Motorola MVME197LE - Manual (Page 72)

EIA-232-D Interconnections
A-4 User’s Manual
A
be provided by a pull-up resistor or gate as described before (see Figure A-1).
Many modems expect a DTR high signal and issue a DSR. These signals are
used by software to help prompt the operator about possible causes of trouble.
The DTR signal is used sometimes to disconnect the phone circuit in
preparation for another automatic call. It is necessary to provide these signals
in order to talk to 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. Figure A-2 shows a way that an EIA-232-D
connector can be wired to enable a computer to connect to a basic terminal
with only three wires. This is because most terminals have a DTR signal that is
ON and can be used to pull-up the CTS, DCD and DSR signals. Two of these
connectors wired back-to-back can be used. It must be realized that all the
handshaking has been bypassed and possible diagnostic messages do not
occur. Also, the TX and RX lines may have to be crossed since TX from a
terminal is outgoing but the TX line on a modem is an incoming signal.