Introduction - Motorola MVME167-32 User Manual

Introduction

The EIA-232-D standard is the most widely used 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. Many times 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.
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 A-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.
MVME167/D3A-1
INTERCONNECTIONS
EIA-232-D
A