RS-232 Bus Description and EIA232 Pinout

EIA-232 Bus

Interface Between Data Terminal Equipment and Data Circuit Terminating Equipment Employing Serial Binary Data Interchange

[RS232 Description] [RS232 Standard Organizations]
[RS232 Interface ICs] [RS232 Pin-Out] ['D' Connectors]
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EIA-232 Description

The RS232 specification 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. The RS-232 interface uses Asynchronous Framing [Known data width, 8bits] with NRZ encoding.

The RS232 interface is synchronous when the clocks are used (DA / DB), otherwise its asynchronous.

The RS232 interface 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.

The RS-232 interface 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. Normally an even parity generator circuit will produce a logic '1' at its output if the data word contains an odd number of ones. When the data word contains an even number of ones then the output of the parity generator will be low. 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. Remember TIA/EIA232 does not define the protocol or parity. Additional comments about the future of RS232, or here; the future of RS232.

RS-232 Voltage Levels

Cable Length
EIA-232 Cable Distance vs. Bus Speed
Graph extracted from a Texas Instruments "TI" Application note

{RS232 Bus Index}

EIA/TIA-232-F Standard Organizations

EIA/TIA-232-F Interface Between Data Terminal Equipment and Data Circuit Terminating Equipment Employing Serial Binary Data Interchange

EIA/TIA-561 8 Position Non-Synchronous Interface Between Data Terminal Equipment and Data Circuit Terminating Equipment Employing Serial Binary Data Interchange

EIA/TIA-574 9 Position Non-Synchronous Interface Between Data Terminal Equipment and Data Circuit Terminating Equipment Employing Serial Binary Data Interchange

Electronic Industries Alliance {EIA}
Telecommunications Industry Association {TIA}

V.28: Electrical characteristics for unbalanced double-current interchange circuits

ITU {International Telecommunication Union}

TIA/EIA-694 is another electrical standard which is very similar to TIA/EIA-232-F, but supports higher data rates (512 kbps). It is an electrical only standard, which is intended to be referenced by complete standards, such as TIA/EIA-723. TIA/ EIA-694 specifies an unbalanced, unidirectional, point-to-point interface. This standard supports inter-operability with TIA/EIA-232-F devices.

MIL-STD-188C [Military Standard 188C] is similar to TIA/ EIA-232-F as an unbalanced point-to-point interface. MIL-STD-188C uses a maximum driver output of 7 volts.

EIA-334-A defines signal quality terms for synchronous serial DTE/DCE interfaces. This standard is referenced by the complete synchronous standards.
EIA-363 defines signal quality terms for non-synchronous serial DTE/DCE interfaces. This standard is referenced by the complete non-synchronous standards.
EIA-404-A defines signal quality for start-stop non-synchronous DTE/DCE interfaces.

{RS232 Bus Index}

EIA-232 Bus Pin-Out

The RS232 specification only defines the pin-out for a 25 pin D-sub connector; how ever, the 9 pin is used more often (defined by EIA-574). A 26 pin connector is also called out in the RS232 spec. EIA-561 calls out an 8 pin connector (RJ-45). RS232C indicates a DB25, RS232D indicates an RJ45. Although this site, in general, calls out DB9 and DB25 style connectors, the real term for a 9-pin D-sub is DE9. A listing of Dsub connector manufacturers is listed on the D-Sub Connector Manufacturers page.
D-Sub Shell Dimensions, D-Sub Insert Arrangments Many RS232 pinouts listed here are also listed on the RS232 Pinout page.

**RS232 DB-9 Connector Pin Out**
Pin #	Signal Name	Signal Description
1	CD	Carrier Detect
2	RXD	Receive Data
3	TXD	Transmit Data
4	DTR	Data Terminal Ready
5	GND	Signal Ground / Common
6	DSR	Data Set Ready
7	RTS	Request To Send
8	CTS	Clear To Send
9	RI	Ring Indicator

**RS232 Pinout, DB-25 Connector Signal Assignments**
Pin #	Signal Name	Signal Function	Pin #	Signal Name	Signal Function
1	----	Protective Ground	14	SBA	2nd Transmitted Data
2	TXD	Transmitted Data	15	DB	DCE Element Timing
3	RXD	Receive Data	16	SBB	2nd Received Data
4	RTS	Request To Send	7	DD	Received Element Timing
5	CTS	Clear To Send	18	-----	Unassigned
6	DSR	Data Set Ready	19	SCA	2nd Request To Send
7	GND	Signal Ground/Common	20	DTR	Data Terminal Ready
8	CD	Carrier Detect	21	CG	Signal Quality Detector
9	---	+Voltage	22	RI	Ring Detector
10	---	-Voltage	23	CH/CI	Data Signal Rate Detector
11	---	----	24	DA	DTE Element Timing
12	SCF	2nd Line Detector	25	----	Unassigned
13	SCB	2nd Clear To Send	--	---	----

**RS232 RJ-45 Connector Pin Out**
Pin #	Signal Name	Signal Description
1	RI	Ring Indicator
2	DCD	Carrier Detect
3	DTR	Data Terminal Ready
4	GND	Signal Ground / Common
5	RXD	Receive Data
6	TXD	Transmit Data
7	CTS	Clear To Send
8	RTS	Request To Send

TXD: Transmit Data; The data sent from the Data Terminal and received by the Data Set. RXD: Receive Data; The data sent from the Data Set and received by the Data Terminal. DTR: Data Terminal Ready; Used by the Data Terminal to signal to the Data Set that it is ready for operation, active high. DSR: 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. RTS: 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. CTS: 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. CD: Carrier Detect; Used by the Data Set to indicate to the Data Terminal that the Data set has detected a carrier (of another device). RI: Ring Indicator; Used by the Data Set to indicate to the Data Terminal that a ringing condition has been detected. GND: Ground; The common return for all signals on the interface. When using Software Flow Control(XON, XOFF); you only need 3 lines, TX (data), RX (data), and GND. XON being equal to ready, XOFF equal to not ready.

**RS232 RJ-45 - 8 and 4 wire Pin Out**
DB9, DTE		RJ45		DB9, DCE		******	DB25		RJ45		DB9
Pin	Signal	Pin	Signal	Pin	Signal	******	Pin	Signal	Pin	Signal	Pin	Signal
7	RTS	1	DCD	1	DCD	******	4	RTS	1	DCD	7	RTS
8	CTS	2	RTS	7	RTS	******	N/A	N/A	2	N/A	N/A	N/A
Shell	GND	3	GND	Shell	GND	******	Shell	GND	3	GND	Shell	GND
3	RxD	4	TxD	3	TxD	******	3	RxD	4	TxD	2	RxD
2	TxD	5	RxD	4	RxD	******	2	TxD	5	RxD	3	TxD
5	GND	6	GND	5	GND	******	7	GND	6	GND	5	GND
4	DTR	7	CTS	8	CTS	******	N/A	N/A	7	N/A	N/A	N/A
6	DSR	8	DTR	4	DTR	******	N/A	N/A	8	N/A	N/A	N/A
RJ45 to DTE, DB9				N/A		******	RJ45 to DB25				N/A
N/A		RJ45 to DCE, DB9				******	N/A		RJ45 to DB9
@ 8 wire 8 pin RJ45						******	@ 4 wire 8 pin RJ45

Computers or peripherals which use serial cables for their communication are split into two categories. These are DCE (Data Communications Equipment) and DTE (Data Terminal Equipment.) Data Communications Equipment are peripheral devices such as a modem, or plotter while Data Terminal Equipment is the Computer or Terminal.

An RS232 Null Modem cable is used to connect two DTE devices together. The table below provides the connector pinout used on each of a Null Modem cable. A Null Modem cable only really requires three wires; TD to RD, RD to TD, and SG to SG. The other pins may be crossed at each terminating connector [DTR to DSR, CD] and [RTS to CTS]. A LoopBack plug may be constructed by tying TD to RD, DTR to DSR/CD, and RTS to CTS.

**RS232 Null Modem Cable {25 pin D Connector Pin Out}**
Pin #	Signal Name	Signal Description	Cable	Pin #	Signal Name	Signal Description
2	RXD	Receive Data	Cross	3	TXD	Transmit Data
3	TXD	Transmit Data	Cross	2	RXD	Receive Data
4	RTS	Request To Send	Cross	5	CTS	Clear To Send
5	CTS	Clear To Send	Cross	4	RTS	Request To Send
6	DSR	Data Set Ready	Cross	20	DTR	Data Terminal Ready
7	GND	Ground	Cross	7	GND	Ground
8	CD	Carrier Detect	Cross	20	DTR	Data Terminal Ready
20	DTR	Data Terminal Ready	Cross	6	DSR	Data Set Ready
20	DTR	Data Terminal Ready	Cross	8	CD	Carrier Detect

The serial port found on a Personal Computer uses either a 9-pin D or 25-pin D connector. The pinout for both those connector types is listed above. This page also lists both of these connector types as they relate to the Personal Computer; RS232 pinout. Serial cables are longer then parallel cables because of the data transmission used.

{RS232 Bus Index}

EIA-562 Bus Description

EIA-562: Electrical Characteristics for an Unbalanced Digital Interface
EIA-562 or TIA-562 is the low voltage version of the RS232 standard. EIA-562 operates at 20kbps over a single-ended line just as RS232 does.
How ever; EIA562 has a minimum output of +/- 3.7 volts, and a maximum output of +/- 25 volts. The TIA/EIA-562 specification does allow for inter-operation with RS232.

{RS232 Bus Index}

EIA-232 Bus Interface IC Vendors

Analog Devices {RS 232 - RS232 xceviers}

Fairchild Semiconductor, Corp. {Line Driver / Receiver IC Manufacturer}

Freescale Semiconductor, Inc. {RS232 IC Manufacturer}

Intersil Semiconductor, Corp. {Radiation Hardened Line Driver / Receiver ICs}

Linear Technology {RS232 IC Manufacturer}

Maxim {RS232 Transmitter / Receiver-Transceiver IC Manufacturer}

Microchip {RS232 Transceiver ICs}

National Semiconductor {RS232 Driver / Receiver ICs}

NEC {RS-232 Line Driver / Receiver ICs}

NXP {MC145406 EIA-232-D/V.28 driver/receiver}

ON Semiconductor (Quad Line EIA-232D Driver; MC1488, Quad Line EIA-232D Receiver MC1489)

Texas Instruments 'TI' {RS-232 Differential Driver / RS232 Differential Receiver - all IC types}

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IC Manufacturers {All other Integrated Circuit device types}

{Back to RS232 Bus Index}

Engineering Design Key words: RS232, EIA-232, Peripheral Interconnect, Signaling, Electrical layer, Standard, Serial Interface, Interface Standard Data Bus, Standard, Specification, Spec, Interface, IC, Physical Interface, Pin Out, RS232 Pinout, Serial Mouse Port, Unbalanced, Description, Personal Computer, PC, Semiconductor Manufacturers, Integrated Circuit, Data Terminal Equipment, Circuit Terminating Equipment, DE9, Shell, Obsolete, Out-dated, DTE, DCE.


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