Motorola MVME197LE user manual Ring Indicator RI is sent by the modem to

		EIA-232-D Interconnections
	Table A-1. EIA-232-D Interconnections (Continued)
Pin	Signal	Signal Name and Description
Number	Mnemonic
22	RI	RING INDICATOR - RI is sent by the modem to the
		terminal. This line 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.
23	---	Not used.
24	TXC	TRANSMIT CLOCK - Same as TXC on pin 15.
25	BSY	BUSY - a positive EIA signal applied to this pin causes the
		modem to go off-hook and make the associated phone busy.

A

Notes

1.High level = +3 to +15 volts. Low level = -3 to -15 volts.

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

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

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 applications, TRS is turned around and returned as TRS 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. It 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 CARRIER NOT PRESENT to help the user to diagnose failure to communicate. Obviously, if the system is designed properly to use this signal, and it is not connected to a modem, the signal must

MVME197LE/D2 A-3