PLC's-Programmable Logic Controllers Chapter 2
2 sections of a process module
CPU and memory
Bits
(binary digits) memory elements measures in K
CJC
(cold conjunction compensating) integrated in the terminal block to obtain accurate readings from each of the channels
most common analog I/O
4 to 20 milliamps
byte
8 bits
communications module
In industrial automation applications communication modules are used as connectors for sending different electric signals, including converting RS-232, RS-422/485 signals and making the series information of servo and control devices compatible with one another.
R/W memory
RAM temporary storage area of data
bridge rectifier
a rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one
Addressing
a system where the address is a label or number that indicates where a certain piece of information is located in the PLC's memory
I/O group
a word in the output image table file and its corresponding word in the input image table file
voltage sensing
for a given current flow, a proportional magnetic field is produced around the current carrying conductor
input impedance and capacitance
for analog i/os, these values must be matched to the external device connected to the module. typical ratings are in the meg-ohm and pico-farad range
noise
generally caused by electromagnetic interference, radio frequency interference, and ground loops
HMI
human machine interface that gives the operator the ability to view the operation in real time
high density
may have up to 64 inputs or outputs
discrete type
most common, connects field input devices of the ON/OFF nature
5 rungs
most hand held device will show on screen
common-mode rejection
noise that is picked up equally in parallel wires is rejected because the difference is zero. twisted pair wires are used to ensure that this type of noise is equal on both wires
coil or contact
one location per...
PLC memory capacitance
one program at a time
rack/slot-based
one type of addressing schemes
current sourcing
output devices that allow current flow from the load through and then to the ground
PC
personal computer
REM position
places the processor in remote mode
discrete input module
power section and logic section
writing
process of storing data in memory
PLC power supply
provides the necessary power (typically 5 VDC) to the processor and I/O modules plugged into the backplane of the rack
ROM
read only memory
current sinking
refers to an output device that allows current flow from the load through the output to ground
memory location
refers to the address in the CPU's memory where a binary word can be stored
memory utilization
refers to the number of memory locations required to store each type of instruction
nonvolatile
retains stored information even if there is a loss or removal of power
reading
retreiving of data from memory
modes of operation
run, program, and REM
operating signals
signal that comes out of an electronic system that encodes a message
bipolar
signals swing between a maximum negative value and a maximum positive value
resolution
specifies how accurately an analog value can be represented digitally. this will determine the smallest measurable unit of current or voltage change that can be detected
logic section
specifies the logical operations
memory
the element that stores information and data in a PLC
chassis
the hardware assembly that houses I/O modules, processor modules, and power supplies
hold up time
the length of time a PLC can tolerate a power loss (typically ranges from 10 milliseconds to 3 seconds)
slot (number)
the physical location of the I/O module
remote rack
the rack when it is located away from the processor module
input/output section of a PLC
the section to which all field devices are connected and provides the interface between them and the CPU
resolution
the smallest distinguishable increment into which a quantity is divided
input current/voltage range(s)
these are the voltage or current signal ranges that an analog input module is designed to accept
output current/voltage range(s)
this specification defines the current or voltage signal ranges that a particular analog output module is designed to output under program control
unipolar and bipolar
two types of analog voltage input modules
bit oriented
type of input/output where each bit represents a complete information element in its self and provides the status of some external contact or advises of the presence or absence of power by some field supplied voltage source
central processing unit (CPU)
typically a plug-in module built into single-unit fixed PLC's
UV EPROM
ultraviolet erasable programmable read only memory
status information
up to date information
modular types
use internal I/O modules that plug into the PLC
word and bit
use to identify the actual terminal connection in a particular I/O module
word
used by analog - allows the entire word to be addressed
transistor
used for DC devices only
triac
used only for AC devices-a semiconductor device that allows current to pass through it when it has a small "trigger" current signal applied to its gate
NPN and PNP
used to describe a current signal flow relationship between field input and output devices in a control system and their power supply
CMOS-RAM
user memory maintained for 2 to 5 years
discrete module
usually uses only one word
RAM
volatile random access memory
zener diode
voltage rating that sets the minimum threshold level of voltage that can be detected
interposing relay
when a control relay can be used to control a larger load or motor starter
channels per module
whereas individual circuits on discrete I/O modules are specified as points per module, circuits on analog I/O modules are specified as channels per module
input interface modules
accept signals from the machine or process devices and convert them into signals that can be used by the controller
program position
allows program entry and editing, but prevents the processor from scanning or executing the ladder program
redundant (two) processors
allows transfer of control to the second processor in the event of a processor fault
logical rack
an addressable unit consisting of 128 input points and 128 output points, uses 8 words in input and 8 words in output image file
relay
an older electrical operated device that has external inputs, outputs, counters, timers, and other circuits wired on it that need to be mechanically operated
analog output interface module
an output circuit that employs a digital-to-analog converter to convert a digital value, sent from the processor, to an analog value that will control a connected analog device
input protection
analog input circuits are usually protected against accidentally connecting a voltage that exceeds the specified input voltage range
tag-based
another type of addressing scheme
bit
binary digit that can either be 1 or 0
unipolar
can accept an input signal that varies the positive direction only
output interface
can control external devices
hand held device
compact, inexpensive device to program PLC's
output interface modules
convert controller signals into external signals used to control the machine or process
electrostatic voltages
degrades performances
type
determines if an input or output is being addressed
D/A
digital to analog converter - the main element of analog output module
volatile
easily altered and will lose its stored information if all operating power is lost
EEPROM
electrical erasable programmable read only memory
EPROM
erasable programmable read-only memory
run position
executes ladder program and energizes output devices while preventing online program editing
field supplied
external power source
flash EEPROM
fast backup storage
sourcing field devices
field devices connected to the (+) side of the field power supply
sinking field devices
field devices connected to the (-) side or DC common of the field power supply
current sensing
for a given current flow, a proportional magnetic field is produced around the current carrying conductor
