Do you need information about RS-232C?

The following information is intended to collect together in one place, and explain in relatively simple terms, enough of the details of the RS-232 standard to allow the construction and/or debuggging of interfaces between any two RS-232 compatible devices.

A more detailed coverage of this subject may be found in the book "Technical Aspects of Data Communication" by John E. McNamara (1977, Digital Press).

BACKGROUND

RS-232C is the most recent version of the EIA (Electronics Industry Association) standard for low speed serial data communication. It defines a number of parameters concerning voltage levels, loading characteristics and timing relationships.

The actual connectors which are almost universally used (DB-25P and DB-25S, rarely called "EIA connectors") are recommended, but not mandatory. Typical practice requires mounting the female (DB-25S) connector on the chassis of communication equipment, and male (DB-25P) connectors on the cable connecting two such devices.

There are two main classes of RS-232 devices, namely DTE (Data Terminal Equipment), such as terminals, and DCE (Data Communication Equipment), such as modems.

Typically, one only interfaces a DTE to a DCE, as opposed to one DTE to another DTE, or one DCE to another DCE, although there are ways to do the latter two configurations by building non-standard cables.

Rarely if ever are more than two devices involved in a given interface (multidrop is not supported).

A serial port on a computer may be implemented as either DTE or DCE, depending on what type of device it is intended to support.

RS-232 is intended for relatively short (50 feet or less), relatively low speed (19,200 bits per second or less) serial (as opposed to parallel) communications.

Both asynchronous and synchronous serial encoding are supported. As 'digital' signals (switched D.C. voltage, such as square waves) are used, as opposed to 'analog' signals (continuously varying voltage, such as sine waves) a very wide bandwidth channel (such as direct wire) is required.

A limited bandwidth channel (such as a phone circuit) would cause severe and unacceptable distortion with consequent loss of information.

RS-232 supports simplex, half-duplex or full-duplex type channels.

In a simplex channel, data will only ever be travelling in one direction, e.g. from DCE to DTE. An example might be a receive only printer.

In a half-duplex channel, data may travel in either direction, but at any given time data will only be travelling in one direction, and the line must be 'turned around' before data can travel in the other direction. An example might be a Bell 201 style modem.

In a full-duplex channel, data may travel in both directions simultaneously. An example might be a Bell 103 style modem.

Certain of the RS-232 'hand-shaking' lines are used to resolve problems associated with these modes, such as which direction data may travel at any given instant.

If one of the devices involved in an RS-232 interface is a real modem (especially a half-duplex modem), the 'hand-shaking' lines must be supported, and the timing relationships between them are quite important.

These lines are typically much easier to deal with if no modems are involved. In certain cases, these lines may be used to allow one device (which is receiving data at a higher rate than it is capable of processing indefinitely) to cause the other device to pause while the first one 'catches up'.

This use of the hand-shaking lines was not really intended by the designers of the RS-232 standard, but it is a useful by-product of the way such interfaces are typically implemented.

Much of the RS-232 standard is concerned with support of modems. These are devices which can convert a serial digital data signal into an analogue signal compatible with a narrow bandwidth (e.g. 3.1kHz) channel such as a switched telephone circuit, and back into serial digital data on the other end.

The first process is called MOdulation, and the second process is called DEModulation, hence the term MODEM.

The actual process used (at data rates of up to 1200 bits per second) is FSK (Frequency Shift Keying), in which a constant frequency sine wave (called the 'carrier') is shifted to a slightly higher or slightly lower frequency to represent a logic 0 or logic 1, respectively.

In a half duplex modem, the entire available bandwidth is used for one direction. In a full duplex modem, the available bandwidth is divided into two sub-bands, hence there is both an originate carrier (e.g. for data from the terminal to the computer), and an answer carrier (e.g. for data from the computer to the terminal). The actual frequencies (in Hertz) used on the Bell 103A full duplex modem are:

     SIGNAL         STATE    ORIGINATE  ANSWER

  
     logic 0           SPACE     1180           1850

     carrier                         1080           1750

     logic 1           MARK        980            1650

 

THE STANDARD CIRCUITS AND THEIR DEFINITIONS

For the purposes of the RS-232 standard, a circuit is defined as a continuous wire from one device to the other.

There are 25 circuits in the full specification, less than half of which are at all likely to be found in a given interface.

In the simplest case, a full-duplex interface may be implemented with as few as 3 circuits.

There is a certain amount of confusion associated with the names of these circuits, partly because there are three different naming conventions (common name, EIA circuit name and CCITT circuit name).

The table below lists all three names, along with the circuit number (which is also the connector pin with which that circuit is normally associated on both ends).

Note that the signal names are from the viewpoint of the DTE (e.g. Transmit Data is data being sent by the DTE, but received by the DCE).

 

     PIN      NAME      EIA      CCITT       DTE  DCE     FUNCTION



      1        CG           AA        101              ---             Chassis Ground

      2        TD           BA         103               -->             Transmit Data

      3        RD           BB         104     <-- Receive Data 4 RTS CA 105>                         Request To Send

      5        CTS         CB         106     <-- Clear To Send 6 DSR CC 107 <-- Data Set Ready 7 SG AB 102 Signal Ground 8 DCD CF 109 <-- Data Carrier Detect 9* <-- Pos. Test Voltage 10* <-- Neg. Test Voltage 11 (usually not used) 12+ SCDC SCF 122 <-- Sec. Data Car. Detect 13+ SCTS SCB 121 <-- Sec. Clear To Send 14+ STD SBA 118>                         Sec. Transmit Data

     15#      TC          DB          114     <-- Transmit Clock 16+ SRD SBB 119 <-- Sec. Receive Data 17# RC DD 115 <-- Receive Clock 18 (not usally used) 19+ SRTS SCA 120>                         Sec. Request To Send

     20       DTR        CD          108.2            -->             Data Terminal Ready

     21*      SQ          CG          110     <-- Signal Quality 22 RI CE 125 <-- Ring Indicator 23* CH 111>                         Data Rate Selector

                            CI            112     <-- Data Rate Selector 24* XTC DA 113>                         Ext. Transmit Clock

     25*                                                    -->             Busy

 

In the above, the character following the pin number means:

     *    rarely used

     +    used only if secondary channel implemented

     #    used only on synchronous interfaces

 

Additionally, the direction of the arrow indicates which end (DTE or DCE) originates each signal, except for the ground lines (---).

For example, circuit 2 (TD) is originated by the DTE and received by the DCE.

Certain of the above circuits (11, 14, 16, and 18) are used only by (or in a different way by) Bell 208A modems.

A secondary channel is sometimes used to provide a very slow (5 to 10 bits per second) path for return information (such as ACK or NAK characters) on a primarily half duplex channel.

If the modem used suppports this feature, it is possible for the receiver to accept or reject a message without having to turn the line around, a process that usally takes 100 to 200 milliseconds.

On the above circuits, all voltages are with respect to the Signal Ground(SG) line.

The following conventions are used:

     Voltage        Signal     Logic     Control

     +3 to +25      SPACE        0           On

     -3 to -25      MARK         1           Off

 

Note that the voltage values are inverted from the logic values (e.g. the more positive logic value corresponds to the more negative voltage).

Note also that a logic 0 corresponds to the signal name being 'true' (e.g. if the DTR line is at logic 0, that is, in the +3 to +25 voltage range, then the Data Terminal IS Ready).

ELECTRICAL CHARACTERISTICS OF EACH CIRCUIT

DEFINITION OF THE MOST COMMON CIRCUITS

1    CG    Chassis Ground



     This  circuit  (also called Frame Ground) is a mechanism  to

     insure  that the chassis of the two devices are at the   same

     potential, to prevent electrical shock to the operator. Note

     that  this circuit is not used as the reference for   any  of

     the other voltages. This circuit is optional. If it is used,

     care should be taken to not set up ground loops.



2    TD   Transmit Data



     This  circuit is the path whereby serial data is  sent   from

     the DTE to the DCE.  This circuit must be present if data is

     to travel in that direction at any time.



3    RD   Receive Data



     This  circuit  is the path whereby serial data is sent   from

     the DCE to the DTE.  This circuit must be present if data is

     to travel in that direction at any time.



4    RTS  Request To Send



     This  circuit  is  the signal that indicates   that  the  DTE

     wishes  to  send data to the DCE (note that no such line  is

     available  for the opposite direction,  hence the   DTE  must

     always be ready to accept data).  In normal  operation,   the

     RTS  line  will be OFF (logic 1 / MARK).  Once the   DTE  has

     data  to  send,  and has determined that the channel is  not

     busy,  it will set RTS to ON (logic 0 / SPACE), and await an

     ON condition on CTS from the DCE,  at which time it may then

     begin  sending.  Once the DTE is through  sending,   it  will

     reset  RTS  to  OFF (logic 1 / MARK).  On a   full-duplex  or

     simplex  channel,  this  signal  may be set to  ON  once  at

     initialization and left in that state.  Note that some   DCEs

     must  have  an incoming RTS in order to  transmit   (although

     this  is  not strictly according to the standard).   In  this

     case,  this  signal must either be brought across   from  the

     DTE,  or provided by a wraparound (e.g. from DSR) locally at

     the DCE end of the cable.



5    CTS  Clear To Send



     This  circuit is the signal that indicates that the   DCE  is

     ready to accept data from the DTE.  In normal operation, the

     CTS line will be in the OFF state. When the DTE asserts RTS,

     the  DCE  will do whatever is necessary to allow data to  be

     sent (e.g.  a modem would raise carrier,  and wait until  it

     stabilized).  At this time,  the DCE would set CTS to the ON

     state, which would then allow the DTE to send data. When the

     RTS from the DTE returns to the OFF state,  the DCE releases

     the channel (e.g.  a modem would drop carrier), and then set

     CTS back to the OFF state. Note that a typical DTE must have

     an  incoming CTS before it can transmit.  This   signal  must

     either  be  brought  over from the DCE,  or   provided  by  a

     wraparound  (e.g.  from  DTR) locally at the DTE end of  the

     cable.



6    DSR  Data Set Ready



     This circuit is the signal that informs the DTE that the DCE

     is alive and well. It is normally set to the ON state by the

     DCE  upon power-up and left there.  Note that a typical   DTE

     must  have  an incoming DSR in order to  function   normally.

     This  line  must either be brought over  from   the  DCE,  or

     provided by a wraparound (e.g.  from DTR) locally at the DTE

     end  of  the cable.  On the DCE end of the   interface,  this

     signal  is almost always present,  and may be   wrapped  back

     around (to DTR and/or RTS) to satisfy required signals whose

     normal function is not required.



7    SG   Signal Ground



     This  circuit is the ground to which all other voltages   are

     relative. It must be present in any RS-232 interface.



8    DCD  Data Carrier Detect



     This  circuit is the signal whereby the DCE informs the   DTE

     that it has an incoming carrier.  It may be used by the   DTE

     to  determine  if the channel is idle,  so that the DTE  can

     request  it  with  RTS.  Note that some DTEs   must  have  an

     incoming DCD before they will operate.  In this  case,   this

     signal must either be brought over from the DCE, or provided

     locally  by a wraparound (e.g.  from DTR) locally at the DTE

     end of the cable.



15   TC   Transmit Clock



     This circuit provides the clock for the transmitter  section

     of  a synchronous DTE.  It may or may not be running at   the

     same  rate  as  the receiver clock.  This   circuit  must  be

     present on synchronous interfaces.



17   RC   Receiver Clock



     This  circuit provides the clock for the receiver section of

     a synchronous DTE.  It may of may not be running at the same

     rate as the transmitter clock.  Note that both TC and RC are

     sourced  by  the  DCE.  This  circuit   must  be  present  on

     synchronous interfaces.



20   DTR  Data Terminal Ready



     This  circuit provides the signal that informs the DCE   that

     the  DTE  is alive and well.  It is normally set to  the  ON

     state  by the DTE at power-up and left there.  Note   that  a

     typical  DCE  must  have  an incoming   DTR  before  it  will

     function  normally.  This signal must either be brought over

     from the DTE,  or provided by a wraparound (e.g.  from   DSR)

     locally at the DCE end of the cable.  On the DTE side of the

     interface,  this signal is almost always present, and may be

     wrapped back around to other circuits (e.g.  DSR, CTS and/or

     DCD)  to  satisfy  required hand-shaking   signals  if  their

     normal function is not required.

 

Note that in an asynchronous channel, both ends provide their own internal timing, which (as long as they are within 5% of each other) is sufficient for them to agree when the bits occur within a single character.

In this case, no timing information need be sent over the interface between the two devices.

In a synchronous channel, however, both ends must agree when the bits occur over possibly thousands of characters. In this case, both devices must use the same clocks.

Note that the transmitter and receiver may be running at different rates.

Note also that BOTH clocks are provided by the DCE. When one has a synchronous terminal tied into a synchronous port on a computer via two synchronous modems, for example, and the terminal is transmitting, the terminal's modem supplies the Transmit Clock, which is brought directly out to the terminal at its end, and encodes the clock with the data, sends it to the computer's modem, which recovers the clock and brings it out as the Receive Clock to the computer.

When the computer is transmitting, the same thing happens in the other direction.

Hence, whichever modem is transmitting must supply the clock for that direction, but on each end, the DCE device supplies both clocks to the DTE device.

All of the above applies to interfacing a DTE device to a DCE device.

In order to interface two DTE devices, it is usually sufficient to provide a 'flipped' cable, in which the pairs (TD, RD), (RTS,CTS) and (DTR,DSR) have been flipped.

Hence, the TD of one DTE is connected to the RD of the other DTE, and vica versa.

It may be necessary to wrap various of the hand-shaking lines back around from the DTR on each end in order to have both ends work.

In a similar manner, two DCE devices can be interfaced to each other.

An RS-232 break-out box is particularly useful in solving interfacing problems.

This is a device which is inserted between the DTE and DCE.

Firstly, it allows you to monitor the state of the various hand-shaking lines (light on = signal ON / logic 0), and watch the serial data flicker on TD and/or RD.

Secondly, it allows you to break the connection on one or more of the lines (with dip-switches), and make any kind of cross-connections and/or wraparounds (with jumper wires).

Using this, it is fairly easy to determine which line(s) are not functioning as required and quickly build a prototype cable that will serve to interface the two devices.

At this point, the break-out box can be removed and a real cable built that performs the same function.

An example of this kind of device is available from BlackBox.

Care should be taken with this type of device to connect the correct end of it to the DTE device, or the lights and switches do not correspond to the actual signals.