|
|
| Voltage levels
with respect to ground represent the RS 232 signals. There is a wire for
each signal, together with the ground signal (reference for voltage
levels). This interface is useful for point-to-point communication at
slow speeds. For example, port COM1 in a PC can be used for a mouse,
port COM2 for a modem, etc. This is an example of point-to-point
communication: one port, one device. Due to the way the signals are
connected, a common ground is required. This implies limited cable
length - about 30 to 60 meters maximum. (Main problems are
interference and resistance of the cable.) Shortly, RS 232 was
designed for communication of local devices, and supports one
transmitter and one receiver.
One of the most universal parts
of the PC is its serial port. You can connect a mouse, a
modem, a printer, a plotter, another PC, etc. But its
usage (both software and hardware) is one of the best-kept secrets for
most users, besides that it is not difficult to understand how to
connect devices to it and how to program it.
Here is the list of
all signals specified in the RS232C standard. Its letters identifies
each signal. Pin number on a DB-25 and DB-9 connector and its signal
name. The circuit letters associated to each signal are devised by the
following:
|
- If the first letter is A,
this is a common circuit
- If the first letter is B,
this is a signal circuit
- If the first letter is C,
this is a control circuit
- If the first letter is D,
this is a timing circuit
- If the letters are preceded by an S, this
is a secondary channel
|
|
|
|
|
|
|
|
CIRCUIT
|
V.24 CIRCUIT #
|
DB-25 Pin #
|
DB-9 Pin #
|
SIGNAL NAME
|
|
AA
|
101
|
1
|
-
|
Protective
Ground
|
|
AB
|
102
|
7
|
5
|
Signal
Ground
|
|
BA
|
103
|
2
|
3
|
Transmitted
Data
|
|
BB
|
104
|
3
|
2
|
Received
Data
|
|
CA
|
105
|
4
|
7
|
Request
to send
|
|
CB
|
106
|
5
|
8
|
Clear
to send
|
|
CC
|
107
|
6
|
6
|
Data set
ready
|
|
CD
|
108
|
20
|
4
|
Data terminal
ready
|
|
CE
|
125
|
22
|
9
|
Ring
detector
|
|
CF
|
109
|
8
|
1
|
Carrier
detect
|
|
CG
|
110
|
21
|
-
|
Signal quality
detect
|
|
CH /
CI
|
111 /
112
|
23
|
-
|
Data signal
rate selector
|
|
DA
|
113
|
24
|
-
|
Transmitter signal
timing (DTE)
|
|
DB
|
114
|
15
|
-
|
Transmitter signal
timing (DCE)
|
|
DD
|
115
|
17
|
-
|
Receiver signal
timing
|
|
SBA
|
118
|
14
|
-
|
Secondary
TX
|
|
SBB
|
119
|
16
|
-
|
Secondary
RX
|
|
SCA
|
120
|
19
|
-
|
Secondary
RTS
|
|
SCB
|
121
|
13
|
-
|
Secondary
CTS
|
|
SCF
|
122
|
12
|
-
|
Secondary
CD
|
|
|
9
|
-
|
Reserved
Positive test
|
|
|
10
|
-
|
Reserved
Negative test
|
|
|
11
|
-
|
N/C
|
|
|
18
|
-
|
N/C
|
|
|
25
|
-
|
N/C
|
|
|
|
|
|
| AA: Protective
ground |
| This line is
connected to the power ground of the serial adapter. It
should not be used as signal ground. Connect this line to
the screen of the lead wire (if applicable). By connecting this line on
both sides you make sure that no large currents flow through the signal
ground in case of an insulation defect or other defect on either side.
On the other side, when two devices are separated by great distances you
may not wish to use its signal, because of different ground potential
and it is possible that it may carry a substantial current as a ground
loop. If it is great enough, it may cause electrical interference. |
|
| AB: Signal ground |
| This is the
logical ground, which is used as a point of reference for all signals
received or transmitted. This signal is very important and must be
present for all communications. |
|
BA: Transmitted
data  |
| This line is
used to transmit data from the DTE to the DCE. It is
maintained at a logical 1 state when nothing is
transmitted. The terminal will start to transmit when a logical 1 is
present on all of the following lines: |
- Clear To Send
- Data Terminal Ready
- Data Set Ready
- Data Carrier Detect
|
|
BB: Received
data  |
| This circuit is
used to receive data from the DCE to the DTE. The terminal will start to
transmit when a logical 1 is present on all of the following lines: |
- Request To Send
- Data Terminal Ready
- Data Set Ready
- Data Carrier Detect
|
| The standard
specifies the output levels as being -5 to -15 Volts for
logical 1 and +5 to +15 Volts for logical 0, and the input
levels as being -3 to -15 Volts for logical 1 and +3 to +15
Volts for logical 0. This ensures data bits to be read
correctly even at maximum lengths between DTE and DCE,
which are specified as 50 feet although you could probably go too much
greater distances without any problems. As you may have noticed, logical
1 are represented by a negative tension and vice versa. There's no
particularly good reason for the inversion except that it's the way
things have always been done, why change when it works! |
|
CA: Request To
Send  |
| On this line,
the DTE will send a signal when it wants to receive data from the DCE. |
|
CB: Clear To
Send  |
| Here the DCE
will send a signal when it's ready to receive data from the DTE.
(Example: When your local modem connects to another modem via telephone
lines). |
|
CC: Data Set
Ready |
| At a logical
level of 1, this line indicates to the DTE that the DCE is ready to send
data. (Ex. When a modem has established a connection with a remote
modem and is in transmission mode). |
|
CD: Data
Terminal Ready  |
| When a logical
level 1 is sent from the DTE the DCE can start to send and receive data.
When this line passes to logical level 0 the DCE will stop all
communications. (Ex. A modem would stop all communications and would
disconnect from the line, you will often see "DROP DTR" in communication
programs). |
|
CF: Data Carrier
Detect  |
| On this line the
DCE indicates to the DTE that it has established a carrier
with a remote device. |
|
CE: Ring
Indicator  |
| This line is
used mostly by communications software when the modem is
not in "auto answer" mode and will indicate to the
software that a remote device is calling. This is signal is
optional when not using software that will answer a phone call
automatically. |
|
CG: Signal
quality  |
| This line
although rarely used serves to indicate to the DTE that the quality of
the signal is poor or just not good enough to keep a good
connection. |
|
CH: Data signal
rate selector  |
| In this case
where a modem able of multiple connection rates, the DTE
could choose the speed at which it is connected. Usually
this line is kept a logical level 0 which selects the highest speed. |
|
CI: Data signal
rate selector  |
| This signal is
the same as CH but in this case the modem selects the speed at which the
DTE communicates. |
|
|
|
|
| In synchronous
mode, it is necessary to have some way to exchange clock signals, here
are three timing circuits used in the RS-232 protocol. |
DA & DB:
Transmitter signal timing
- DA: DTE towards DCE (clock
part of the DTE).
- DB: DCE towards DTE (clock part of the
DCE).
 These two circuits are used to synchronize the
flow of data. Timing is given by the DTE or DCE but never from both
at the same time. Usually data is transmitted to the modem
or it's own clock control on the DB circuit.
- DD: Receiver signal timing
DCE
- DD: DCE towards DTE (clock
part of the DCE). This circuit is used to
synchronize data received from the DTE. The clock signal received on
this line indicates to the DTE at which instant to sample the received
data on the BB line
|
|
|
|
|
|
|