Pinouts of Connectors

RS232 interface pinout

layout
schematic diagram

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RS232 is a serial interface. It can be found in many different applications where the most common ones are modems and Personal Computers.

25 pin D-SUB MALE connector pin-out & layout
25 pin D-SUB MALE connector  at the DTE (Computer)

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.

The RS232 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. +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. Modern 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 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.

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

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.

Some specs

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

Note: Direction is DTE (Computer) relative DCE (Modem).
Note: Do not connect SHIELD(1) to GND(7).

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25 pin D-SUB FEMALE connector pin-outs & layouts
25 pin D-SUB FEMALE connector  at the DCE (Modem)
Pinouts.ru > Pinouts of serial ports >  Pinout of RS232 interface and layout of 25 pin D-SUB MALE connector and 25 pin D-SUB FEMALE connector

Source(s): Hardware Book, Rs232 interface, from Hardware Book, Petr Krc
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