rs232 introduction

RS-232 IntroductionRS-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

Signal = 1 (HIGH) < -3.0V

Notes:

1. Signal voltages in the range >-3.0V to +3.0V is regarded as the 'dead area' and allows 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 in terms of 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 the specific 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 interface and may have to SPOOF certain signals. Our signal primer page

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 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. So, if you want to amaze

your friends over the dinner table you can read more here 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.

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

smaller connector) alternate . We suggest that if you come across one of these that 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) 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 the 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 is called the Data Communications Equipment (DCE) as shown in the diagram below.

Serial Communications with a modem

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).

NULL Modem Connections

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

consequently certain signals 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 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

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 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.

Pin

No.

Nam

e

Dir Notes/Description

1 - - Protective/shielded ground

2 TD OU

T

Transmit Data (a.k.a TxD, Tx) (ASYNC)

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

4 RTS OU

T

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 OU

T

Secondary Transmit Data. Only needed if second channel being used.

15 DB OU

T

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 OU

T

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

20 DTR OU

T

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

answer applications.

23 CH/CI OU

T

Signal Rate selector.

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

25 - - Unassigned

NOTE: Leave all pins not specified above unconnected.

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.

Pin

No.

Nam

e

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 OU

T

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

4 DTR OU Data Terminal Ready. Raised by DTE when powered on. In auto-answer mode raised

T only when RI arrives from DCE.

5 SGND - Ground

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

7 RTS OU

T

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.

DTE raised DTR to answer.

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

Pin

No.

Name Notes/Description

1 DSR/RIData 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 with 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. 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.

RJ45 Male Connector Pin Numbering

RS232 DB25 NULL Modem Pinout

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 DB25 sections.

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

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.

RS232 DB9 NULL Modem Pinout

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.


SPOOF connections.


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

5 SGND 5 SGND

9 RI 9 RI

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


NOTE:






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 normally constructed

in the DB shell or using a diagnostic light-box.

DB9 Loopback

DB9 Signal Loopback

to

Signal

2 RD 3 TD

3 TD 2 RD

4 DTR 6,1,9DSR, 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 Loopback

to

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 6,8,22DSR, 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.

RS232 DB9 NULL Modem Pinout on CAT5

This is in response to a number of recent emails asking how to wire both ends of a DB9 connection

using cat5(e) 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(e) 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(e) (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(e) 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.


SPOOF connections.

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, DCDBrown-

white

6,1 DSR, DCDBrown-

white4 DTR Green

7 RTS Blue-white 8 CTSGreen-

white

8 CTSGreen-

white7 RTS Blue-white

5 SGND Orange 5 SGND Orange

9 RIOrange-

white9 RI

Orange-

white



NOTE:






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

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

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 hideously huge connectors such 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 of balanced 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 Shiel

d

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 Tramsmit 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

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!

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.

RJ45

Pin

Signal Notes

1RX1 (Ring -

negative)

2 RX2 (TIP - positive)

3 FGND (RX GND)Ground/

Shield

4TX1 (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)

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rs232 introduction

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