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:
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:
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:
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:
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).
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).
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.
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.
This line is the same as CH, but in this case the modem selects the speed of connection with DTE.
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:
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: 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.