RS232 Cables

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RS-232 Cables, Wiring and Pinouts

Brief tutorial and pinouts for RS-232, T1/E1 and V.35.

Contents

DTE (PC) and DCE (Modem)

DB9 and DB25 Male and Female Pin Numbering

RS232 on DB25 Pinout (RS-232C)

RS232 on DB9 Pinout (EIA/TIA - 574)

RS232 on RJ45 (RS-232D EIA/TIA-561)

RS232 DB25 NULL Modem Pinout


RS232 DB9 and DB25 Loopback Pinout

RS232 DB9 NULL Modem Pinout using Cat5(e)

RS232 DB9 to DB25 Pinout

RS232 DB9 to DB25 NULL Modem PinoutEIA/TIA RS-530-A (DB25 using RS-422, 423 and 485)

V.35 on a DB25

DBx - Designations for D type sub-miniature connectors

T1/E1 Pinout (RJ-48C)

RS-232 standards(EIA-232) are defined by EIA/TIA (Electronic Industries Alliance

/Telecommunications Industry Association). RS-232 defines both the physical and electrical

characteristics of the interface. RS-232 is practically identical to ITU V.24 (signal description

and names) and V.28 (electrical). RS232 is an Active LOW voltage driven interface and

operates at +12V to -12V where:

Signal = 0 (LOW) > +3.0V (SPACE)

Signal = 1 (HIGH) < -3.0V (MARK)

Notes:

1. Signal voltages in the range >-3.0V to +3.0V are regarded as being in the 'dead

area' (indeterminate value) and allow for absorption of noise. For more on the use of

signals and other heavy stuff.

2. The power level on RS232 pins is defined by TIA for short circuit protection to be

100mA. Most RS232 drivers will provide lower short circuit protection (especially for

laptops). A max of 50mA PER PIN may be available but the data sheet for thespecific interface/chip should be consulted before commiting to externally powered

designs.

3. We received an email recently pointing out some issues with NULL modem cables.

The pinouts shown below will gnerally work. However there are many permutations

of signal sets that can be used by either end of a connection and they may not be

SYMMETRIC. One end may expect something (a signal) that the other end cannot

generate. This typically happens with CTS/RTS (and perhaps DCD) and DTR/DSR. If

you suspect this is the case then unfortunately you need to UNDERSTAND the

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interface and may have to SPOOF certain signals. Our signal primerpage may help

you. Finally, if you are having serious problems, splash out on a light box or some

other device that will show you which signals are being activated.

4. Serial communications equipment may be either a DTE (Data Terminal Equipment - a

terminal or PC) or a DCE (Data Communications Equipment - for example, a modem)

and have a direction depending on the type. All the diagrams below define the

interface from the DTE perspective.5. The terms Data Carrier Detect (DCD) and Received Line Signal Detect (RLSD) are

one and the same. We use DCD throughout 'cos we think it's more common.

6. Like most folks we use the term DB9 which is widely - but erroneously - used to

describe a 9-pin serial connector. We got an email pointing out the error of our ways

(hint: it is really a DE-9P). So, if you want to amaze your friends over the dinner

table you can read more and use the technically correct terms in the future. While

we get away with it most of the time (with common or garden PCs), sometimes it is

essential to know EXACTLY what connector type you are talking about. And following

a recent email we discovered that the thread on RS-232 (DB9 and 25) receptacles

is UNC 4-40.

7. RS-232-E is normally defined to be used with a DB25 connector, but does have a 26

pin (a much smaller) alternative . We suggest that if you come across one of thesethat you do the decent thing - use an expletive. Alternatively, with your luck, you

could consider buying a lottery ticket.

8. We have received a number of emails recently asking how to wire DB9's using

cat5(e)/cat 6 cable. We guess there is a lot of LAN cable lying around these days so

folks naturally want to use it. We have added a null modem only section to cover this

wiring. There is absolutely no standard to cover this form of wiring. This section is

simply offered as one of many possible ways to do it.

9. We got an email asking about TTY 20ma current loop interfaces. This was an old

method used to connect teletype devices and uses current (normally 20ma but

sometimes 60ma) to indicate mark and space. A TTY system CANNOT be connected

to RS-232 (which is a voltage driven interface) and has no standard. You will need to

get the manufacturer's specifications and start reading!

DTE (PC) and DCE (Modem)

In serial communications the terminal end (PC) is called the Data Terminal Equipment (DTE)

and the modem end is called the Data Communications Equipment (DCE) as shown in the

diagram below.

Serial Communications with a modem

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RS-232 signals have a direction (in or out) depending on whether they are with respect to a

DTE or a DCE. In all the pinout diagrams below the signal direction is with respect to the

DTE (PC) end.

NULL Modem Connections

When PCs are connected back-to-back each end is acting as a DTE (there is no DCE in this

case) and consequently certain signals may have to be looped in the connection to satisfy

any input signal requirement. This is called a NULL (no) modem configuration. For example,

when the DTE raises Request to Send (RTS) it typically expects Clear to Send (CTS) from

the DCE. Since there is no DCE to raise CTS, the outgoing RTS signal is looped in the NULL

modem cable to the incoming CTS to satisfy the DTE's need for this signal. This is shown in

the diagram below.

Serial Communications with a NULL modem configuration

DB9 and DB25 Male and Female Pin Numbering

These diagrams show the male (grey background) and female (black background) pin

numbering for DB9 and DB25 sub-miniature connectors. Generally Pin 1 is marked on the

front of the connector right next to the pin - though you may need a magnifying glass to

read it. Some manufacturers mark each pin number on the plastic housing at the rear of the

connector. The male connector has the pins sticking out!

DB25 Male and Female

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DB25: View looking into male connector

DB25: View looking into female connector

DB9 Male and Female

DB9: View looking into male connector

DB9: View looking into female connector

RS232 on DB25 (RS-232C)

The use of each pin including methods for spoofing signals is described in our Signal/pin

primer. The RS-232 DB25 connector is capable of supporting two separate connections -

each with its own optional clock when used in Synchronous mode or Bit-Synchronous mode.

If you are using the interface purely for Asynchronous communications then you only need

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those marked with (ASYNC) below or you can use even fewer (if you understand what is

happening). The column marked Dir shows the signal direction with respect to the DTE.

Note: This is NOT the same as the DB25 Parallel port on a PC.

PinNo. Name Dir Notes/Description

1 - - Protective/shielded ground

2 TD OUT Transmit Data (a.k.a TxD, Tx) (ASYNC)

3 RD IN Receive Data (a.k.a RxD, Rx) (ASYNC)

4 RTS OUT Request To Send (ASYNC)

5 CTS IN Clear To Send (ASYNC)

6 DSR IN Data Set Ready (ASYNC)

7 SGND - Signal Ground

8 CD IN Carrier Detect (a.k.a DCD).

9 - - Reserved for data set testing.

10 - - Reserved for data set testing.

11 - - Unassigned

12 SDCD IN Secondary Carrier Detect. Only needed if second channel being used.

13 SCTS IN Secondary Clear to send. Only needed if second channel being used.

14 STD OUT Secondary Transmit Data. Only needed if second channel being used.

15 DB OUT Transmit Clock (a.k.a TCLK, TxCLK). Synchronous use only.

16 SRD IN Secondary Receive Data. Only needed if second channel being used.

17 DD IN Receive Clock (a.k.a. RCLK). Synchronous use only.

18 LL - Local Loopback

19 SRTS OUT Secondary Request to Send. Only needed if second channel being used.

20 DTR OUT Data Terminal Ready. (ASYNC)

21 RL/SQ - Signal Quality Detector/Remote loopback

22 RI IN Ring Indicator. DCE (Modem) raises when incoming call detected used for auto answerapplications.

23 CH/CI OUT Signal Rate selector.

24 DA - Auxiliary Clock (a.k.a. ACLK). Secondary Channel only.

25 - - Unassigned

NOTE: Leave all pins not specified above unconnected.

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view - looking into male connector

(male and female connector diagrams)

RS232 on DB9 (EIA/TIA 574)

Signal functions are described in detail in our Signal/pin primer. The column

marked Dir shows the signal direction with respect to the DTE.

PinNo.

Name Dir Notes/Description

1 DCD IN Data Carrier Detect. Raised by DCE when modem synchronized.

2 RD IN Receive Data (a.k.a RxD, Rx). Arriving data from DCE.

3 TD OUT Transmit Data (a.k.a TxD, Tx). Sending data from DTE.

4 DTR OUT Data Terminal Ready. Raised by DTE when powered on. In auto-answer mode raised onlywhen RI arrives from DCE.

5 SGND - Ground

6 DSR IN Data Set Ready. Raised by DCE to indicate ready.

7 RTS OUT Request To Send. Raised by DTE when it wishes to send. Expects CTS from DCE.

8 CTS IN Clear To Send. Raised by DCE in response to RTS from DTE.

9 RI IN Ring Indicator. Set when incoming ring detected - used for auto-answer application. DTEraised DTR to answer.

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DB9 (EIA/TIA 574): View - looking into male connector


RS232 on RJ45 (RS-232D)

More properly EIA/TIA - 561. Use when connecting to or from a serial port with a 8 position

Modular Jack (RJ45). If you are cross-connecting from a DB9 or a DB25 use the signal

names to cross connect the appropriate connections.

Signal/pin primer

PinNo.

Name Notes/Description

1 DSR/RI Data set Ready/ring indicator

2 DCD Data Carrier Detect

3 DTR Data Terminal Ready

4 SGND Signal Ground

5 RD Receive Data

6 TD Transmit Data

7 CTS Clear to Send

8 RTS Request to Send

Note: Pin 1 is a multi-function pin sharing DSR (Data Set Ready) and RI (Ring Indicator).

This means it is impossible to differentiate between a incoming ring signal and when the

modem has finally connected and synched up. With local (null modem connections) or if the

modem is run in auto-answer mode this is not normally a problem. If used with a modem

and the DTE (the computer end) wants to control the connection the problem is more real.

DSR would normally indicate the 'connected and synched-up' state following DTR from the

DTE whereas RI simply indicates a ring voltage is present on the line and would normally be

the trigger for the DTE to raise DTR if it wants to accept the call. DCD will indicate that a

carrier has been received but does not indicate synchronization of both ends. In most cases

however CTS (Clear To Send) in response to RTS (Request To Send) will not normally be

returned until an end-to-end connection is available (equivalent to the DSR state).

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RJ45 Male Connector Pin Numbering


Use when connecting two systems (e.g. PCs) via their DB25 interfaces without a modem(i.e. back-to-back). See the full signal names in the DB25sections.

If this pinout does not work for you then you could try our Signal/pin primer because you

may need to SPOOF connections.

Note: This DB25 is NOT the same as the DB25 Parallel port on a PC which is defined here.

DB25 Signal DB25 Signal

3 RD 2 TD

2 TD 3 RD

20 DTR 6,8 DSR, DCD

6,8 DSR, DCD 20 DTR

4 RTS 5 CTS

5 CTS 4 RTS

7 SGND 7 SGND

22 RI 22 RI

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DB25: View - looking into male connector


NOTE:

1. Leave all pins not specified above unconnected.2. We have received email suggesting that the above pinout looks like DTR from one

side is driving into DSR/DCD on the other side - not normally a healthy situation. The

emails miss the point that since this is a NULL modem connection both ends are

DTEs. The two peer DTE's treat DSR/DCD signals as RX (INPUT) only. The INPUT

DSR/DCD on one side is created by cross connecting the OUTPUT DTR signal for the

other peer.


Use when connecting two systems, for example two PCs, via their DB9 interfaces without a

modem. Typically called a back-to-back or NULL modem connection. See the full signal

names in the DB9 section.




2 RD 3 TD

3 TD 2 RD

4 DTR 6,1 DSR, DCD

6,1 DSR, DCD 4 DTR

7 RTS 8 CTS

8 CTS 7 RTS

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5 SGND 5 SGND

9 RI 9 RI

DB9 TIA/EIA 574: View - looking into male connector


NOTE:

1. We have received email suggesting that the above pinout looks like DTR from one





other peer.

RS232 DB9 and DB25 Loopback Pinout

Loopback is a method of testing the RS232 connector and interface circuitry to ensure it is

functioning correctly, that is, in layman's jargon - it ain't broke! If communication fails to

occur between two machines the question that immediately arises is - which end is broken?

In the worst case both ends could even be broken in which case ritual suicide may be the

best solution. Loopback works by testing each end of the connection independently. Data is

sent and received on the same RS232 connector - which may be either DB9 or DB25. The

test normally consists of using some program to transmit data. The program then checks to

ensure exactly the same data was received. Loopback testing gives you a binary result - it

works, in which case the end under test is good, or it does not, in which case the end under

test is broken. Pinouts are shown for both DB9 and DB25. The loopback is normallyconstructed in the DB shell or using a diagnostic light-box.

DB9 Loopback

DB9 Signal Loopbackto

Signal

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2 RD 3 TD

3 TD 2 RD

4 DTR 6,1,9 DSR, DCD, RI

7 RTS 8 CTS

5 SGND 5 SGND

(DB9 male and female connector diagrams)

NOTE:

1. We show 4 (DTR) being looped to 6 (DSR), 1 (DCD) and 9 (RI). RI (9) is included

because we understand that certain test programs use this to ensure a more

complete test of the interface signal set.

DB25 Loopack

DB25 Signal Loopbackto

Signal

3 RD 2 TD

2 TD 3 RD

4 RTS 5 CTS

5 CTS 4 RTS

7 SGND 7 SGND

15 DB 17 DD

20 DTR 6,8,22 DSR, DCD, RI

23 CH/CI 23 CH/CI


NOTE:

1. For the sake of simplicity this loopback will only work for the primary channel. Full

DB25 interfaces allow a secondary channel. If a complete interface loopback is

required you will need to add pins 12, 13, 14, 16, 19, 24.2. By looping the primary channel clocks (15 and 17) both synchronous and

asynchronous capabilities can be tested. If only asynchronous tests are being

performed omit this, and the pin 23 loopback

3. We show 20 (DTR) being looped to 6 (DSR), 8 (DCD) and 22 (RI). RI (22) is included

because we understand that certain test programs use this to ensure a more

complete test of the interface signal set.

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RS232 DB9 NULL Modem Pinout on CAT5/CAT5(e)/CAT6

This is in response to a number of recent emails asking how to wire both ends of a DB9connection using cat5, cat5(e) or cat6 cable. This must not be confused with DB9 to RJ45

(RS232D). We have shown a null modem (back-to-back PCs) only configuration. And if you

want to use cat5, cat5(e) or cat 6 with a real modem (a DB25 connector)? Our advice -

don't.

Warning:. There is, as far as we know, no standard to cover the use of cat5, cat5(e) or cat

6 (8 conductor) wiring when used with two DB9 connectors. Any such wiring scheme is

therefore non-standard - that includes the wiring scheme below. Specifically this means that

both ends of the cable must be wired in the same way and that no assumptions can be

made about how the other end is wired. You will have to manually inspect both ends of the

connection. Damage can result from mis-matched wiring.

A DB9 clearly has 9 connections and a cat5, cat5(e) and cat 6 cable has 8

conductors. RS232D has chosen to use Pin 1 as a multi-function pin (DSR/RI) to provide

maximum flexibility with modems - in particular it allows for DCD which is a meaningful

signal from a modem but not, we suggest, from a peer PC. We have chosen to use a minor

variation on the normal DB9 Null modem pinout above - specifically we have allowed for RI

which could be used from a peer PC to commence a transmission sequence. The colors used

are unimportant but the suggested configuration is one way to provide the shortest use of

the adjacent (twisted) pairs.



PC1 Peer PC2 Peer

DB9 Signal cat5(e)Color

DB9 Signal cat5(e)Color

2 RD Brown 3 TD Blue

3 TD Blue 2 RD Brown

4 DTR Green 6,1 DSR, DCD Brown-white

6,1 DSR, DCD Brown-white 4 DTR Green

7 RTS Blue-white 8 CTS Green-white

8 CTS Green-white 7 RTS Blue-white

5 SGND Orange 5 SGND Orange

9 RI Orange-white 9 RI Orange-white

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NOTE:

1. We have received email suggesting that the above pinout looks like DTR from one





other peer.

RS232 DB9 to DB25 Pinout

Use when connecting a DB9 (e.g. a PC) to a DB25 (e.g. a modem) interface. See the full

signal names in the DB9 and DB25 section.

Signal/pin primer

DB9 Signal DB25

1 DCD 8

2 RD 3

3 TD 2

4 DTR 20

5 SGND 7

6 DSR 6

7 RTS 4

8 CTS 5

9 RI 22

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View - looking into male connector





RS232 DB9 to DB25 NULL Modem Pinout

Use when connecting two systems (e.g. PCs) when one has a DB9 interface and the other a

DB25 interface without a modem. Typically called a back-to-back or NULL modem

connection. See the full signal names in the DB9 and DB25 sections.

Signal/pin primer


2 RD 2 TD

3 TD 3 RD

4 DTR 6,8 DSR, DCD

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6,1 DSR, DCD 20 DTR

7 RTS 5 CTS

8 CTS 4 RTS

5 SGND 7 SGND

9 RI 22 RI






EIA/TIA RS-530-A (DB25)

RS 530-A defines the pinout when using either balanced RS-422 (and RS-485)

or unbalanced RS-423 electrical interfaces using a DB25 connector. By using a DB25

connector RS-530 is now frequently used to replace many older standards which defined

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hideously huge connectors such as V.35.(used a whopping 35 pin connector) and RS-449

(used a pretty serious 37 pin connector).

V.35 on DB25 (RS-530-A)

The original V.35 specification defined use ofbalanced signals over a huge 35 pin connector.

V.35 has been obsolete for years (replaced with V.10 and V.11) though the term is still

frequently used. Most modern systems that call themselves V.35 use a DB25 connector

which has more modest dimensions. The A (+) and B (-) below refer to each signal pair

used in balanced serial interfaces. When used with RS-423 (unbalanced) the B (-) are tied

to a common ground. Signals marked U under Bal/Ubal are not balanced since they typically

change very infrequently (for example once per session) and therefore do not affect TX/RX

performance sensitivity - hence speed. BEWARE: RS-530 (without the A suffix) is an earlier

standard and is wired differently. This is the 530-A pinout spec.

Signal/pin primer

Pin

No.

Name Bal/Ubal Notes/Description

1 Shield Cable Shield, connected at DTE only.

2 BA Transmit Data (A+) (a.k.a TxD)

3 BB Received Data (A+) (a.k.a. RxD)

4 CA/CJ RTS (A+) Request To Send

5 CB CTS (A+) Clear To Send

6 CC U Data Communications Equipment Ready (modem/CSU) (a.k.a DSR)

7 AB Signal Ground

8 CF Data Carrier Detect (A+) (a.k.a DCD, CD or RLSD)

9 DD Receiver Signal Element Timing (B-) RX Clock

10 CF - Data Carrier Detect (B-) (a.k.a DCD, CD or RLSD)

11 DA - Ext. Transmit Clock (B-)

12 DB Transmit Signal Element Timing (B-) TX CLOCK

13 CB CTS (B-) Clear to Send

14 BA Transmit Data (TD) (B-) (a.k.a TxD)

15 DB Transmit Signal element Timing (A+) TX CLOCK

16 BB Received Data (B-) (a.k.a RxD)

17 DD Receiver Signal Element Timing (A+) RX CLOCK

18 LL U Local Loopback

19 CA/CJ RTS (B-) Request to Send

20 CD U DTE Ready (a.k.a DTR)

21 RL U Remote Loopback

22 RI Ring Indicator

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23 AC Signal Ground

24 DA Ext TX Clock (A+)

25 TM U Test Mode

NOTES:

Leave any pins not specified above unconnected.

In balanced mode signals with the same name are the paired set, for example, pins

2 and 14 are both named BA and form the Transmit Data pair. Each signal of the pair

is either a high (A+) or low (B-)

When used with RS-485 in half-duplex, multi-dropped environments a simple three

signal arrangements is frequently used - one pin is used as a GND and RX/TX is

alternately switched onto a balanced pair of wires which can be either the BA (TX) or

BB (RX) pair.


(DB25 male and female connector diagrams)

DB - Designations for D-subminiature Connectors

This lists the designations for DB connectors (supplied by Rob Recny - Thanks). Any errors

in this list are ours not Rob's.

A - 15-pin 2-row joystick connector.

B - 25-pin 2-row serial or parallel connector - also 44-pin high-density 3-row. C - 37-pin connector - sometimes found on multi-port serial or data acquisition

boards.

D - 50-pin connector - a little longer than C, but three rows using the same pins as

the 2-row connectors.

E - 9-pin 2-row serial - also 3-row VGA.

So a DB9 is more properly a DE-9P. Isn't knowledge a wonderful thing!

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The thread size on an RS232 receptacle (the jackscrew) is UNC 4-40.

T1/E1 Pinout (RJ-48C)

T1/E1 wiring may use either a RJ45, DB15 or BNC connectors. The pinout shown uses RJ45

connectors - its formal name is USOC RJ-48C and is defined in ANSI T1-403-1989. T1 is a

North America (primarily) digital service providing 1.544 Mbps. E1 is a European/Rest of

World standard providing digital service at 2.048 Mbps. CATegory 5(e) cabling is used to

provide balanced pairs. The color coding for Cat 5(e) cabling may be 568A or 568B.

RJ45Pin

Signal Notes

1 RX1 (Ring - negative)

2 RX2 (TIP - positive)

3 FGND (RX GND) Ground/Shield

4 TX1 (Ring - negative)

5 TX2 (TIP - positive)

6 FGND (TX GND) Ground/Shield

7 NC Unused

8 NC Unused

NOTES:

1. NC = Not connected.2. There are a confusing number of pinouts for use with an RJ45/48C connector. Some

specs show use of pins 7,8 for Grounds. Always consult any equipment specification

if available.

3. The telecom world loves its Tip and Ring designations. Tip is assumed to carry a

positive voltage (and would carry the transmission signal), Ring a negative voltage

(and would carry the inverted transmission signal)

RS232 Cables

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