Activity on Data signals

In this activity you will look at the signals that a computer send thorugh the serial port using an oscilloscope. Each group of students needs a computer with Hyperterminal installed, an oscilloscope and a cable.
Before You Start:
Answer the following questions to the best of your ability before doing the experiment.
What do you think a serial data signal will look like?
Which format (NRZ, RZ, Manchester, Bipolar) would you expect to be the best choice for a serial port? Why or why not?
Do you think a computer 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?

Set up your computer

In your Windows Accesories, run Hyperterminal.
When prompted for a connection name, write a name, for exmaple act6.
When prompted for a phone number, go to the box for "Connect using" and select "Direct to COM1".
Configure the port for a speed of 2400 bps, 8 data bits, No parity (none) 1 stop bit and
no handshaking (none).

Set up the oscillos
Connect your cable to CH1
Above the CH1 connector you will find a switch labeled AC GND DC. Select DC.
Set the CH1 knob so that the 5 is in the 1X mark. This will select a vertical scale of 2 Volts per division.
Set the horizontal knob to 2.0 ms. This will select a horizontal scale of 2.0 ms per division.
Make sure the Trigger Level knob is in it's center position. Your instructor will tell you about additional settings for triggering.
Plug your computer and press any character. You should see the bits for that character appear in the scope screen.

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 computer 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 voltage). Using these definitions of 1 and 0, write down the binary code for the letter j.
Keep in mind that the first bit is allways a start bit (higher voltage level), followed by 8 data bits. The final bit is a stop bit. You should count 10 bits. The first one allways up and the last one allways down.

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

5. How much time does one letter take to send? What is the width of each bit? Compare it to your configuration of 2400 bits per second.

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 serial port (NRZ, RZ, . . . ) and this bitrate, approximately what bandwidth will you need to send the characters at typical typing speeds?

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

How fast does your signal rise or fall (optional) ?

With the time/division setting of 0.5 ms, 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. Set up a new hyperterminal connection at the highest speed your computer allow. Set the horizontal scale so that two bits fill the entire screen. When the bits fills the screen, do you think the fall and the rise still appear instantaneous? Sketch what you 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 serial port 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 (optional)?

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. Using 1 to represent the lower voltage and 0 for the higher voltage, record the codes for the following letters: e, r, t, y, u, i, f, g, h, j.
Keep in mind that the least significant bits come first and a 1 is a low voltage level.

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

15. Look at the ASCII table (copied below). Based on your answers in questions 12, 13, and 14, 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 computer 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 voltage). Using these definitions of 1 and 0, write down the binary code for the letter j. Keep in mind that the first bit is allways a start bit (higher voltage level), followed by 8 data bits. The final bit is a stop bit. You should count 10 bits. The first one allways up and the last one allways down.
4.	How many bits does the code for j appear to require?
5.	How much time does one letter take to send? What is the width of each bit? Compare it to your configuration of 2400 bits per second.
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 serial port (NRZ, RZ, . . . ) and this bitrate, approximately what bandwidth will you need to send the characters at typical typing speeds?
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.	Using 1 to represent the lower voltage and 0 for the higher voltage, record the codes for the following letters: e, r, t, y, u, i, f, g, h, j. Keep in mind that the least significant bits come first and a 1 is a low voltage level.
14.	Does each letter use the same number of bits? How can you tell?
15.	Look at the ASCII table (copied below). Based on your answers in questions 12, 13, and 14, is the keyboard signal related to ASCII? Justify your answer, using information from all three specified questions.