RS-232 OVERVIEW

This file is still under construction. Please bear with us.

In trying to gain a technical understanding of RS-232, most technical people need the following information

1. DTE and DCE
2. The names of the signals involved in RS-232
3. What the RS-232 signalsmean
4. What RS-232 conditions are required for data transmission

• Thedistance / speed tradeoff
• Null modem cables and modem cables
• Common connector pinouts for DB-25DB-9RJ45ATI
• Pricing of RS-232 components
• Commontesting ideas

EIA/TIA RS-232 is the most common interface in use today. It is used for everything from modems and terminals to PC's and printers. It is an industry standard throughout the world and in Europe is known as V.24. The standard defines both physical and protocol matters. EIA/TIA RS-232 is almost impossible to distroy with wiring errors because all outputs are protected against continuous short circuits.

DEFINATIONS

The terminal is called the 'Data Terminating Equipment' (DTE). This may be an asynchronous terminal such as a WYSE or DEC terminal, or a printer with a serial interface or the COM: port on a P.C.

The modem is also called 'Data Set' or 'Data Communications Equipment' (DCE) Typical characteristics of DCE include:

• Transmits data on pin 3 and receives data on pin 2
• Is a modem
• DSR (pin 6) and/or CTS (pin 5) and/or DCD 9 (pin 8) are high.
• Raises RI (pin 22) to initiate a handshake
• Raises DSR (pin 6) and/or CTS (pin 5) and/or DCD (pin 8) after DTR (pin 20) and/or RTS (pin 4) are raised.
• Uses a straight through cable to a terminal
• Uses a crossover (null modem) cable to connect to a modem

• Transmits data on pin 2 and receives data on pin 3.
• Is a terminal or printer
• DTR (pin 20) and/or RTS (pin 4) are high.
• BUSY (pin 25) is raised
• Drops and re-raises DTR (pin 20) after a drop of DCD (pin 8).
• Uses a straight through cable to a modem.
• Uses a cross over (null modem) cable to connect to a terminal.

Half Duplex (HDX): the ability to communicate in both directions, but in only one direction at a time. Radios such as C.B. radios are an example of half duplex communications.

Two Wire Line

• The two wire line can be half duplex or full duplex depending on the modems used.
• However, as an approximation, and when in doubt, equate:

Four Wire Line

• A four wire circuit can always be used as a full duplex circuit. The modems may work as a half duplex devices due to improper configuration (jumpers or software command setting), but can always be changed (at both ends, please) to be full duplex. Some modems will still switch RTS on and off even when running on full duplex circuits, although this is not required and may slow down communications. . The modem options should be set for CTS always on.

STANDARDS

PHYSICAL

CONNECTORS

DB-25

This connector was standardized in the original EIA RS-232 specification. It is composed of a shell (body) that may or may not include the pins and a shoud (cover) that can be composed of plastic or metal. Metal shouds are used in applications where there is great sensitivity of interfearance (EMI). According to the standard, male ends are supposed to be on DCE devices and female ends on DTE devices. In practice, this standard is not always honored. WARNING: PC's use the DB-25 female connector for parallel outputs. Not all DB-25 connectors are for RS-232. This is the only connector that can be used for synchronous connections.

DB-9

This connection was developed by IBM for asynchronous communications on it's XT class PC's. Because it does not have room for the two clocking signals usually found on pins 15 and 17, it cannot be used for synchronous signals. It was finally standardized in EIA/TIA 574.This chart shows both the pin configuration and the wiring for a DB-25 to DB-9 converter. The IBM PC or compatable has a male end on it.

	DB-9		Function		DB25

	1		Carrier Detect		8

	2		Receive Data		3

	3		Transmit Data		2

	4		Data Terminal Ready	20

	5		Signal Ground		7

	6		Data Set Ready          6

	7		Request to Send          4

	8		Clear to Send	        5

	9		Not defined 

RJ45

This connection was standardized by EIA/TIA 561. Like the DB-9 connector, it cannot be used for synchronous connections. This standard also explicitly permitted speeds of up to 38,400 bps thereby recognising long standing practice. Pin 1 is on the left when holding a male connector in your hand with the locking tab down and the cable towards your body.

	RJ45		Function		DB-25

	1		Ring Indicator		22

	2		Data Carrier Detect	8

	3		Data Terminal Ready	20

	4		Signal Ground		7

	5		Receive Data		3

	6		Transmit Data		2

	7		Clear to Send		5

	8		Request to Send         4

ATI

This connector is only used by Hewlett Packard for asynchronous communications . It is a three wire connection on an unusual DB style connector. Ground is pin 1, Transmit data is pin 2 and Received data is by itself on the bottom row of the connector as pin 3.

CONNECTIONS

When a DCE is connected to a DTE as the RS-232 specification intended, then the cable is wired as 'straight through' This means that pin 2 is wired to pin 2, pin 3 to pin 3 and pin 7 to pin 7 as a minimum cable. This cable is also refered to as a 'modem' cable.

When a DTE is connected to a DTE, then we must change the cable. This cable is known as a 'null modem' cable because it eliminates the need for the two modems (DCE's) in the circuit. In this configuration, the minimum cable connects transmit on one DTE (pin 2) to receive on the other DTE (pin 3) and vice versa. The grounds on pin 7 will still connect straight across. For DCE to DCE connections, this type of cable is also used. These sample cables are only useful for three wire asynchronous RS-232 devices. For synchronous devices, we must add timing or clocking signals. These are typically on pin 15 and 17 of the DB-25 cable.

ELECTRICAL

• 3 to -25Vdc represents 'off', or logical 1 (mark)
• 3 to +3Vdc is undefined. Constant readings in this range represent a failure of the electronics, however, this type of failure is very uncommon.

  The interface must be able to sustain a short circuit indefinatley without sustaining any damage. This means that in practice, it is almost impossible to destroy RS-232 circuitry with an incorrectly wired cable. However, there is one exception to this rule. NEVER CONNECT A PARALLEL PORT TO AN RS-232 CONNECTION. The wisps of smoke will alert you to the fact this has happened and that electrical damage in inevitible.

  TESTING

  Two basic test tools are required for easy RS-232 testing, A breakout box and an RS-232 signal monitor. You can get away without the monitor if you have a meter and only need to check one signal at a time, but the signal monitor is necessary for someone who expects to be doing this more than once.

  A breakout box is a device with two RS-232 connections and some method of connecting and disconnecting the signals between them. It is used to

  An RS-232 monitor is a pair of DB25 connections with a number of LED's monitoring some or all of the twenty five signals. With this tool, you can determine if a device is behaving as a DCE or DTE

  One of the more difficult things to remember is that when you are dealing with RS-232 test equipment, the usual practice is for RED leds typically represent 'on', while GREEN leds typically represent 'off'. So an RS-232 breakout box with a lot of red leds on it is usually a good thing, however, it tends to scare the hell out of customers. An RS-232 test kit should contain a breakout box, an 8 led monitor, a 25 pin straight through cable, a male to male gender changer, and a female to female gender changer. Optionally, you may need to include loopback connectors for specific equipment as well as several other gender changers and and ATP cable if you work with Hewlett Packard equipment.

  DISTANCE and CABLE

  RS-232 signals have been sucessfully run on everything from barbed wire to Catagory 5 cable. Instead of asking which cable is 'best' or 'correct', the question should be rephrased to enquire about the type of cable and the amount of cable. Barbed wire may work at 300 baud for distances of 100 feet in dry weather indoors. However, to properly examine this issue, we need to look at the specification to see what it says.

  There is an unfortunate misconception that RS-232 is limited to a fifty foot distance restriction. In fact, the specified limitation is for a range of 50 feet OR 2500 pF. . This means that cables longer than 50 feet can be acceptable to the standard.. As examples, the following cable types are presented:

  
                    CABLE                          pF / ft.   2500 pF Limit 
  
  Belden 8135, Shielded, low capacitance,            11           227 ft.
  
  Belden 8335, Shielded                             30            83 ft.
  
  Belden 1229A, Unshielded Twisted Pair            17.5            142 ft.
  
  TSB-36 Catagory 3 UTP specification                20  maximum   125 ft.
  
  TSB-36 Catagory 5 UTP specification                17  maximum   147 ft. 
  
  

  The average technician does not have a capacitance meter to measure the actual cable and technical specifications frequently omit this number. For that reason, some practical guidelines have been developed. These guidelines rely on the the fact that the slower the baud rate is, the longer the cable may be. Texas Instruments published these numbers years ago. They are shown in the attached graph.

  RS-232 LEADS

  RS-232 signals fall into several broad catagories.

  Grounds supply both a safety connection against electrical shock (pin 1) and a return path for data signals (pin 7). In some devices there is an option jumper that connects these two grounds together. Every RS-232 link will have at least one ground.

  Data leads are the two leads that actually move data across the RS-232 link. Every RS-232 link will have one or more data leads being used.

  Control leads are a number of leads that can be used to indicate the status of a device. Status messages such as being ready to work (Data Set Ready and Data Terminal Ready) or ready to send data (Request to Send) or receive data (Clear to Send). The use of these signals is completely optional and this is the area that leads to most of the confusion in RS-232 work.

  Timing signals can also be used in RS-232 for synchronous circuits only.

  Secondary signals not used in RS-232 circuits today. They were used ten or twenty years ago to allow the 'secondary' end of the circuit to send the 'primary' end full duplex data. With today's full duplex circuits, this is seldom necessary today.

  
  Pin  Cct   Source              Name                   Mnemonics
  
  No.  Ref                                                          
  
  
  
  1      AA               Chassis (Frame) Ground              FG
  
  2*    BA   DCE    Transmitted Data                          TD, Tx
  
  3*    BB   DTE     Received Data                           RD, Rx
  
  4*    CA   DTE    Request to Send                           RTS, RS
  
  5*    CB   DCE     Clear to Send                         CTS, CS, CB
  
  6*    CC   DCE     Data Set Ready                          DSR, MR
  
  7*    AB               Signal Ground                      SG
  
  8*    CF    DCE     Carrier On                              CO,CD,DCD,RLSD
  
  9               DCE     + d.c. power
  
  10             DCE     - d.c. power
  
  11                           unassigned
  
  12   SCF   DCE     Secondary Carrier On
  
                                        or
  
                                Data Rate Indicator
  
  13   SCB   DCE     Secondary Clear to Send                       SCTS
  
  14   SCA   DTE     Secondary Transmitted Data                  STD
  
  15*  DB    DCE     Transmitter Clock                       TC, SCT
  
  16   SBB   DCE     Secondary Received Data                      SRD
  
  17*  DD    DCE     Received Clock                   RC, SCR
  
  18                            unassigned
  
  19   SCA   DTE     Secondary Req. to Send                        SRTS
  
  20*  CD    DTE     Data Terminal Ready                        DTR, DR, TR
  
  21   CG    DCE       Signal Quality Detector                   SQ
  
  22*  CE    DCE      Ring Indicator                     RI
  
  23   CH    DTE      Data Rate Selector                 XC, SR
  
  24   DA    DTE     Transmitter Clock        
  
  25                                not used
  
  
  
  * shows leads that are commonly used Primary Circuits.
  
  
  
  Cct Ref:  Used by ITU-T (formerly CCITT) to reference the pin on
  
  V.24 connections.
  
  The first letter of the circuit reference indicates the type of
  
  circuit
  
  
  
       A    -    Ground
  
       B    -    Data
  
       C    -    Control
  
       D    -    Timing    
  
       S    -    Secondary signals
  
  
  
  

  Common RS-232 Lead Discriptions

  This sections shows the most common RS-232 leads, their abberviations, full spelling, DB25 pin number, direction of signal flow, and the meaning of the signal.

• RTS - Request to Send Pin 4. DTE to DCE. On indicated DTE requests permission to transmit. In a local configuration it could be used as a flow control signal, although this is rare.

• CTS - Clear to Send CTS Pin 5. DCE to DTE. On indicates DCE has permission to transmit. In two wire circuits, this signal goes high (on) after RTS went high. Typical delays are less than 300 mSec. In a local configuration it could be used for flow control, although this is rare.

• DSR - Data Set Ready DSR Pin 6. DCE to DTE. On indicates that DCE is ready to communicate. For local configurations, it can be used as a flow control signal to indicate that this end is ready.

• CO - Carrier On CO Pin 8. DCE to DTE. On indicates that the DCE is receiving carrier tone from the far end DCE. RD is invalid unless CO is on.

• DTR - Data Terminal Ready DTR Pin 20. DTE to DCE. On indicates that DTE is ready to communicate. In dial-up mode, this signal can be turned on by the terminal to tell the modem to answer an incoming call. In a local or dedicated line arrangement, this can be used as a flow control signal.

• RI - Ring Indicator RI Pin 22. DCE to DTE. On indicates that the telephone is ringing. Note that typical telephone ringing is 2 seconds on and 4 seconds off. The RI signal will follow this cadence. However, there are options for the telephone system such as "Distinctive Ringing" that can modify this ring cadance.

• Data Rate Selector Pin 23. DTE to DCE. Depending on options: On selects the higher of the two data speeds, or On indicates the higher of the two data speeds.

  CONDITIONS FOR DATA TRANSFER

  The easiest connection type is the three wire connection. This means that only transmit, receive and ground are connected between the two devices. Hewlett Packard commonly uses this type of connection with its async. equipment. Even though you only require three wires between the two devices, it may be possible that the equipment at one end or the other may require some RS-232 signals before it will send or receive data. The most common RS-232 signals that are required before data will move are:
• Data Set Ready (DSR) must be on solidly from the DCE.
• Data Terminal Ready (DTR) must be on solidly from the DTE. For a modem, this signal is required only after the telephone has been answered. In the case of a PC, this signal will only become high after the PC is powered on and the communications software has been started.

  CLOCKING

  Clocking refers to the speed that a line runs at. In async. communications, the clock rate is set at both ends of the RS-232 connection. The clocks are synchronized by the 'start' and 'stop' bits in each byte of data. If the rate is not the same at both ends, then the data will not be able to be read. For async. connections, there is no need for a wire between the two devices to transport the clock signal. For synchronous (sync) communications, the clock rate must only be set at one end of the RS-232 connection. The other end just reads the clock signal that is being sent out and gets its speed and synchronization information from that. For that reason, any cable that uses pins 15 and 17 (the clocking signals) obviously can be used for synchronous communications. It is also true that without pins 15 and 17 the cable is inappropriate for synchronous communications. The following information on clocking is appropriate for synchronous communications.

• Serial Clock Receive (SCR) must be changing if the DCE is supplying clock.

• Receiver Clock (RC) must be changing if the DTE is supplying clock.

• Serial Clock Transmit (SCT) must be changing if the DCE is supplying clock.

• Transmitter Clock (TC) must be changing if the DTE is supplying clock. (Note: the DCE is typically responsible for clocking, but there is usually jumper in both ends of each RS-232 interface to select this option. Make sure that the options in DCE and DTE are the same.)
  1. Internal clocking
  2. Same speed
  Clear to Send (CTS) must be on before the DTE can transmit data. CTS is either on solid as a modem option in a 4 wire circuit or it is a delayed response to Request to Send (RTS).

  TYPICAL COSTS

  The costs shown below should not be taken as high or lowest costs, but only as typical costs as of the date shown.
  • Prices are in Canadian Dollars.
  • Retail prices must have 7 % GST and 8% PST added to them.

  
     PART                DATE    RETAIL    MANUFACTURER
  
  DB9m-DB25f, 10 ft, 3 wire 02/93   $14.28
  
  DB9m/f, 4 ft., 9 wire     02/94   $32.70
  
  RS-232, 8 wire            07/91  $138.00
  
  Switch, AB, DB25          05/95   $18.00    Lextek   25D12
  
  Switch, ABC, DB25         05/95   $24.00    Lextek   25D13
  
  Switch, ABCD, DB25        05/95   $28.80    Lextek   25D14
  
  Switch, ABCDE, DB25       05/95   $61.20    Lextek   25D15
  
  Switch, ABCDEF, DB25      05/95   $67.20    Lextek   25D16
  
  Switch, AB, DB25          04/94   $13.20    Allied   FS-2500AB
  
  ca, 16 pin                         $1.92
  
  ca, 3 pr. low cap.        03/94    $0.13/ft.Allied
  
  ca, 6 pr., low cap,shield 09/95    $0.28    Allied   AL-020672
  
  ca, 7 pin                 05/89    $0.96
  
  pins, female              01/94    $0.11
  
  pins, male                01/94    $0.11    Allied   CD-PPC001
  
  shell, DB15               07/94             Allied   CD-DB15PC
  
  shell, DB25               07/94    $1.14    Allied   CD-DB25PC
  
  shell, DB9                04/94             Allied   CD-DB09SC
  
  shroud, DB25              02/90    $1.92
  
  gender changer, DB25      05/94    $4.20    Allied   FI-113325-G
  
  ca,25 wire,m/f,20 ft.  03/95                LexTech  SER101-20
  
  ca,25 wire,m/f,40 ft.locap03/95             LexTech  SER101-40
  
  

  REFERENCES

  RS-232D specifications can be ordered from Global Engineering Documents at 1-800-854-7179. Request part number EIA/TIA-232-E, last published July 1991 and costing about $63 plus shipping.

  RS-232 specifications could be found in a technical library under McGraw-Hill publications such as xxx.

  Cable information can be found in the cableFAQ or on the news

  Modem details can be found at US Robotics web site

  PC and UART information regarding comm ports can be found here

  ITU-T (formerly CCITT) home page

  V.24 standards ordering

  Campbell, Joe C Programmer's Guide to Serial Communications 1994, Sams Publishing, IBDN 0-672-30286-1 A technical treatment of PC communications including UART operations, PC command sets and 'C' level programming of the interface. Recommended even if you are not a 'C' programmer.

  ACKNOWLEDGEMENTS

  Tel Serv wishes to acknowledge the following original sources:

  Did You Know...DTE or DCE, Connections newsletter, Equinox Systems Inc. Apr. 1990, page 4 .

  Pricing assistance from Mr. Arnold Smith of Allied Wire and Cable, Mississauga, Ontario 1-800-463-WIRE

  Master Catalog Belden Wire and Cable 1989. Their product engineering department can be contracted at 1-317-983-5200. Return to Tel Serv home Page

  Last modified: Sunday, 1995 December, 03 - 10:41:07 AM