PLC Setup

Lab Setup

• Power Supply

• SLC 5/03 CPU

• DC input card

• Relay output card

Our lab setup consists of 8 stations each having an IBM-PC compatible computer, 24VDC power supply and the followingPLC hardware

Power Supply

PLCs typically include a power supply module in the rack. Thepower supply only supplies backplane power. This will powerthe internal electronics of the module.

External devices, such as sensors, relays, solenoids must have a different external power source.

CPU front

Channel 0 connects to IBM-PCserial port

Channel 1 (when used), connects to the datahighway

LED will flash when data is beingtransferred to/from PLC

CPU

• SLC 5/03 Processor

• 8K memory

• version 3.02 of firmware

In the Allen-Bradley SLC 500 PLC family there are a number ofdifferent processors. Each processor will support a core set of instructions and features. However, every processor is differentin terms of the advanced instructions that it will support, amountof memory available and speed.

Going from least to most powerful the family starts with the SLC 500 (non-modular), 5/01, 5/02, 5/03, 5/04 and 5/05.

We have the

DC input card

• 16 point

• Uses “sinking” to activate

• Each input is activated by bringing the terminal to ground

External connections

• The PLC power supply doesn't / cannot supply power for the I/O

• 24 VDC external supply for the module

• Serial cable to PC, COM 1

Connecting outputs

• Inductive/Capacitive proximity

• Photoelectric

• Shaft encoders

• Relays & Solenoids

• Mechanical counters

PLC I/O devices

• Work by using a tuned circuit

• Insert RLC diagram here

• The circuit is De-tuned by bringing an object near it

Inductive/Capacitive proximity

• Works by sending and receiving a stream of light pulses

• If the stream is interrupted, the sensor activates

• Typically operate in the infra-red region

• Often have both sink and source outputs

Photoelectric

Mechanical counters

• Pretty much tamper proof

• Slow, 20-50 mSec per increment

• Retain display when power is lost/off

PLC Scan

• All inputs are read at the beginning of the scan

• Inputs that change before the next scan are not detected

Inputs

Description

• In the SLC CPUs the placement of rungs within the program does matter

Logic solved

• Communications with the PC

• Remote I/O racks

• Device Net

Remote I/O

• Updating timers, counters

• Updating “forces”

• Diagnostics

• Watchdog timer– Checks to make sure a scan doesn’t take too

long

Housekeeping

In small programs housekeeping time may account for 90%+ ofthe scan time.

• At the end of the scan, the information from the output table is copied to the physical outputs

Outputs

DH 485

Network communication

There are two major protocols for network communication. Token passing and CSMA/CD.

Imagine a classroom full of people who would all like to participate in a group discussion. With token passing, someone (maybe the instructor) would start, he/she would beable to talk until they were done. They would then pass thetoken (in this example the token could be a microphone), to the next person who could then speak. This process continuesuntil everyone has had a chance to talk. If someone has nothingto say, they pass the token on. This approach is also called “Round Robin”.

Token passing

CSMA/CD

Instead of using tokens to control who has the right to communicatewe could apply the following rules:1. If no one is speaking, then anyone who wants to can speak.2. If someone is speaking, then wait3. If two or more people start to speak at the same time, everyonestops, pauses a random amount of time and try again.

CSMA/CD stands for Carrier sense (is anyone speaking), multipleaccess (many people can use the system) with collisiondetect (if two people start to speak at the same time, I.e. you heargarbled speech, stop). Ethernet uses this protocol.