Step 1: Understand RS-232 Connections &
Signals
RS-232C, EIA RS-232, or simply
RS-232, refers to the same standard defined by the Electronic
Industries Association in 1969 for serial communication.
DTE and DCE
DTE stands for Data Terminal
Equipment. A computer is a DTE. DCE stands for Data Communication
Equipment. A modem is a DCE.
DTE normally
comes with a Male Connector, while DCE comes with a Female
Connector. However, that is not always true. Use the simple
way below to confirm:
Measure Pin 3 and
Pin 5 of a DB-9 Connector with a Volt Meter, if you get a voltage
of -3V to -15V, then it is a DTE device. If the voltage is on Pin
2, then it is a DCE device.
Note: The result for a DB-25 Connector is reversed (Please refer
to DB-9 to DB-25 conversion table below).
RS-232 Pinouts
(DB-9)
 |
A male DB-9
connector viewed from the front. Reverse or back view of male
connector for Female Connector. |
| |
|
DTE Pin Assignment (DB-9) |
DCE Pin
Assignment (DB-9) |
| 1 |
DCD |
Data Carrier
Detect |
1 |
DCD |
Data Carrier
Detect |
| 2 |
RxD |
Receive Data |
2 |
TxD |
Transmit Data |
| 3 |
TxD |
Transmit Data |
3 |
RxD |
Receive Data |
| 4 |
DTR |
Data Terminal
Ready |
4 |
DSR |
Data Set Ready |
| 5 |
GND |
Ground (Signal) |
5 |
GND |
Ground (Signal) |
| 6 |
DSR |
Data Set Ready |
6 |
DTR |
Data Terminal
Ready |
| 7 |
RTS |
Request to Send |
7 |
CTS |
Clear to Send |
| 8 |
CTS |
Clear to Send |
8 |
RTS |
Request to Send |
| 9 |
RI |
Ring Indicator |
9 |
RI |
Ring Indicator |
DB-9 to DB-25
Conversion
|
DB-9 |
DB-25 |
Function |
| 1 |
8 |
DCD |
Data Carrier
Detect |
| 2 |
3 |
RxD |
Receive Data |
| 3 |
2 |
TxD |
Transmit Data |
| 4 |
20 |
DTR |
Data Terminal
Ready |
| 5 |
7 |
GND |
Ground (Signal) |
| 6 |
6 |
DSR |
Data Set Ready |
| 7 |
4 |
RTS |
Request to Send |
| 8 |
5 |
CTS |
Clear to Send |
| 9 |
22 |
RI |
Ring Indicator |
RS-232 Connections
A straight-through cable is used
to connect a DTE (e.g. computer) to a DCE (e.g. modem), all
signals in one side connected to the corresponding signals in the
other side in a one-to-one basis.
A crossover (null-modem) cable is
used to connect two DTE directly, without a modem in between.
They cross transmit and receive data signals between the two sides
and there are many variations on how the other control signals are
wired, below is one of them:
|
Straight-through
(DB-9) |
|
Crossover
(Null-Modem) (DB-9) |
|
(DTE) |
|
(DCE) |
|
(DTE) |
|
(DTE) |
| 1 |
DCD |
------- |
DCD |
1 |
|
1 |
DCD |
|
DCD |
1 |
| 2 |
RxD |
------- |
TxD |
2 |
|
2 |
RxD |
------- |
TxD |
3 |
| 3 |
TxD |
------- |
RxD |
3 |
|
3 |
TxD |
------- |
RxD |
2 |
| 4 |
DTR |
------- |
DSR |
4 |
|
4 |
DTR |
------- |
DSR |
6 |
| 5 |
GND |
------- |
GND |
5 |
|
5 |
GND |
------- |
GND |
5 |
| 6 |
DSR |
------- |
DTR |
6 |
|
6 |
DSR |
------- |
DTR |
4 |
| 7 |
RTS |
------- |
CTS |
7 |
|
7 |
RTS |
------- |
CTS |
8 |
| 8 |
CTS |
------- |
RTS |
8 |
|
8 |
CTS |
------- |
RTS |
7 |
| 9 |
RI |
------- |
RI |
9 |
|
9 |
RI |
|
RI |
9 |
| |
|
 |
Null-Modem (Model: CVT-Null-1) |
RS-232 Signals
RS-232 Logic Waveform (8N1)
The graphic above illustrates a
typical RS-232 logic waveform (Data format: 1 Start bit, 8 Data
bits, No Parity, 1 Stop bit). The data transmission starts with a
Start bit, followed by the data bits (LSB sent first and MSB sent
last), and ends with a "Stop" bit.
The voltage of Logic "1" (Mark)
is between -3VDC to -15VDC, while the Logic "0" (Space) is between
+3VDC to +15VDC.
RS-232 connects the Ground of 2
different devices together, which is the so-called "Unbalanced"
connection. An unbalanced connection is more susceptible to noise,
and has a distance limitation of 50 ft (which is around 15 meters).
Step 2:
Learn about the Protocol
Step 3: Start controlling your RS-232 devices
by using 232Analyzer
|
