Page 45 - Programmable Logic Controllers, Fifth Edition

+
Coil Contact AC/DC AC DC
–

Load Load Load
Relay output Triac output Transistor output
Figure 2-18 Relay, transistor, and triac switching elements.

whereas transistor outputs can be used only for control thereby allowing it to be set to whatever the field devices
of DC devices. The discrete relay contact output module require. Sinking and sourcing terminology applies only to
uses electromechanical as the switching element. These DC input and output circuits.
relay outputs can be used with AC or DC devices, but they Allen-Bradley delineates between the various digi-
have a much slower switching time compared to solid- tal DC modules by sorting them into two categories:
state outputs. Allen-Bradley modules are color-coded for Sinking and Sourcing. These terms are used to describe
identification as follows: a current signal flow relationship between field input and
output devices. If a device provides current when it is
ON, it is said to be sourcing current. Conversely, if a
device receives current when it is ON, it is said to be
Color Type of I/O sinking current.
Figures 2-19 and 2-20 show device connections for
Red AC inputs/outputs both sourcing and sinking configurations:
Blue DC inputs/outputs
Orange Relay outputs • Conventional current (+ to −) is assumed.
Green Specialty modules • In sinking devices, current flows into the device’s
Black I/O wiring; terminal blocks terminal from the module (the module provides, or
are not removable sources the current).
• In sourcing devices, current flows out of the
device’s terminal into the module (the module
Certain DC I/O modules specify whether the module
is designed for interfacing with current-source or current- receives, or sinks, the current).
sink devices. If the module is a current-sourcing module, • A sourcing I/O device or I/O module will always
then the input or output device must be a current-sinking have a connection directly to the positive side of the
device. Conversely, if the module is specified as current- DC power supply.
sinking, then the connected device must be current- • A sinking I/O device or I/O module will always
sourcing. Some modules allow the user to select whether have a connection directly to the negative side of the
the module will act as current sinking or current sourcing, DC power supply.

Sinking input Sourcing input
module module
0 0
1 1
2 2
Sourcing 3 Sinking 3
sensor sensor
4 4
POS (+) 5 NEG (–) 5
Power 6 Power 6
supply supply
7 7
NEG (–) (–) POS (+) (+)
Common Common
Current Current
Figure 2-19 Sinking and sourcing inputs.

26 Chapter 2 PLC Hardware Components

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