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pinout (распиновка)

RS232 interface

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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  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
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
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)

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