TRICONEX 3701A CPU card

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 such as CARRIER NOT PRESENT to help the user 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

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

Sample Configurations

 Figure E-1 is a good middle-of-the-road 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 shows a way of wiring an EIA-232-D connector to enable a computer to connect to a basic terminal with only three lines. This is feasible because most terminals have DTR and RTS signals that are ON, and which can be used to pull up the CTS, DCD, and DSR signals. Two of these connectors wired back-to-back can be used. In this implementation, however, diagnostic messages that might otherwise be generated do not occur because all the handshaking is bypassed. In addition, 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.

Proper Grounding 

Another subject to consider is the use of ground pins. There are two pins labeled GND. Pin 7 is the SIGNAL GROUND and must be connected to the distant device to complete the circuit. Pin 1 is the CHASSIS GROUND, but it must be used with care. The chassis is connected to the power ground through the green wire in the power cord and must be connected to the chassis to be in compliance with the electrical code

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