ProfitPress Mega CDROM2 …eeware (MSDOS)(1992)(Eng)/ProfitPress-MegaCDROM2.B6I/COMM/MISC/TELEDX10.ZIP/RS232

Text File | 1992-02-06 | 11.1 KB | 261 lines

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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.

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  connec-
tors") 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  Equip-
ment),  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  support-
ed,  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  indefinite-
ly)  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   frequen-
cy  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  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  inter-
face 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




          PIN  NAME EIA  CCITT  DTE  DCE     FUNCTION

          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).

                                                                RS232 DATA
                                                                LINE
                                                                DEFINITIONS
                                                                <232DEF>