Lines and Their Usage:
There are four kinds of line defined in the RS232 specification. They are Control, Data Timing and Ground. Not all of them are needed all the time. This is possible to set up a very simple communication by using very few lines. While looking at the lines and their functions it is necessary to remember that they are described for a connection between a modem (the data set or communications equipment) and a terminal or computer (data terminal equipment) in mind. All of the lines have directions, and when utilized in this way a one to one cable correctly operates. The most apparent lines are the data lines. There are two of these, each one for data travelling in each of the direction. Transmit data is carried on pin 2 and the receive data is carried on line three. The most fundamental of the control circuits is Data Carrier Detected (DCD). This illustrates when the modem has detected a carrier on the telephone line and a connection seem to have been made. It generates a high, which is maintained till the connection is lost. Data Set Ready (DSR) and Data Terminal Ready (DTR) are the main control circuits. They convey the main information among the terminal and modem. While the terminal is ready to begin handling data it flags this on the DTR line. If the modem is also ready then it returns its signal on the DSR line. These circuits are mainly utilized for telephone circuits. After a connection has been established the modem will be linked to the line by using DTR. This connection will remain till the terminal is switched off when the DTR line is dropped off. The modem will detect this and release the telephone line. Sometimes pin 20 is not assigned to DTR. Rather than another signal named, Connect Data Set to Line (CDSTL) is utilized. This is virtually the same as DTR, but differs in that DTR purely enables the modem to be switched onto the telephone line. CDSTL commands the modem to switch, in spite of anything else it may be doing. A further two circuits, Request To Send (RTS) and Clear To Send (CTS) are also used. This pair of circuits is utilized together. The terminal equipment will flag that it contains data to send. Then the modem will return the CTS signal to give all clear after a short delay. This signalling is utilized specifically when switched carriers are used. This means that the carrier is only present on the line when there is data to send. It determines its uses when one central modem is servicing several others at remote locations.