Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 34)

Publication 1785-UM012D-EN-P - July 2005
2-2 Selecting and Placing I/O
Guidelines for Selecting I/O Modules
Selecting I/O Module
Density The density of an I/O module is the number of controller input or output
image-table bits to which it corresponds. A bidirectional module with 8 input
bits and 8 output bits has a density of 8. I/O module density helps determine
your I/O addressing scheme. See chapter 4 for more information about I/O
addressing.
Use these guidelines for selecting I/O module density:
Choose this Type of
I/O Module For these Types of Field Devices or Operations
(examples)
Explanation
Discrete input module
and block I/O module Selector switches, pushbuttons, photoelectric eyes, limit
switches, circuit breakers, proximity switches, level
switches, motor starter contacts, relay contacts,
thumbwheel switches Input modules sense on/off or opened/closed
signals. Discrete signals can be either ac or dc.
Discrete output module
and block I/O module Alarms, control relays, fans, lights, horns, valves, motor
starter, or solenoids Output module signals interface with on/off or
opened/closed devices. Discrete signals can be
either ac or dc.
Analog input module Temperature transducers, pressure transducers, load cell
transducers, humidity transducers, flow transducers, and
potentiometers Convert continuous analog signals into input values
for the PLC controller.
Analog output module Analog valves, actuators, chart recorders, electric motor
drives, analog meters Interpret PLC controller output to analog signals
(generally through transducers) for field devices.
Specialty I/O modules Encoders, flow meters, I/O communication, ASCII, RF type
devices, weigh scales, bar-code readers, tag readers,
display devices Are generally used for specific applications such as
position control, PID, and external device
communication.
Choose this I/O Density If You
8-point I/O module
•
currently use 8-point modules
•
need integral, separately-fused outputs
•
want to minimize cost per module
16-point I/O module
•
currently use 16-point modules
•
need separately-fused outputs with a special
wiring arm
32-point I/O module
•
currently use 32-point modules
•
want to minimize number of modules
•
want to minimize the space required for I/O
chassis
•
want to minimize cost per I/O point