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EIA-232-D Interconnections
A
Signal Adaptations
Sample Configurations
A-4
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 A-1.
CTS is also frequently jumpered to an MC1488 gate which has its
inputs grounded (the gate is provided for this purpose).
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
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 A-1).
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 A-1).
Figure A-1 is a good minimum configuration that almost always
works. If the CTS and DCD signals are not received from the
modem, the jumpers can be moved to artificially provide the
needed signal.
to help
CARRIER NOT PRESENT
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