly instantaneous. In order to measure the time needed for the bit to change, we define a fall time and a rise time. The fall time of a signal is defined as the time for a signal to fall from 90% of its maximum value to 10% of its maximum value. Similarly, the rise time of a signal is defined as the time for a signal to rise from 10% of its maximum value to 90% of its maximum value.

9. This version of the simulation does not permit the time/division to be changed. If it did, you could change the horizontal scale so that one bit filled the entire screen. When the bit fills the screen, do you think the fall and the rise still appear instantaneous? Sketch what you would expect to see on your paper.

STOP. The class will discuss this question before continuing.

10.	Your on-line reading (based on additional parts of Grant's book) says that the rise time and fall time of a bit must each be less than 70% of the total width of the bit to avoid distortion or blending of bits. Is this condition adequately met by the keyboard signal?
11.	Is the distortion of your bit (due to the rise and fall times) symmetric? What consequences might this have for transferring signals?

How do signals from different keys compare?

If you get this far in the activity, answer the following questions about the signals from different keys.

12. Compare the signals representing lower-case and capital letters. How does the computer know the difference between j and J based on the signals sent from the keyboard?

13. Some keys, such as the number keys, appear twice on the keyboard. Do these repeated keys send the same signal or different signals? Was your earlier prediction correct?

14. Using 1 to represent the lower voltage and 0 for the higher (equilibrium) voltage, record the codes for the following letters: e, r, t, y, u, i, f, g, h, j.

15. Does each letter use the same number of bits? How can you tell?

16. A series of bits representing the signal from a keyboard to a computer is given below, with repeated 0s representing the flat null signal between keystrokes. What sequence of letters has been typed? 00010100101100001001111011000011101001110000100110011000001110010111000

17. Look at the ASCII table (copied below). Based on your answers in questions 12, 14, and 15, is the keyboard signal related to ASCII? Justify your answer, using information from all three specified questions.

Table of ASCII representations for keyboard symbols

BITS	000	001	010	011	100	101	110	111
0000	NUL	DLE	SPACE	0	@	P	`	p
0001	SOH	DC1	!	1	A	Q	a	q
0010	STX	DC2	"	2	B	R	b	r
0011	ETX	DC3	#	3	C	S	c	s
0100	EOT	DC4	$	4	D	T	d	t
0101	ENQ	NAK	%	5	E	U	e	u
0110	ACK	SYN	&	6	F	V	f	v
0111	BEL	ETB	'	7	G	W	g	w
1000	BS	CAN	(	8	H	X	h	x
1001	HT	EM	)	9	I	Y	i	y
1010	LF	SUB	*	:	J	Z	j	z
1011	VT	ESC	+	;	K	LEFTSQUARE	k	{
1100	FF	FS	,	<	L	\	l	\|
1101	CR	GS	-	=	M	RIGHTSQUARE	m	}
1110	SO	RS	.		N	^	n	~
1111	SI	US	/	?	)	_	o	DEL

1.	Is the signal sent from the keyboard decimal, binary, or something else? Justify your answer, using a sketch if helpful.
2.	Does the signal use NRZ, RZ, Manchester, or Bipolar coding? Justify your answer, using a sketch if helpful.

3.	The keyboard signal should look like a series of short pulses, either representing 1 (lower voltage) or 0 (higher equilibrium voltage). Using these definitions of 1 and 0, write down the binary code for the letter j.
4.	How many bits does the code for j appear to require?
5.	The horizontal scale of the simulated oscilloscope represents time. The time per centimeter division appears at the top of the screen. It will probably be 250ms/div, meaning that one centimeter division represents 250 x 10^-6 seconds. How much time does one letter take to send?
6.	How many letters could be sent from the keyboard to the computer in one minute? Compare this to typical typing speeds.
7.	The number you calculated in the previous question is the bitrate of your signal. Based on the coding format used by the keyboard (NRZ, RZ, . . . ) and this bitrate, approximately what bandwidth will you need to send this signal?
8.	Can you determine yet WITHOUT LOOKING AT THE ASCII TABLE whether the keyboard uses ASCII?

12.	Compare the signals representing lower-case and capital letters. How does the computer know the difference between j and J based on the signals sent from the keyboard?
13.	Some keys, such as the number keys, appear twice on the keyboard. Do these repeated keys send the same signal or different signals? Was your earlier prediction correct?
14.	Using 1 to represent the lower voltage and 0 for the higher (equilibrium) voltage, record the codes for the following letters: e, r, t, y, u, i, f, g, h, j.
15.	Does each letter use the same number of bits? How can you tell?
16.	A series of bits representing the signal from a keyboard to a computer is given below, with repeated 0s representing the flat null signal between keystrokes. What sequence of letters has been typed? 00010100101100001001111011000011101001110000100110011000001110010111000
17.	Look at the ASCII table (copied below). Based on your answers in questions 12, 14, and 15, is the keyboard signal related to ASCII? Justify your answer, using information from all three specified questions.