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Signal Adaptations
One set of handshaking signals frequently implemented are RTS and CTS. CTS is used in many systems to inhibit transmission until the signal is high. In the modem application, RTS is turned around and returned as CTS after 150 microseconds. RTS is programmable in some systems to work with the older type 202 modem (half duplex). CTS is used in some systems to provide flow control to avoid buffer overflow. This is not possible if modems are used. It is usually necessary to make CTS high by connecting it to RTS or to some source of +12 volts such as the resistors shown in Figure
Another signal used in many systems is DCD. The original purpose of this signal was to inform the system that the carrier tone from the distant modem was being received. This signal is frequently used by the software to display a message like CARRIER NOT PRESENT to help the user to diagnose failure to communicate. Obviously, if the system is designed properly to use this signal and is not connected to a modem, the signal must be provided by a pullup resistor or gate as described above (see Figure
Many modems expect a DTR high signal and issue a DSR response. These signals are used by software to help prompt the operator about possible causes of trouble. The DTR signal is sometimes used to disconnect the phone circuit in preparation for another automatic call. These signals are necessary in order to communicate with all possible modems (see Figure