CCITT V.24 defines the functions
of the V.28 voltage signals and their category .Each pin on the D-Type ISO 2110
connector is assigned a function and given a number between 100 and 199. The
interface signals are always defined with respect to the terminal (DTE) .
Transmitted data on pin 2 of the terminal is output on pin 2 of the DTE and
goes directly as input on pin 2 of the DCE (modem). The Signal Ground on pin 7
serves as a common reference so that both devices can recognise positive and
negative voltages.
In an asynchronous environment a minimum of 3
connections are required pins 2 Transmit , 3 Receive and pin 7 Ground . A more
sophisticated interface will use pin 20 DTR , 6 DSR , 4 RTS ,5 CTS and pin 8
DCD . These signals allow DTE and DCE to keep each other informed of their
current status (Handshaking). Asynchronous means that timing/clock is part of
the data and not provided separately.
Synchronous circuits have separate
timing/clocking sources to synchronise data being transmitted.Depending on the
source chosen for clocking pins 15 ,17 or 24 are used.
CCITT V.24 Circuits
Explained
101 Protective
Ground
- Rarely used protective/shield ground extended
from DTE to DCE.
102 Signal Ground
- Common signal return/ground for all other
signals.
103 Transmit Data
- Data transmitted out from the DTE.
104 Receive Data
- Data receive input to the DTE from modem or other
equiptment.
105 Request to
Send
- When the DTE wishes to send data it turns 105 on
, in a full duplex system this circuit is permanently on.
106 Clear to Send
- Response 105 by modem (DCE) to indicate modem is
ready to send data over the modem link to the distant
modem(DCE).
107 Data Set Ready
- This circuit indicates that the DCE is connected
to line and is a response to the DTE signal DTR (108).
108 DTR / CDSTL
- There are two options for this
circuit:
- 108/2 DTR provides the terminal
with indirect control over switching the modem to line.An ON condition on this
circuit from the DTE informs the DCE that the terminal is ready to operate.This
can also be used when the modem has automatic answering facilities.When waiting
for a call 108/2 remains ON,then when the RI (125) is turned on the modem is
automatically switched to line,the modem turns 107 on.
- 108/1 CDSTL is used to give the
terminal direct control over switching the modem to the telephone line .The
call is dialled with 108/1 off and when the call is answered the circuit is
turned on by the Terminal (DTE) .The ON condition will connect the modem to
line.Turning 108/1 off disconnects the modem from line.
109 Data Carrier
Detect
- This circuit is turned ON by the DCE to indicate
it has received carrier.
111 Data Signalling
Rate Selector
- Some modems allow different speeds to be selected
(1200/75) and when so , this circuit being ON selects the higher
rate.
113 Transmit Signal
Element Timing
- Transmit Signal Element Timing DTE -->
DCE terminal provides timing.
114 Transmit Signal
Element Timing
- Transmit Signal Element Timing DCE --> DTE
modem provides timing . This is the usual usage and it should be noted that
only one of 113 or 114 can be used not both!
115 Transmit Signal
Element Timing
- Receive Signal Element Timing DCE -->
DTE.Clocks the received data on 104 and is derived from the incoming modulated
signal and is thus synchronised with the timing at the source
transmitting.
118 Transmit
Backward Channel Data
- Equivalent to Transmit Data on 103 DTE -->
DCE.
119 Receive Backward
Data Channel
- Equivalent to Receive
Data on 104, DCE --> DTE.
120 Secondary RTS
- Secondary Request to Send equivalent to 105, DTE
--> DCE.
121 Secondary CTS
- Secondary Clear to Send equivalent to 106, DCE
--> DTE.
122 Secondary DCD
- Secondary Data Carrier
Detect equivalent to 109 , DCE --> DTE.
125 Ring
Indicator
- This circuit reacts to an incoming call to the
modem ,ie it is on when the ringing current is being sent by an exchange to the
modem..
126 Select Transmit
Frequency
- Designed for old V.21 200/300 baud modems to
select which speed this side would use to transmit / modulate its
data.
140 Initiate Remote
Digital Loop
- ON requests the remote modem to provide a Digital
Loopback if it has this option enabled.
141 Initiate Local
Analogue Loop
- ON requests the locally connected modem to
provide an analog loop.
142 Test
Indicator
- ON indicates the local modem is in test
mode!
|
