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Table A-1. EIA-232-D Interconnections (Continued)
Pin
Signal
Number
Mnemonic Signal Name and Description
22
RI
23
24
TxC
25
BSY
Notes:
1. A high EIA-232-D signal level is +3 to +15 volts. A low level is -3
to -15 volts. Connecting units in parallel may produce out-of-range
voltages and is contrary to specifications.
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:
A middle-of-the-road approach is illustrated in Figure A-1.
RING INDICATOR. Output from the modem to the terminal;
indicates to the terminal that an incoming call is present. The
terminal causes the modem to answer the phone by carrying
DTR true while RI is active.
Not used.
TRANSMIT CLOCK (DTE). Input to modem from terminal;
same function as TxC on pin 15.
BUSY. Input to modem from terminal. A positive EIA signal
applied to this pin causes the modem to go off-hook and make
the associated phone busy.
Automatic dialing
Automatic answering
Synchronous transmission
Levels of Implementation
A
A-3
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