Levels Of Implementation; Signal Adaptations - Motorola MVME167 Series User Manual

2. The EIA-232-D interface is intended to connect a terminal to a modem. When
computers are connected without modems, one must be configured as a modem
and the other as a terminal.

Levels of Implementation

There are several levels of conformance that may be appropriate for typical EIA-232-
D interconnections. The bare minimum requirement is the two data lines and a ground.
The full implementation of EIA-232-D requires 12 lines; it accommodates automatic
dialing, automatic answering, and synchronous transmission. A middle-of-the-road
approach is illustrated in Figure A-1.

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 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 tell 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
to help the user to diagnose failure to communicate. Obviously, if the system
PRESENT
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. 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. It is necessary to provide these signals in order to talk to all possible modems (see
Figure A-1).
MVME167/D3
Levels of Implementation
CARRIER NOT
A-3
A