Motorola MVME187 manual EEIA-232-D Interconnections, Introduction

EEIA-232-D

Interconnections

E

Introduction

The EIA-232-D standard is the most common terminal/computer and terminal/modem interface, and yet it is not fully understood. This may be because not all the lines are clearly defined, and many users do not see the need to follow the standard in their applications. Often designers think only of their own equipment, but the state of the art is computer-to-computer or computer-to- modem operation. A system should easily connect to any other system.

The EIA-232-Dstandard was originally developed by the Bell System to connect terminals via modems. Several handshaking lines were included for that purpose. Although handshaking is unnecessary in many applications, the lines themselves remain part of many designs because they facilitate troubleshooting.

Table E-1lists the standard EIA-232-D interconnections. To interpret this information correctly, remember that EIA-232-D was intended to connect a terminal to a modem. When computers are connected to each other without modems, one of them must be configured as a terminal (data terminal equipment: DTE) and the other as a modem (data circuit-terminating equipment: DCE). Since computers are normally configured to work with terminals, they are said to be configured as a modem in most cases.

Signal levels must lie between +3 and +15 volts for a high level, and between -3 and -15 volts for a low level. Connecting units in parallel may produce out-of-range voltages and is contrary to EIA-232-D specifications.

E-1