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RS232 Quick Guide
TIA/EIA-232-F Standard
RS232 conveys data over a simple unterminated, multiconductor cable at rates up to 20kB. The RS232 standard specifies the electrical characteristics
and connector for an all encompassing point-to-point modem interface. Although the original specification was intended for modems, subsequent
renderings shed unneeded signals to expand its scope and use as a general purpose serial interface at data rates up to 1MB.
Signal Pinout
DB25 DB9 Name ABBR. DTE ⇔ DCE
1 Frame Ground FG
2 3 Transmit Data TD
⇒
3 2 Receive Data RD
⇐
4 7 Request to Send RTS
⇒
5 8 Clear to Send CTS
⇐
6 6 Data Set Ready DSR
⇐
7 5 Signal Ground SG
8 1 Data Carrier Detect DCD
⇐
9 (Reserved)
10 (Reserved)
11 Unassigned
12 Sec. Carrier Detect (S) CD
⇐
13 Sec. Clear to Send (S) CTS
⇐
14 Sec. Transmit Data (S) TD
⇒
15 Transmitter Clock TC
⇐
16 Sec. Receive Data (S) RD
⇐
17 Receiver Clock RC
⇐
18 Local Loopback
⇒
19 Sec. Request to Send (S) RTS
⇒
20 4 Data Terminal Ready DTR
⇒
21 Remote Loopback
⇒
Signal Quality Detect SQ
⇐
22 9 Ring Indicator RI
⇐
23 Data Rate Select
24 Transmitter Clock (E) TC
⇒
25 Test Mode
⇐
n
The DTE ⇔ DCE column indicates data direction.
n
Pin numbers in bold indicate commonly used signals.
n
Data rate select (Pin 23) can be from DTE or DCE.
Connectors
Specification
RS232
Mode of Operation Single-Ended
Number of Drivers and Receivers Allowed on One Line 1 Driver, 1 Receiver
Maximum Cable Length 50 Feet*
Maximum Data Rate 20kB/s
Maximum Voltage Applied to Driver Output ±25V
Driver Output Signal Minimum Loaded ±5V
Maximum Unloaded ±15V
Termination 3kΩ to 7kΩ
Output Slew Rate 30V/µs (Max)
Receiver Input Voltage Range ±25V Max
Receiver Input Sensitivity ±3V
Receiver Input Resistance 3kΩ to 7kΩ
* For 2500pF cable capacitance, as per IEA 232D for data rates less than 20k baud. For data rates greater than 20k baud, C
LOAD
= 1000pF.
Character FrameRelative Signal Timing
Cable and Adapters
DTE
1
2
3
4
5
6
7
8
15
17
20
DCE
2
3
7
4
5
6
8
20
DTE
2
3
7
4
5
6
8
20
DCE
1
2
3
4
5
6
7
8
15
17
20
FG
Straight Through
Cable
Minimum Straight
Cable
TD
RD
RTS
CTS
DSR
SG
TD
RD
SG
RTS
CTS
DSR
DCD
DTR
RTS
CTS
DSR
DCD
DTR
DCD
TC (SYNC. ONLY)
RC (SYNC. ONLY)
DTR
DTE
1
7
2
3
4
5
6
8
20
DCE
1
7
2
3
4
5
20
6
8
FG
Null Modem Cable
SG
TD
RD
RTS
CTS
DSR
DCD
DTR
TD
RD
RTS
CTS
DTR
DSR
DCD
25-PIN
2
3
4
5
6
7
8
20
22
9-PIN
3
2
7
8
6
5
1
4
9
TD
AT to 25-Pin Adapter
RD
RTS
CTS
DSR
SG
DCD
DTR
RI
A minimum null modem cable is the same as a minimum straight cable except that RD and
TD (Pins 2 and 3) are cross-connected as in the null modem cable.
25 24 23 22 21 20 19 18 17 16 15 14
13 12 11 10 9 8 7 6 5 4 3 2 1
9 8 7 6
5 4 3 2 1
DB25 DB9
Views are from the pin
side of the female
(DCE) connector or
the wire side of the
male (DTE) connector.
DTR
DSR
RTS
CTS
TD
Normal timing sequences during establishment of
communications are shown below. On half-duplex
circuits, RTS is dropped as soon as the data is sent.
This is to signal a turnaround of the circuit.
START
BIT
STOP
BIT
LSB
OTHER DATA BITS
MSB = MOST SIGNIFICANT DATA BIT
LSB = LEAST SIGNIFICANT DATA BIT
OPTIONAL PARITY BIT
• • • MSB
DATA BIT
CHARACTER FRAME
PAR
