When transmitting and receiving data across telephone or other circuits, the equipment which actually generates and uses the data (e.g. a computer or VDU terminal) is known as data terminating equipment (DTE). The equipment which terminates the telephone line and converts the basic data signals into signals which can be transmitted is known as data circuit-terminating equipment (DCE). As far as the user is concerned the interface between DTE and DCE is the most important. ITU-T recommendation V24 defines the signal interchanges and functions between DTE and DCE; these are commonly known as the 100 series interchanges circuits:
Interchange circuit Data Control Timing101 Protective ground
or earth
102 Signal ground or
common return
103 Transmitted data •
104 Received data •
105 Request to send •
106 Ready for •
sending
107 Data set ready •
108/1 Connect data set •
to line
108/2 Data terminal •
ready
109 Data channel •
received line
signal detector
110 Signal quality •
detector
111 Data signalling •
rate selector
(DTE)
112 Data signalling •
rate selector
(DCE)
113 Transmitter signal • element timing
(DTE)
114 Transmitter signal • element timing
(DCE)
115 Receiver signal • element timing
(DCE)
116 Select stand by •
117 Standby indicator •
118 Transmitted •
backward
channel data
119 Received •
backward
channel data
120 Transmit • backward
channel line
signal
121 Backward •
channel ready
122 Backward •
channel
received line
signal detector
123 Backward •
channel single
quality detector
124 Select frequency • groups
125 Calling indicator •
126 Select transmit • frequency
127 Select receive • frequency
128 Receiver signal • element timing
(DTE)
129 Request to •
receive
130 Transmit •
backward tone
131 Received • character
timing
132 Return to •
non-data mode
133 Ready for •
receiving
134 Received data •
present
191 Transmitted voice •
answer
192 Received voice •
answer
The connectors used with 100 series interchange circuits and its pin assignments are defined by international standard ISO 2110 and are (for modems following the ITU-T recommendations V21, V23, V26, V26bis, V27 and V27bis) as follows:
Interchange circuit numbers1 number∗1∗1∗1 Ground
2 103 103 103 TXD
3 104 104 104 RXD
4 105 105 105 RTS
5 106 106 106 RFS
6 107 107 107 DSR
7 102 102 102 Signal return
8 109 109 109 Signal DET
9∗N∗N∗N
10∗N∗N∗N
11 126∗N∗NSTF 12∗F 122 122
13∗F 121 121
14∗F 118 118
15∗F∗2 114
16∗F 119 119
17∗F∗2 115
18 141 141 141
19∗F 120 120
20 108/1-2 108/1-2 108/1-2 DTR 21 140 140 140
22 125 125 125 Call ind. 23∗N 111 111
24∗N∗N 113
25 142 142 142
Notes :
∗1 Pin 1 is assigned for connecting the shields between tandem sections of shielded cables. It may be connected to protective ground or signal ground.
∗F Reserved for future use.
∗N Reserved for national use.
1 number DCD Data carrier detect
2 RXD Receive dataDescription
3 TXD Transmit data
4 DTR Data terminal ready
5 GND Ground
6 DSR Data set ready
7 RTS Ready to send
8 CTS Clear to send
9 RI Ring indicator
A similar series of interchange circuits is defined in ITU-T recommendation V25 for automatic calling answering between modems over the telephone network. This is the 200 series interchange circuits:
Interchange circuit201 Signal ground ••
202 Call request •
203 Data line occupied •
204 Distant station connected •
205 Abandon call •
206 Digit signal (20) •
207 Digit signal (21) •
208 Digit signal (22) •
209 Digit signal (23) •
210 Present next digit •
211 Digit present •
213 Power indication •
The EIA equivalent of CCITT V24 interface is the RS 232C specification, which similarly defines the electrical interface between DTE and DCE. Although the two have different designations, they are to all practical purposes equivalent. The RS 232C interchange circuits are:
Interchange circuit Data Control TimingAA Protective ground
AB Signal
ground/common
return
BA Transmitted data •
BB Received data •
CA Request to send •
CB Clear to send •
CC Data set ready •
CD Data terminal ready •
CE Ring indicator •
CF Received line signal •
detector
CG Signal quality •
detector
CH Data signal rate •
selector (DTE)
CI Data signal rate •
selector (DCE)
DA Transmitter signal • element timing
(DTE)
DB Transmitter signal • element timing
(DCE)
DD Receiver signal • element timing
(DCE)
SBA Secondary •
transmitted data
SBB Secondary received •
data
SCA Secondary request to • send
SCB Secondary clear to • send
SCF Secondary received • line signal
detector
The EIA RS 232C standard, although the most common, is by no means perfect. One of its main limitations is the maximum data rate −18.2 K baud. Various improved interchange circuits (RS 422, RS 423) have been developed. The RS 449 standard is capable of very fast data rates (up to 2 Mbaud):
Interchange circuit Data Control TimingMnemonic SG Single ground
SC Send common
RC Receive common
IS Terminal in service • IC Incoming call • TR Terminal ready • ñ MnemonicDM Data mode •
SD Send data •
RD Receive data •
TT Terminal timing • ST Send timing •
RT Receive timing •ô
ò RS Request to send • CS Clear to send •
RR Receiver ready •
SQ Signal quality •
NS News signal • SF Select frequency •
SR Signalling rate selector •ô
ó SI Signalling rate indicator •ñ SSD Secondary send data •ô
SRS Secondary request to send •
ô SCS Secondary clear to send •ó SRR Secondary receiver ready •
LL Local loopback • RL Remote loopback • TM Test mode •
SS Select standby • SB Standby indicator •
Most personal computers use the Centronics parallel data transfer to a printer. The pin connections of the connector, abbreviations and signal descriptions are shown.
All signals are standard TTL, although not all signals necessarily exist in any given interface.number eviation
1 STROBE
2 DATA1
3 DATA2
4 DATA3
5 DATA4
6 DATA5
7 DATA6
8 DATA7
9 DATA8
10 ACKNLG
11 BUSY
12 PE
13 SLCT
14 AUTO FEED XT
15 NC
16 OV
17 CHASSIS GND
18 NC
19 to 30 GND
31 INIT
32 ERROR
33 GND
34 NC
35 Logic 1
36 SLCT IN
Strobe
Data line 1
Data line 2
Data line 3
Data line 4
Data line 5
Data line 6
Data line 7
Data line 8
Acknowledge data
Busy
Paper end
Select printer
Automatic line feed at end of line
No connection
Logic ground
Printer chassis (not
necessarily the same as logic ground)
No connection
Single ground
Initialize
Error
Signal ground
No connection
Logic 1
Select input to printer