Introduction to Networking: and Data Communications
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Chapter 17. Standard Digital Codes

Computers process information in digital form. Characters are assigned a 7 or 8 bit code to indicate its character. This 7 or 8 bit code becomes a number (usually hexadecimal) that the computer can work with. The characters stored in a computer include the following:

Lower case letters:	a - z
Upper case letters:	A - Z
Digits:	0 - 9
Punctuation Marks:	. , ; : ! ? etc...
Unit Symbols:	# $ % & * etc...
Control Codes:	EOF, etc..

There are 2 major codes existing today: ASCII (pronounced ah-skee) and EBCDIC (pronounced eb-ce-dic).

EBCDIC - Extended Binary Coded Decimal Interchange Code

EBCDIC is used mainly by IBM mainframes and compatibles. It is not common in the PC LAN world unless you are connecting to the IBM mainframe world. In order to connect, you would require either an IBM 3270 terminal emulation program or a device called a gateway.

Table 18-1 shows the EBCDIC translation table. Computers speak in the binary code of 1s and 0s: they do not recognize the letter "A." Instead, they refer to the letter "A" as the binary number 1100 0001. It is not easy for humans to remember binary numbers such as 1100 0001, but it is easier to remember the hexadecimal number C1. The hexadecimal number C1 is equal to the binary number 1100 0001.

The hexadecimal number C1 is equal to the decimal number 193. Table 18-1 shows both the decimal (dec) number and the hexadecimal (hex) number for the capital letter "A". Lower case "a" is represented by the EBCDIC decimal code 129 (or, hexadecimal code 81).

Besides character codes like the previous letter "A", the EBCDIC code also defines control characters. These are characters that have special meaning. For example, the control character FF stands for Form Feed, and is used by printers to either advance a page or to eject a page. The decimal code for FF is 12 and the hexadecimal code is C.

Both hexadecimal and decimal codes are indicated, because in many instances, a program or interface will report the EBCDIC code in one of the formats. You may have to use Table 18-1 to translate from the numerical code to the actual character.

Note: Some EBCDIC codes are not defined--and have no name.

Dec	Hex	Name	Dec	Hex	Name	Dec	Hex	Name	Dec	Hex	Name
0	0	NUL	32	20	DS	64	40	RSP	96	60	-
1	1	SOH	33	21	SOS	65	41	97	61	/
2	2	STX	34	22	FS	66	42	98	62
3	3	ETX	35	23	WUS	67	43	99	63
4	4	SEL	36	24	BYP	68	44	100	64
5	5	HT	37	25	LF	69	45	101	65
6	6	RNL	38	26	ETB	70	46	102	66
7	7	DEL	39	27	ESC	71	47	103	67
8	8	GE	40	28	SA	72	48	104	68
9	9	SPS	41	29	SFE	73	49	105	69
10	A	RPT	42	2A	SM	74	4A	¢	106	6A	\|
11	B	VT	43	2B	CSP	75	4B	.	107	6B	,
12	C	FF	44	2C	MFA	76	4C	108	6C	%
13	D	CR	45	2D	ENQ	77	4D	(	109	6D	-
14	E	SO	46	2E	ACK	78	4E	+	110	6E	>
15	F	SI	47	2F	BEL	79	4F	ê	111	6F	?
16	10	DLE	48	30	80	50	&	112	70
17	11	DC1	49	31	81	51	113	71
18	12	DC2	50	32	SYN	82	52	114	72
19	13	DC3	51	33	IR	83	53	115	73
20	14	RES	52	34	PP	84	54	116	74
21	15	NL	53	35	TRN	85	55	117	75
22	16	BS	54	36	NBS	86	56	118	76
23	17	POC	55	37	EOT	87	57	119	77
24	18	CAN	56	38	SBS	88	58	120	78
25	19	EM	57	39	IT	89	59	121	79	`
26	1A	UBS	58	3A	RFF	90	5A	!	122	7A	:
27	1B	CU1	59	3B	CU3	91	5B	$	123	7B	#
28	1C	IFS	60	3C	NAK	92	5C	*	124	7C	@
29	1D	IGS	61	3D	93	5D	)	125	7D	'
30	1E	IRS	62	3E	SUB	94	5E	;	126	7E	=
31	1F	IUS	63	3F	SP	95	5F	ù	127	7F	"

Table 18-1 EBCDIC code

Dec	Hex	Name	Dec	Hex	Name	Dec	Hex	Name	Dec	Hex	Name
128	80	160	A0	192	C0	{	224	E0	\
129	81	a	161	A1	~	193	C1	A	225	E1	NSP
130	82	b	162	A2	s	194	C2	B	226	E2	S
131	83	c	163	A3	t	195	C3	C	227	E3	T
132	84	d	164	A4	u	196	C4	D	228	E4	U
133	85	e	165	A5	v	197	C5	E	229	E5	V
134	86	f	166	A6	w	198	C6	F	230	E6	W
135	87	g	167	A7	x	199	C7	G	231	E7	X
136	88	h	168	A8	y	200	C8	H	232	E8	Y
137	89	i	169	A9	z	201	C9	I	233	E9	Z
138	8A	170	AA	202	CA	SHY	234	EA
139	8B	171	AB	203	CB	235	EB
140	8C	172	AC	204	CC	236	EC
141	8D	173	AD	205	CD	237	ED
142	8E	174	AE	206	CE	238	EE
143	8F	175	AF	207	CF	239	EF
144	90	176	B0	208	D0	}	240	F0	0
145	91	j	177	B1	209	D1	J	241	F1	1
146	92	k	178	B2	210	D2	K	242	F2	2
147	93	l	179	B3	211	D3	L	243	F3	3
148	94	m	180	B4	212	D4	M	244	F4	4
149	95	n	181	B5	213	D5	N	245	F5	5
150	96	o	182	B6	214	D6	O	246	F6	6
151	97	p	183	B7	215	D7	P	247	F7	7
152	98	q	184	B8	216	D8	Q	248	F8	8
153	99	r	185	B9	217	D9	R	249	F9	9
154	9A	186	BA	218	DA	250	FA
155	9B	187	BB	219	DB	251	FB
156	9C	188	BC	220	DC	252	FC
157	9D	189	BD	221	DD	253	FD
158	9E	190	BE	222	DE	254	FE
159	9F	191	BF	223	DF	255	FF	EO

Table 18-1 EBCDIC code (cont'd)

ASCII - American Standard Code for Information Interchange

ASCII is the most popular code and it is used by the majority of the computing world. ASCII itself is a 7 bit code that allows only 128 characters (27). Most applications follow IBM's Extended ASCII code, which uses 8 bits and allows an addition 128 graphic characters for a total of 256 characters (28). We will be concentrating on 7 bit ASCII codes.

Format effectors

Format effectors control the movement of the cursor on the screen and the print head in a printer. The format effectors are shown below.

BS	Backspace
HT	Horizontal Tab
LF	Line Feed
CR	Carriage Return
FF	Form Feed
VT	Vertical Tab

Communication Controls

Communication Controls are used in controlling data transmission over a communication network. They are used in both Asynchronous and Synchronous Transmissions, and are used in "handshaking".

STX	Start of Text
ETX	End of Text
EOT	End of Transmission
ENQ	End of Inquiry
ACK	Acknowledge
NAK	Negative Acknowledge
EXT	Interrupt
SYN	Synchronous idle
ETB	End of Block
EOF	End of File

Information Separators

Information separators are used to separate database enquiries and files as follows:

FS	File Separator(in a PC - used as cursor R, L, U, D)
GS	Group Separator
RS	Record Separator
US	Unit Separator

Additional Control Codes

Of the remaining codes used by the computer, the most important ones are shown below.

NUL	Nothing character
BEL	Rings the bell!
DC1 - 4	Device Control 1 - 4
ESC	Escape - used for formatting printers & terminals
DEL	Delete - deletes characters under cursor

DC1 & DC2 are used in the Xon/Xoff software handshaking to control data transfer.

Displaying ASCII codes directly to the screen

You can type the ASCII codes directly to the screen on IBM compatible computers: you press the "ALT" key and a 3 digit number on the numeric keypad. The 3 digit number is the ASCII decimal code for the character. You must use the numeric keypad because the QWERTY numbers will NOT work.

For example, the character "A" corresponds to the ASCII decimal code 65. To access the ASCII code directly, hold down the ALT key and type in 065 on the numeric keypad. The letter A will appear on the screen when you release the ALT key.

Table 18-2 shows the ASCII codes according to decimal numbers and hexadecimal numbers. If a network sniffer or analyzer is used it will show raw data in decimal or hexadecimal formats. You may have to perform a manual translation using Table 18-2.

Introduction to Data Communications

Previous	ASCII - American Standard Code for Information Interchange (cont'd)	Next Dec	Hex	Name	Dec	Hex	Name	Dec	Hex	Name	Dec	Hex	Name
0	0	NUL	32	20	Space	64	40	@	96	60	`
1	1	SOH	33	21	!	65	41	A	97	61	a
2	2	STX	34	22	"	66	42	B	98	62	b
3	3	ETX	35	23	#	67	43	C	99	63	c
4	4	EOT	36	24	$	68	44	D	100	64	d
5	5	ENQ	37	25	%	69	45	E	101	65	e
6	6	ACK	38	26	&	70	46	F	102	66	f
7	7	BEL	39	27	¢	71	47	G	103	67	g
8	8	BS	40	28	(	72	48	H	104	68	h
9	9	HT	41	29	)	73	49	I	105	69	i
10	A	LF	42	2A	*	74	4A	J	106	6A	j
11	B	VT	43	2B	+	75	4B	K	107	6B	k
12	C	FF	44	2C	,	76	4C	L	108	6C	l
13	D	CR	45	2D	-	77	4D	M	109	6D	m
14	E	S0	46	2E	.	78	4E	N	110	6E	n
15	F	S1	47	2F	/	79	4F	O	111	6F	o
16	10	DLE	48	30	0	80	50	P	112	70	p
1	11	DC1	49	31	1	81	51	Q	113	71	q
18	12	DC2	50	32	2	82	52	R	114	72	r
19	13	DC3	51	33	3	83	53	S	115	73	s
20	14	DC4	52	34	4	84	54	T	116	74	t
21	15	NAK	53	35	5	85	55	U	117	75	u
22	16	SYN	54	36	6	86	56	V	118	76	v
23	17	ETB	55	37	7	87	57	W	119	77	w
24	18	CAN	56	38	8	88	58	X	120	78	x
25	19	EM	57	39	9	89	59	Y	121	79	y
26	1A	SUB	58	3A	:	90	5A	Z	122	7A	z
27	1B	ESC	59	3B	;	91	5B	[	123	7B	{
28	1C	FS	60	3C	92	5C	\	124	7C	\|
29	1D	GS	61	3D	=	93	5D	]	125	7D	}
30	1E	RS	62	3E	>	94	5E	^	126	7E	~
31	1F	US	63	3F	?	95	5F	_	127	7F	DEL

Table 18-2 ASCII code

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