Introduction - Motorola MVME167 Series Installation Manual

EEIA-232-D
E
Interconnections

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-D standard 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-1 lists 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