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Introduction to PLC

Using the SLC-500

May, 2006

Instructor: Ricky Bryce

(http://www.LearnAutomation.com)

“Customized Automation Training”

 

















Copyright (c)  1999 Ricky Bryce.

Permission is granted to copy, distribute verbatim copies of this document, commercially or non-commercially with no front cover texts, or back cover texts.  Changing this document is not allowed.





Disclaimer:
This document is written in the hope that you can utilize for your own education to gain knowledge of PLC systems (should you decide to utilize this document).  
Although I believe the information in this document to be accurate, it is YOUR responsibility to verify this information before implementing it in any way, especially when damage to personnel or equipment could result.  
By continuing to read this document, you agree to hold no one who writes, modifies, or distributes this document liable in any way (even negligence).  
Due to the wide variety of plant applications, some of the examples in this document may be prohibited at your location, or could cause damage to equipment, or harm personnel.

Glossary

PLC: Programmable Logic Controller. Allows for change in the operation

of equipment simply by changing a program in the processor.

Old-style relay logic would require the panels to be rewired for

changes in operation.

Addressing:

Bit: Smallest unit of information the PLC can process– ON or OFF

Word: Memory location consisting of 16 Bits of information

Hardware:

Input module: Reads the STATUS of field devices

Output module: CONTROLS field devices

Discrete module: Reads or controls devices which only have 2 states:

on or off

Analog Module: Reads or controls devices which have a range

such as 0 to 10 volts or 4 to 20 milliamps

Power Supply: Provides control power to modules on the backplane

Chassis: The physical device that modules are plugged into.

Local Chassis: The chassis where the processor resides.

Processor: The 'Brain' of the PLC which contains the machine program.

Troubleshooting Tools:

Cross Referencing:

Allows the troubleshooter to quickly navigate through the

program by listing all locations in ladder logic where a

particular address is located.

Usually the troubleshooter will cross reference a false condition on a

rung of logic to find the output that will turn the referenced bit on.

You can Access Cross Referencing by right clicking a particular address. (Note: You must be on the address, not the instruction)

Custom Data Monitor Utility:

Allows the troubleshooter to gather data from various

memory locations onto one screen for easy troubleshooting. For

example: One can create a custom data monitor for the failure of

a particular motor. Next time the motor fails, the troubleshooter

can simply look down the list of conditions that must be met, and

see in real time which condition is causing the failure.

.

Force:

Simulates real world jumpers. Use care while performing a force.

You must understand fully how a force is going to affect your system.

In most cases, only addresses starting with an I: or an O: can be forced.

To force an input or output, you can right click on the address in logic, then choose force on or force off.

After the force is installed, forces can then be enabled from

the online toolbar.

Trending:

Trending acts somewhat like a 'software chart recorder', and

allows you to track an analog signal over time.

Trending can be found toward the bottom of the project tree.

Communication terminology:

RSLinx: This is the communication server. If RSLinx is not set up properly, RSLogix will not communicate to the processor.

Driver: Allows RSLinx to communicate with a particular hardware device. The most common drivers are the DF1 driver to communicate with

Channel 0, the PCMK driver to communicate with channel 1.

Configure drivers by clicking communication on the menu bar.

RSWho: A graphical screen which will display what devices RSLinx has

established communication with. Access RSWho by clicking

communication on the menu bar. Then click on the name of

the driver you wish to use for communication. The right hand

side of the screen will reveal devices the driver has communication

with.

RSLogix: The software which allows you to troubleshoot or program a processor.

Online: Actively communicating with the processor (ladder spinning)

Download: If a program was changed offline, it must be downloaded (

or sent to) the processor. When downloading the processor must

be in program or remote program mode. A good way to download

once RSLinx is properly set up is to click COMMS on the menu bar, and then go to Who Active. Click the driver name, highlight your processor, then click DOWNLOAD.

The Basics – What is a PLC?

PLC is a pneumonic for Programmable Logic Controller. Below is a photo of a typical SLC (small logic controller) chassis.

Input Modules will read data from field devices such as limit switches and pushbuttons. These inputs are used in the program to decide what outputs to energize. Output devices such as relay coils and solenoids are wired to the Output Modules. The processor usually resides in the first slot of a chassis, and is responsible for controlling the process. The processor is where the PLC Program resides.

The Logic in the SLC is designed to simulate relay logic to some extent. A big advantage of using a PLC, however, is that the operation of the equipment can be altered without making hardware and wiring changes to the machine itself.

PLC's were originally designed as a solution in the automotive industry for plants that needed to quickly make equipment changes. The PLC can often be used to replace a panel containing thousands of relays. Tools built into the software allow the troubleshooter to quickly troubleshoot problems. The software used to program and troubleshoot the SLC system is called RSLogix 500.

The Local Chassis is considered the chassis where the processor resides. The SLC processor is ALWAYS placed in Slot 0 of the Local Chassis.

Hardware – Discrete Input Modules

The purpose of the discrete input module is to read the status of field devices. When a voltage is detected on the terminal of an input module with respect to common, the corresponding status light is energized, and during the processor scan, the value of 1 is placed into the input data table. Examples of input devices include switches, pushbuttons, or auxilliary contacts on a motor starter.

Please answer the following questions:

1. What is the catalog number of your DC Input module?

2. Name at least three field devices that can be connected to the DC Input module?

3. What do the status lights indicate on the front of the DC Input Module?

4. What slot number is your DC Input module located in?

Below is a typical wiring diagram of a PLC Input module. You will notice that we just supply a common to the module, and we detect an input is present at any point the + side of the supply is connected to the module. This module is acting as 16 individual voltage testers.

Discrete Output Modules

The purpose of the discrete output module is to control field devices. The discrete input module requires power from an external source. When a 1 is placed into the output table of the PLC (in run mode), a status light is energized on the module, and a connection is made between the source, and the corresponding output terminal. Examples of output devices include: lights, solenoids, motor starter coils, and contactors. If you have an inductive load as the output, be sure to use the proper surge supression.

Please answer the following questions:

1. What is the catalog number of your DC Output module?

2. Name at least three field devices that can be connect to the DC Output module:

3. What do the status lights indicate on the DC Output module?

4. What slot number is your DC Output module located in?

5. If the load on the DC output card is inductive, what should be done across the load to minimize the effects of inductive kick?

Below is a diagram of an output module. You will notice that we must supply the module with an external source of power. When the PLC calls for this output to energize, the output module merely connects the external source of power to the field device.

Analog Modules

Analog modules are used to control and read the status of analog devices. Analog devices have a range of states instead of just on/off states like discrete devices.

Some analog modules have switches which determine whether the input channels are to be set up for voltage or current. Some analog modules are configured through software.

Examples of analog inputs include: Potentiometer, Pressure Transducers, Variable speed drives, and with a thermalcouple module, temperature can be read into the processor's memory.

Examples of analog outputs include: Meters, Variable Speed Drives, Valve Positioners, and chart recorders.

An analog signal cannot be expressed with a single bit, and therefore analog values will consume a word of memory.

Please answer the following questions:

1. What is the catalog number of your analog module?

2. How many channels of Input, and how many channels of Output are available on this module?

3. How do you set up the input channels to accept either a current or a voltage input?

4. What range voltage or current will the Input channels accept on your module? What range of voltage will the output channels accept?

5. Name at least three devices that are analog inputs:

6. Name at least three field devices that are analog outputs:

7. What slot number is your analog output module located in?

An Analog Module – 1747-NIO4V

The Chassis

The chassis is the device which holds modules. Allen Bradley makes the SLC chassis available in 4, 7, 10, or 13 slots, and the PLC-5 chassis in 4, 8, 12, or 16 slots. For the SLC, the processor is included in the slot count. For the PLC-5 chassis, the processor is not included in the slot count.

Both the SLC and the PLC support extended local I/O. This is more common in the SLC, however. For the SLC system, a maximum of 3 chassis can be connected together using extended local I/O, but not to exceed the valid slot count of 0 to 31. The 1746-C7 cable can be used to connect two chassis together which are mounted horizontally (side by side), and the 1746-C9 cable is longer for chassis that are mounted vertically (one above the other).

Here are some chassis:

The PLC-5 Chassis (With modules): (For dip switch settings on this chassis, refer to page 4-1 and 4-2 of the PLC-5 Quick Reference Guide.)

The SLC-500 Chassis (With modules):

The ControlLogix Chassis (With Modules):

The Flex Chassis:

The Power Supply

The power supply supplies power to the modules on the backplane. Generally power from field devices DOES NOT come from the power supply. The power supply only provides control power to modules on the backplane. Power for field devices come from a separate source which is connected to the output module. The power supply merly provides the power needed to shut a contact, or fire a triac or transistor to pass power from this external source to the field device.

The power supply has a fuse which protects the AC side of the power supply. If this fuse blows, the power supply is probably defective and in need of repair.

You will also find a jumper on the power supply to switch between low and high voltage. Be sure this jumper is set up properly.

Please answer the following questions:

1. Where does power come from to power field devices such as solenoids, lights, and motor starters?

2. What does a blown fuse indicate on a power supply?

3. If you get a new power supply out of the stock room, what must you check on the new power supply before putting it in your system?

The Processor

The processor is the main part of your SLC 500 system. The processor is where the program is stored that reads the status of your equipment, and based on certain status, makes a decision on what to control. For example: The processor is reading the status of a switch. When the operator energizes the switch, the processor might call for solenoid to energize that extends a cylinder. When the cylinder reaches the end of it's travel, it might close a limit switch. The processor will see that a limit switch has been closed, and shut off the solenoid.

The processor consists of several components:

1. The battery: The battery retains the processor's program when the PLC is powered down. Certain AB documentation states that the shelf life of the battery is up to 2.5 years. When the battery is low, you will see a BATT light on the front of the processor. A status bit is also set in the memory of the processor (S:5/11 for the SLC). Once you have an indication of a low battery, change the battery within one to two days. If the battery is on the front of the processor, change the battery while the processor is powered up. If the battery is on the back of the processor (5/03 and higher), you must power down, and remove the processor from it's chassis. Once the old battery is removed a capacitor will hold the processor's memory for up to 30 minutes. This gives you time to connect the new battery before loosing your program. If you desire to reset the processor back to factory defaults (clear the program), disconnect the battery, and on the back of the main board, you will find two terminals that must be shorted for 1 minute: VBB and GND.

2. Next to the battery, you will find a socket for an EEPROM. The EEPROM is an onboard backup of the processor's memory, and must be purchased separately. Changes to the PLC program do not automatically go to the EEPROM chip. You must manually store changes to the EEPROM from the COMMS menu in RSLogix.

3. On the front of the processor, you will find several status lights:

   1. RUN – Indicates when when processor is in RUN Mode

   2. FAULT – If flashing, usually indicates a software problem. Go online to get a description of the fault. You will find the description in the S2 status file on the ERRORS tab. If the fault light is solid red, this could indicate a hardware problem. You can try the following: reseat the processor, replace eeprom, clear memory and reload program, or replace processor.

   3. BATT-- Indicates the battery is low or missing

   4. FORCE – If flashing indicates forces are installed but not enabled... If solid indicates forces are installed, and enabled in the processor.

   5. Communication lights-- You may also have some indicators to indicate when communications are active depending on the type of processor you have.

The 5/05 processor:

Shown below with ports at default settings:

Factory Default the processor:

On the 5/03 and higher, the processor will hold it's RAM for about 30 minutes (could be less, most of the time much longer). To get the processor back to factory defaults, you can perform the following actions:

1. Disconnect the battery on the processor.

2. Short VBB to Ground, for 60 seconds.

3. Connect the battery and power up the processor.

VBB and Ground are shown below:

The Scanner Module

The scanner module is used to interface the SLC system with Remote I/O. Remote I/O is a legacy protocol used to interface with remote devices such as robots, drives, or other chassis. Remote I/O uses the Belden 9463 blue hose. (Allen Bradley catalog # 1770-CD)

The Remote I/O Scanner supports three baud rates: 57.6k, 115.2k, and 230.4k. These baud rates can be configured by DIP switches on the module itself. Allen Bradley's specification is that the blue wire of the belden cable is connected to terminal #1 of the three pin pheonix connector.

The scanner has two status lights:

The FAULT LED: If the Fault LED is flashing red, this usually indicates a configuration error on the scanner. Check the G file configuration. If the Fault LED is solid red, the scanner should be replaced.

The COMM LED: If the Comm LED is flashing green, the scanner is not communicating with one or more devices. If the Comm LED is flashing red, the scanner has lost communication with all devices. If the Comm LED is solid red, the scanner should be replaced.

The dip switches must be configured on the 1747-SN module for the correct baud rate. Baud is a term to describe how many bits per second are transferred over the network. Below is a photo of the switches. You will find a sticker inside the door of the scanner displaying the correct switch configuration for each available baud rate.

Addressing

An Address in the SLC simply points to a memory location in the processor. Some memory locations reflect the state of real world devices, such as inputs and outputs, and some memory locations are internet. Memory locations are basically broken down like this:

Internal memory locations are addressed in a similar manner:

Here is a bit from the binary file:

Or for a timer...

Memory Layout:

Data Files: A section of the processor's memory that stores information. You can also think of Data Files as variables.

Program Files: A section of the processor's memory which holds programs.

Programs hold Routines. Logic in Program Files is used to manipulate

Data Files.