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A Practical Guide to RS-232 Interfacing

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             A Practical Guide to RS-232 Interfacing

                       Lawrence E. Hughes

                       Mycroft Labs, Inc.
                          P.O. Box 6045
                      Tallahassee, FL 32301

   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 a technician to construct
and/or  debug  interfaces  between any  two  "RS-232  Compatible"
devices.  A more detailed coverage of the subject may be found in
the  book  "Technical Aspects of Data Communication" by  John  E.
McNamara (1977, Digital Press).

   This  guide  is necessary due to the casual way  that  vendors
implement  "RS-232"  interfaces,   sometimes  omitting   required
signals,  requiring optional ones, or worse, implementing signals
incorrectly.  Due  to  this,  and  a lack  of  readily  available
information   about  the  real  EIA  standard,   there  is  often
considerable confusion involved in trying to interface two RS-232
devices.

                           BACKGROUND

   RS-232-C  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,  sometimes 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
later  two  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  and
consequent loss of information.

   RS-232 will support 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  analog signal compatible  with  a  narrow
bandwidth  (e.g.  3  kHz) 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

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           THE STANDARD CIRCUITS AND THEIR DEFINITIONS

   For  the  purposes  of the RS-232  standard,  a  'circuit'  is
defined  to  be 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

also, 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

   The following criteria apply to the electrical characteristics
of each of the above lines:

1) The magnitude of an open circuit voltage shall not exceed 25V.

2)  The driver shall be able to sustain a short to any other wire
in the cable without damage to itself or to the other  equipment,
and the short circuit current shall not exceed 0.5 ampere.

3)  Signals shall be considered in the MARK (logic 1) state  when
the  voltage is more negative than -3V with respect to the Signal
Ground.  Signals shall be considered in the SPACE (logic 0) state
when  the  voltage is more positive that 3V with respect  to  the
Signal  Ground.  The range between -3V and 3V is defined  as  the
transition region, within which the signal state is not defined.

4)  The  load impedance shall have a DC resistance of  less  than
7000 ohms when measured with an applied voltage of from 3V to 25V
but more than 3000 ohms when measured with a voltage of less than
25V.

5) When the terminator load resistance meets the requirements  of
Rule 4 above,  and the terminator open circuit voltage is 0V, the
magnitude of the potential of that circuit with respect to Signal
Ground will be in the 5V to 15V range.

6)  The  driver  shall  assert a voltage  between  -5V  and  -15V
relative  to  the  signal  ground  to  represent  a  MARK  signal
condition.  The driver shall assert a voltage between 5V and  15V
relative  to  the  Signal  Ground to  represent  a  SPACE  signal
condition.  Note  that this rule in conjunction with Rule 3 above
allows for 2V of noise margin.  Note also that in practice,  -12V
and 12V are typically used.

7)  The  driver  shall change the output voltage at  a  rate  not
exceeding 30 volts per microsecond, but the time required for the
signal to pass through the -3V to +3V transition region shall not
exceed  1 millisecond,  or 4 percent of a bit time,  whichever is
smaller.

8) The shunt capacitance of the terminator shall not exceed  2500
picofarads,  including  the capacitance of the cable.  Note  that
when  using  standard  cable with 40 to 50  picofarads  per  foot
capacitance,  this  limits  the cable length to no more  than  50
feet. Lower capacitance cable allows longer runs.

9) The impedance of the driver circuit under power-off conditions
shall be greater than 300 ohms.

   Note that two widely available integrated circuit chips  (1488
and 1489) implement TTL to RS232 drivers (4 per chip),  and RS232
receivers  to TTL (also 4 per chip),  in a manner consistent with
all of the above rules.

             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 of a 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 the International
Data Sciences, Inc. Model 60 'Modem and Terminal Interface Pocket
Analyzer'  (also called a 'bluebox').  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.