


                  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 capaci-
     tance 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  initializa-
tion 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  synchro-
nous 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.
