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25 pin D-SUB male connector layout
25 pin D-SUB male connector
at the DTE (Computer)
25 pin D-SUB female connector layout
25 pin D-SUB female connector
at the DCE (Modem)
RS-232 is the standard serial communications interface found on all types of equipment such as computers, modems, printers, Microcontrollers, eprom programmers, and a host of other devices. Identical to the CCITT V.24/V.28, X.20bis/X.21bis and ISO IS2110 standards. RS232 pinout may be varied.

RS-232 is simple, universal, well understood and supported. It was introduced in 1962, and despite rumors for its early demise, has remained widely used through the industry.

RS-232 key features

RS-232 details

RS-232 standard defines electrical signal characteristics (voltage levels, timing, signaling rate, short-circuit behavior, cable length), mechanical characteristics of RS-232 interface, RS-232 pinouts and connectors, plus some other details. RS-232 data transmission consist of time-series of bits. Both synchronous and asynchronous transmissions are supported, but asynchronous link sending seven or eight bits packets is most common configuration on PC. RS-232 devices may be classified as Data Terminal Equipment (DTE) or Data Communications Equipment (DCE) - this defines which wires will be sending and receiving each signal. The standard recommended but did not make mandatory the common D-subminiature 25 pin connector. Personal computers are usually equipped with simplified version of RS-232 interface.

RS-232 signals

The RS232 pinout signals are represented by voltage levels with respect to a system common (power / logic ground). The idle state (MARK) has the signal level negative with respect to common, and the active state (SPACE) has the signal level positive with respect to common. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol.

The RS-232 interface presupposes a common ground between the DTE and DCE. This is a reasonable assumption when a short cable connects the DTE to the DCE, but with longer lines and connections between devices that may be on different electrical busses with different grounds, this may not be true. RS232 data is bi-polar. The standard specifies a maximum open-circuit voltage of 25 volts, but common signal levels are 5 V, 10 V, 12 V, and 15 V. Circuits driving an RS-232-compatible interface must be able to withstand indefinite short circuit to ground or to any voltage level up to 25 volts. From +3 to +12 volts indicates an ON or 0-state (SPACE) condition while A -3 to -12 volts indicates an OFF 1-state (MARK) condition. Some computer equipment ignores the negative level and accepts a zero voltage level as the OFF state. In fact, the ON state may be achieved with lesser positive potential. This means circuits powered by 5 VDC are capable of driving RS232 circuits directly, however, the overall range that the RS232 signal may be transmitted/received may be dramatically reduced. The output signal level usually swings between +12V and -12V. The dead area between +3v and -3v is designed to absorb line noise. In the various RS-232-like pinout definitions this dead area may vary. For instance, the definition for V.10 has a dead area from +0.3v to -0.3v. Many receivers designed for RS-232 are sensitive to differentials of 1v or less.

RS232 pinout

Pin RS-232 pin name ITU-T Direction
DTE to DCE
RS-232 pinout Description
1 GND 101 --- Shield Ground
2 TXD 103 --> Transmit Data
3 RXD 104 <-- Receive Data
4 RTS 105 --> Request to Send. Used by the Data Terminal to signal the Data Set that it may begin sending data. The Data Set will not send out data with out this signal, active high.
5 CTS 106 <-- Clear to Send. Used by the Data Set to signal the Data Terminal that it may begin sending data. The Data Terminal will not send out data with out this signal, active high.
6 DSR 107 <-- Data Set Ready. Used by the Data Set to signal to the Data Terminal that it is ready for operation and ready to receive data, active high.
7 GND 102 --- System Ground
8 CD 109 <-- Carrier Detect. Used by the Data Set to indicate to the Data Terminal that the Data set has detected a carrier (of another device).
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. Used by the Data Terminal to signal to the Data Set that it is ready for operation, active high.
21 RL 140 --> Remote Loop Control
22 RI 125 <-- Ring Indicator. Used by the Data Set to indicate to the Data Terminal that a ringing condition has been detected.
23 DSR 111 --> Data Signal Rate Selector
24 XCK 113 --> Transmit Signal Element Timing
25 TI 142 <-- Test Indicator

RS232 pinout details

Data is transmitted and received on pins 2 and 3 respectively. Data Set Ready (DSR) is an indication from the Data Set (i.e., the modem or DSU/CSU) that it is on. Similarly, DTR indicates to the Data Set that the DTE is on. Data Carrier Detect (DCD) indicates that a good carrier is being received from the remote modem.

Pins 4 RTS (Request To Send - from the transmitting computer) and 5 CTS (Clear To Send - from the Data set) are used to control. In most Asynchronous situations, RTS and CTS are constantly on throughout the communication session. However where the DTE is connected to a multipoint line, RTS is used to turn carrier on the modem on and off. On a multipoint line, it's imperative that only one station is transmitting at a time (because they share the return phone pair). When a station wants to transmit, it raises RTS. The modem turns on carrier, typically waits a few milliseconds for carrier to stabilize, and then raises CTS. The DTE transmits when it sees CTS up. When the station has finished its transmission, it drops RTS and the modem drops CTS and carrier together.

Clock signals (pins 15, 17, & 24) are only used for synchronous communications. The modem or DSU extracts the clock from the data stream and provides a steady clock signal to the DTE. Note that the transmit and receive clock signals do not have to be the same, or even at the same baud rate.

RS232 data flow diagram

RS232 data usually is sent as a packet with 7 or 8 bit words, start, stop, parity bits (may be varied). Sample transmission shown on picture: Start bit (active low, usually between +3v and +15v) followed by data bits, parity bit (depends on protocol used) and finished by stop bit (used to bring logic high, usually between -3v and -15v).

+15V	|    0 1 0 0 0 0 0 0 1 0 1 1
	|    _   ___________   _
	|   | | |           | | |
	|   | | |           | | |
	|   | | |           | | |
	|   | | |           | | |
+3V	|   | | |           | | |
0V	|-  | | |     -     | | |      -
-3V	|   | | |           | | |
	|   | | |           | | |
	|   | | |           | | |
	|   | | |           | | |
	|---| |_|           |_| |____-----
        |
-15V	|  Start    Data       P Stop

RS-232 specifications

SPECIFICATIONS RS232
Mode of Operation SINGLE
-ENDED
Total Number of Drivers and Receivers on One Line 1 DRIVER
1 RECVR
Maximum Cable Length 50 FT.
Maximum Data Rate 20kb/s
Maximum Driver Output Voltage +/-25V
Driver Output Signal Level (Loaded Min.) Loaded +/-5V to +/-15V
Driver Output Signal Level (Unloaded Max) Unloaded +/-25V
Driver Load Impedance (Ohms) 3k to 7k
Max. Driver Current in High Z State Power On N/A
Max. Driver Current in High Z State Power Off +/-6mA @ +/-2v
Slew Rate (Max.) 30V/uS
Receiver Input Voltage Range +/-15V
Receiver Input Sensitivity +/-3V
Receiver Input Resistance (Ohms) 3k to 7k

 

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