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The devices using sequential cables for connection are divided into
two groups. They are DCE (Data Communications Equipment) and DTE (Data
Terminal Equipment.). Data Communications Equipment are modem, TA
adapter, plotter devices etc, and Data Terminal Equipment is a
terminal or a computer.
The electric characteristics of RS-232
serial port are contained in EIA (Electronics Industry Association).
It contains the following parameters:
- "Space" (logic 0) should be between +3 & +25 volt;
- A "Mark" (Logic 1) should be between -3 & -25
volt;
- The range from -3 to +3 volt is not defined;
- The curcuit voltage shouldn't exceed 25 volt regarding earth;
- The cyrcuit consumption current shouldn't exceed 500mA. It is
needed for the dvice to work correctly. (This condition must
always be met!).
The scheme below shows the sygnal levels at the receiver and the
transmitter of the serial port.
The information given above is only the basis of the EIA standard.
Besides these parameters there's also line capacitance, max baud rates
etc.To obtain full information refer to the EIA RS-232-C standard.
Standing aside is the fact that the EIA RS-232-C standard limits
the max baud rates with 20000 bps, but nowadays it is rather slow.
That's why today the new RS-232-D standard is mainly used.
The serial ports have two plug "sizes", they are 25-pin
D-type plug and 9-pin D-type plug. Both these types have male type of
sockets, that's why the female connectors are required for connection.
Below is the table describing the contacts for 9-and 25-pin D-type
plugs.
The table contains the list of signals mentioned in theRS-232-C
standard, each signal is identified with the cirquit symbols according
with V.24 description (complies with CCITT standard), DB-25 and DB-9
pin number andname according to the name of the signal. The cirquit
characters connected with the signal are described as follows:
- A - general output;
- B - signal output;
- C - control output;
- D - timing output;
- S - secondary channel.
AA:Protective ground
This line is connected with the power ground of the serial adapter.
This connection cannot be used as signal ground. This line must be
connected to the main cable screen (if there is one). When using this
line in the cable you must make sure that this wire doesn't contact
with other wires as it may cause drain into ground. If the drain is
substantial it may cause electric signal distortions (contact with
other wires may lead to undesirable consequences including even aying
up the serial port). If this line is not used, you should remember
that when connecting two remote devices you can't use timing signals
because of the different device potentials.
AB: Signal ground
This line is signal ground which is used as a reference point for
received and transmitted signals. This line is needed for any
connection type.
BA: Transmitted data
This line is used as the transmitting line from DTE to DCE. '1' is
always present in this line when there's no connection. The terminal
usually initializes the transmission when the logical 1 is present in
the following lines:
- Clear To Send
- Data Terminal Ready
- Data Set Ready
- Data Carrier Detect
Output signal levels are defined by the standard.
BB: Received data
This line is used for receiving data from DCE by DTE. The terminal
usually initializes the transmission when 1 is present in the
following lines:
- Request To Send
- Data Terminal Ready
- Data Set Ready
- Data Carrier Detect
Output signal levels are defined by the standard.
CA: Request To Send
This line is needed for DTE to send the signal about the necessity
of transmitting data for DCE.
CB: Clear To Send
This line is needed for DCE to signal that it is ready to receive
data from DTE (there is an exceptional case when your modem is
connected with another modem by a telephone line).
CC: Data Set Ready
If the signal on this line is set to '1' it means that DCE is ready
to send data to DTE (there is an exceptional case when your modem has
established connection with a remote modem and is in transmission
mode).
CD: Data Terminal Ready
If the signal on this line is set to '1' it means that DCE can
receive and send data. If the signal on the line is set to logical 0,
DCE will stop the transmission (the modem is an exception as it will
disconnect the communications lines, that's why you will often see the
"DROP DTR" messagein connection programmes).
CF: Data Carrier Detect
With the help of this line DCE reports to DTE that the connection
with the remote device is established.
CE: Ring Indicator
This line is mainly used by connection programmes to identify that
the remote device is calling, this signal is unavailable in 'auto
response' mode. This signal is not necessary; if the connection
software is not used the modem will automaticaly respond.
CG: Signal quality
This line is used to show DTE that the line quality is poor or not
good enough to maintain satisfactory connection.
CH: Data signal rate
selector
This line allows DTE to select the optimal connection speed if the
modem supports such a selection. Usually this signal is set to the
logical 0 and the highest rate is chosen.
CI: Data signal rate
selector
This line is the same as CH, but in this case the modem selects the
speed of connection with DTE.
Timing circuits
In synchronous exchange mode it is necessary to have a way of
exchanging timing signals; there are three time selection circuits
used in the RS-232 protocol:
- DA & DB: Timing transmitter signals are used;
- DA: DTE to DCE (DTE synchronization).
- DB: DCE to DTE (DCE synchronization).
These two cirquits are used to synchronize the data flow. Timing
selection is given to DTE or DCE, but only one device is allowed to
carry out synchronization. Usually data is transmitted to the modem or
it's own clock control on the DB circuit..
DD: Receiver signal timing
DCE
DD: DCE to DTE (DCE synchronization).
This line is used to syncrinize the data flow received from DTE.
Timing signals received in this line inform DTE when to make selection
of data received in BB line.
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