25 pin D-SUB MALE connector at the DTE (Computer) | The description of RS232 is an EIA/TIA norm and is identical to CCITT V.24/V.28, X.20bis/X.21bis and ISO IS2110. The only difference is that CCITT has split the interface into its electrical description (V.28) and a mechanical part (V.24) or Asynchronous (X.20 bis) and Synchronous (X.21 bis) where the EIA/TIA describes everything under RS232.
|
Pin
|
Name
|
ITU-T
|
Dir
|
Description |
|
1 |
GND
|
101
|
|
Shield Ground
|
|
2 |
TXD
|
103
|
|
Transmit Data
|
|
3 |
RXD
|
104
|
|
Receive Data
|
|
4 |
RTS
|
105
|
|
Request to Send
|
|
5 |
CTS
|
106
|
|
Clear to Send
|
|
6 |
DSR
|
107
|
|
Data Set Ready
|
|
7 |
GND
|
102
|
|
System Ground
|
|
8 |
CD
|
109
|
|
Carrier Detect
|
|
9 |
- |
|
- |
RESERVED
|
|
10
|
- |
|
- |
RESERVED
|
|
11
|
STF
|
126
|
|
Select Transmit Channel
|
|
12
|
S.CD
|
? |
|
Secondary Carrier Detect
|
|
13
|
S.CTS
|
? |
|
Secondary Clear to Send
|
|
14
|
S.TXD
|
? |
|
Secondary Transmit Data
|
|
15
|
TCK
|
114
|
|
Transmission Signal Element Timing
|
|
16
|
S.RXD
|
? |
|
Secondary Receive Data
|
|
17
|
RCK
|
115
|
|
Receiver Signal Element Timing
|
|
18
|
LL
|
141
|
|
Local Loop Control
|
|
19
|
S.RTS
|
? |
|
Secondary Request to Send
|
|
20
|
DTR
|
108
|
|
Data Terminal Ready
|
|
21
|
RL
|
140
|
|
Remote Loop Control
|
|
22
|
RI
|
125
|
|
Ring Indicator
|
|
23
|
DSR
|
111
|
|
Data Signal Rate Selector
|
|
24
|
XCK
|
113
|
|
Transmit Signal Element Timing
|
|
25
|
TI
|
142
|
|
Test Indicator
|
The RS232 spec defines both the Mechanical, Electrical, and Functional characteristics.
RS232 is an Unbalanced (Single Ended), unidirectional (point-to-point) interface
- signal is referenced to ground. RS232 drivers feature a controlled slew rate.
Normal output levels are +5 volts. RS-232 uses Asynchronous Framing [Known data
width, 8bits] with NRZ encoding. All signals are measured in reference to a common ground, which is called the signal ground (AB). A positive voltage between 3 and 15 Vdc represents a logical 0 and a negative voltage between 3 and 15 Vdc represents a logical 1.
This switching between positive and negative is called bipolar. The zero state is not defined in RS232 and is considered a fault condition (this happens when a device is turned off).
According to the above a maximum distance of 50 ft or 15 m. can be reached at a maximum speed of 20k bps. This is according to the official specifications, the distance can be exceeded with the use of Line Drivers.
The interface is synchronous when the clocks are used (DA / DB), otherwise
its asynchronous.
RS232 is rated to operate up to 20kbps. Use TIA/EIA-562 (low voltage version
of RS232) or TIA/EIA 423 for higher data rates.
The maximum cable length is not defined, but the maximum line capacitance is;
at 2500pF, with a load impedance of 3K to 7K ohms. This produces a maximum cable
length of something less then 20 meters.
RS-232 does not define the [Layer 2] protocol used. Normally data is sent as
7 or 8 bit words [least significant bit]. A START bit marks the beginning of
the frame. The start bit is active low [RS232 drivers invert the signaling,
so it"s active high as seen on the RS232 cable; between +3v and +15v]. The figure
above shows a framed 8 bit data word [before inversion]. The data word follows
the start bit; a logic high will appear as a low voltage between -3v and -15v
when probed on the bus. A parity bit may follow the data word depending on the
protocol used. A mark parity bit [always set high] may be used, a space parity
bit [always set low] may be used, or an even/odd parity bit may be used. The
even parity bit will be a 1 if the number of ones/zeros is even, or a zero if
there are an odd number. The odd parity bit will be high if there is an odd
number of ones/zeros in the data field. No parity bit is used in the example
above. A stop bit will normally follow the data field [or parity bit if used].
The stop bit is used to bring [or insure] the signal rests at a logic high following
the end of the frame; so when the next start bit arrives it will bring the bus
from a high to low ~ remember we will invert, so on the RS232 cable the stop
bit is low and the start bit will transition low to high.
Functional description
| Description | Circuit | Function |
| Shield Ground |
AA |
Also known as protective ground. This is the chassis ground connection between DTE and DCE. |
| Signal Ground |
AB |
The reference ground between a DTE and a DCE. Has the value 0 Vdc. |
| Transmitted Data |
BA |
Data send by the DTE. |
| Received Data |
BB |
Data received by the DTE. |
| Request To Send |
CA |
Originated by the DTE to initiate transmission by the DCE. |
| Clear To Send |
CB |
Send by the DCE as a reply on the RTS after a delay in ms, which gives the DCEs enough time to energize their circuits and synchronize on basic modulation patterns. |
| DCE Ready |
CC |
Known as DSR. Originated by the DCE indicating that it is basically operating (power on, and in functional mode). |
| DTE Ready |
CD |
Known as DTR. Originated by the DTE to instruct the DCE to setup a connection. Actually it means that the DTE is up and running and ready to communicate. |
| Ring Indicator |
CE |
A signal from the DCE to the DTE that there is an incomming call (telephone is ringing). Only used on switched circuit connections. |
| Received Line Signal Detector |
CF |
Known as DCD. A signal send from DCE to its DTE to indicate that it has received a basic carrier signal from a (remote) DCE. |
Data Signal Rate Select
(DTE/DCE Source> |
CH/CI |
A control signal that can be used to change the transmission speed. |
Transmit Signal Element Timing
(DTE Source) |
DA |
Timing signals used by the DTE for transmission, where the clock is originated by the DTE and the DCE is the slave. |
Transmitter Signal Element Timing
(DCE Source) |
DB |
Timing signals used by the DTE for transmission. |
Receiver Signal Element Timing
(DCE Source) |
DD |
Timing signals used by the DTE when receiving data. |
| Local Loopback / Quality Detector |
LL |
|
| Remote Loopback |
RL/CG |
Originated by the DCE that changes state when the analog signal received from the (remote) DCE becomes marginal. |
| Test Mode |
TM |
|
| Reserved for Testing |
|
|
| 
25 pin D-SUB FEMALE connector at the DCE (Modem) |
