PLC Homework
Mohammed Aljohani
chap4_s.pdf
Ch. 4 Programming the Application 1
Chapter 4 Programming the Application This chapter deals with the steps to creating a working program from both the Siemens and A-B
platform. In the last chapter, we dealt with linking the computer to the PLC and establishing the
project or file for the PLC to begin programming. This chapter deals with the creation of
programs for the PLC.
First with Siemens
Starting from the project tree below, the first action is to establish a program block. This almost
always is OB1, the constantly scanned background block that contains the main program for the
PLC. Execution is not timed and occurs as often as possible when other programs have not pre-
empted the cpu’s time.
First, click on Program Blocks
Then, click on Main [OB1]
Fig. 4-1 Getting into Main [OB1]
Ch. 4 Programming the Application 2
Here you may choose a programming language from the list of available languages for OB1.
Here if right click on OB1, the choices of language are:
LAD FBD Other block areas allow a third language, STL.
A first contact is chosen and added to the first network, Network 1:
Fig. 4-2 Choosing LAD as a
Programming Language
Fig. 4-3 Network 1
Ch. 4 Programming the Application 3
Notice the <??.?> above the contact. This signals that an address must be selected for this
contact, much as a name was required for each contact in a ladder diagram. Move to the tree
area PLC tags and choose Show all tags. This area is blank to begin and must be added before the
contact above is complete. Bit, byte and word length tags may be created here. Some care must
be given to the addressing since tags can be programmed over other tags, a problem that will
cause errors later in debugging.
To start, tags will be given generic names such as “a1”, “a2”, etc. This is not a good practice but
will be done to start the process of naming variables. Tags should be given meaningful names
that give the user an idea as to the meaning behind the contact or other instruction. Below,
when the first tag is entered, an address appears of %I0.0. If your tag is to be addressed to the
first input %I0.0 then all is well. Usually, this variable needs to be changed. Here it is changed to
an M identifier. M bits and bytes are used for internal storage, not for inputs or outputs from the
PLC. The first bit of the M table is M0.0. Since this table is addressed in bytes, the succeeding
bits are M0.1, M0.2, M0.3, M0.4, M0.5, M0.6, M0.7, M1.0, etc.
Fig. 4-5 Using M for Internal Data Storage
Fig. 4-4 Adding Tags
Ch. 4 Programming the Application 4
Thus, the first address is entered as M0.0.
Fig. 4-6 We Entered M0.0
And we proceed back to the ladder diagram for Network 1 and add the tag to the contact:
Fig. 4-7 Completed Tag
The final result resembles the following:
Fig. 4-8 Continuing with Network 1
Next, we would like to add an input to the logic. The input is tied to the input point I0.0. In the
PLC tag table, we begin with the name “Input0”. We proceed across with default tag table, Bool
and then see an address of %M0.1 picked. This must be changed. The tag table will
automatically roll to the next available address but we will be using an internal bit for an input
but rather an “I” bit, I0.0.
Ch. 4 Programming the Application 5
Fig. 4-9 Adding the Input
After changing the M address reference to an I address, and entering the correct bit offset, the
table appears as follows:
Fig. 4-10 Completing the Tag for the Input
Do not forget that a real device needs to be wired to an input for this input to perform its correct
function. Usually the input wired is through a NO (normally open) contact. This may change
from time to time but NO is usually chosen.
L+ M G L+ M 1M .0 .1 .2 .3 .4 .5 .6 .7 .0 .1 .2 .3 .4 .5 24VDC 24 VDC DI a DI b
Input Output
24
VDC
0
VDC
PB
Fig. 4-11 Wiring the Input
Ch. 4 Programming the Application 6
In the Program Block, the contact is added and the tag Input0 chosen.
Fig. 4-12 Adding the Tag for the Input
Our program now has a normally closed contact labeled “Input0” address I0.0 in series with a
normally open contact labeled “a1” address M0.0. Next, we would like to add a parallel contact
to the NO contact “a1”. Start with an arrow from the left ladder.
Fig. 4-13 Adding a Parallel Path
Add a tag to the tag table ”a2”. Note that we want this address as an M bit so the next available
M bit is M0.1.
Fig. 4-14 Adding the Tag for the Parallel Path
Ch. 4 Programming the Application 7
We finish the contact by choosing the normally closed contact and adding the “a2” tag from the
tag list:
Fig. 4-15 Adding the Contact for the Parallel Path
The up arrow is chosen to tie the circuit right of “a2” to the circuit above. The circuit is now
complete except for an output coil.
Fig. 4-16 Tying in the Parallel Path
Ch. 4 Programming the Application 8
The output coil is chosen from the instruction list and the tag is added.
Fig. 4-17 Adding the Coil
The tag is chosen from the internal M bits again, this time M0.2.
Fig. 4-18 Adding the Tag to Finish the Coil
Ch. 4 Programming the Application 9
At this time, our circuit is complete. The next rung or circuit is to be programmed in the next
available location. The programmer has a choice of moving to the next network, Network2, or
continuing in the present network. The program will solve the same either way. Usually, the
programmer will continue in the same network for compactness on the screen. More can be seen
at the same time when troubleshooting if more rungs are grouped into the same network.
Fig. 4-19 Completed Circuit (Rung)
The following shows a second circuit input in the same network as the first. This circuit is not
completed but illustrates the ability of the programmer to stack several ideas or circuits into one
network.
Fig. 4-20 Where to Add Another Circuit (Rung)
Ch. 4 Programming the Application 10
The following shows the same circuit but entered in a second network. Here the ideas are more
spread out, usually a less attractive alternative but available as desired.
Fig. 4-21 Alternate Place Add Circuit (Rung)
Ch. 4 Programming the Application 11
Next, Allen-Bradley
Starting with the project tree, the first program to enter is MainRoutine under MainProgram. This
program is equivalent to OB1 in that it is always on and scanning in the background. Execution
occurs as often as possible when other programs are not pre-empting the cpu’s time. This
program is programmed in Ladder. Subroutines and other programs may be programmed in FBD
and STL.
To choose a NO contact, either of the following tabs may be chosen.
Choose either the Favorites tab above or the Bit tab below to show a
NO contact. NC contact and coil are found in both as well.
Fig. 4-23 Alternate Tabs for NO Contact
Fig. 4-22 Beginning a New Rung
Ch. 4 Programming the Application 12
Tags are given the same generic names as with the Siemens processor but care must be taken to
be meaningful to the process being represented. Names generally are less than 30 characters in
length and may have underscore ( _ ). The more well commented, the better in the long run.
Fig. 4-24 Entering the NO Contact
The tag may be entered by right clicking the contact. The new tag will then be entered from the
following screens:
Fig. 4-25 Entering the Tag
The screen below will be entered with Name as a1. A description may be entered if desired.
Since the contact was chosen, the Data Type is Bool. Other options are listed but usually left as
is.
When the tag name is successfully entered, the contact and tag appear as one unit.
Ch. 4 Programming the Application 13
It is worth noting that the A-B tag database has no M offsets similar to the Siemens architecture.
The variables’ offset is hidden from the user. This is more like a computer language in which
the value of a variable’s address may not be known.
Fig. 4-26 Addressing a Tag using A-B
Tags may also be entered from the Program Tags option from the project tree. Here, they are
entered in the Edit Tags mode (see tab at bottom of page). This mode must be properly set to
enter tags or monitor tags. Use the Monitor Tags mode when online and changing variables to
verify the program or enter data to try for a specific result. This tag will be discussed more in the
troubleshooting section.
Ch. 4 Programming the Application 14
Fig. 4-27 Tag Entry from the Program Tags Option
Input and output tags are already defined and may be entered using their address. The addresses
for these devices can be found under the controller tab Controller Tags. This table is set for the
L23E. Other controllers with stacked cards will vary with the card type and number of each. For
our processor, the following I/O list is standard.
Fig. 4-28 I/O Tags Configured for the Processor
Expand the Input tab to find the specific input point to be used. For input point 0, Local:1:I.Data.0
is used.
Ch. 4 Programming the Application 15
Fig. 4-29 Expanding the I/O Table to get Actual Tag
This data address may be copied into the contact directly and used for the address.
Fig. 4-30 Copy/Paste of I/O Tag into Rung
Ch. 4 Programming the Application 16
Again, the device must be wired to an input. The following schematic shows a NO pushbutton
wired to the input programmed above:
24
VDC
0
VDC
PB
In 0
In 1
In 2
In 3
In 4
In 5
In 6
In 7 DC
Com 1 In 9
In 8
In 11
In 10
In 13
In 12
In 15
In 14 DC
Com 2
Local:1:I.Data.0
Fig. 4-31 Wiring the Input Point
Ch. 4 Programming the Application 17
Adding a parallel branch involves the following:
From the Favorites, choose the loop (second choice) and place the loop
before the contact to be branched around. Then drag the cursor around
the contact. Then move the cursor just before the contact to be added.
Fig. 4-32a Adding the Parallel Path
Ch. 4 Programming the Application 18
Fig. 4-32b The Completed Path
Fig. 4-32c Adding the Output
Once the rung has been completed, it is wise to verify the rung for completeness. Right click on
the rung at left. The following will appear. Choose Verify Rung and the eee’s should disappear.
Fig. 4-33 Verify Rung Chosen
Ch. 4 Programming the Application 19
To start a second rung, simply click on the new rung button. The following will appear.
Fig. 4-34 Adding the next Rung
Ch. 4 Programming the Application 20
Fig. 4-33 Main Task Properties
While your program may be set
to operate with no timing
problems, it is wise to check the
Task Properties for MainTask
as shown at left.
The setting of 500 ms is an
acceptable time for the
Watchdog timer. If the program
execution exceeds 500 ms or the
program isn’t allowed to
execute within 500 ms, the
WDT will shut down the
processor.
Ch. 4 Programming the Application 21
You may alias a tag to another name as shown in the example below. Here Input0 is aliased to
Local:1:I.Data.0.
Fig. 4-36 Aliasing of a Tag
Fig. 4-37 How Aliasing Looks in the Program
Ch. 4 Programming the Application 22
Troubleshooting the Siemens Processor
Online mode
In online mode, there is an online connection between your programming device / PC and one or
more devices.
An online connection between the programming device/PC and the device is required, for
example, for the following tasks:
Testing user programs
Displaying and changing the operating mode of the CPU
Displaying and setting the date and time of day of the CPU
Displaying module information
Comparing blocks
Hardware diagnostics
Fig. 4-38 Choosing to Go Online
Ch. 4 Programming the Application 23
Fig. 4-39 May Choose Go Online Here as Well
Fig. 4-40 Choose “Load”
Ch. 4 Programming the Application 24
Several changes appear when in the online mode. Among them are the following:
1. The title bar of the active window now has an orange background.
2. The title bars of inactive windows for the relevant station now have an orange line below
them.
3. An orange, pulsing bar appears at the right-hand edge of the status bar. If the connection
has been established but is functioning incorrectly, an icon for an interrupted connection
is displayed instead of the bar. You will find more information on the error in
"Diagnostics" in the Inspector window.
4. Operating mode symbols or diagnostics symbols for the stations connected online and
their underlying objects are shown in the project tree. A comparison of the online and
offline status is also made automatically. Differences between online and offline objects
are also displayed in the form of symbols.
5. The "Diagnostics > Device information" area is brought to the foreground in the Inspector window.
Fig. 4-41 Online Siemens Display
Ch. 4 Programming the Application 25
Fig. 4-42 Click on the Glasses to see Monitor Mode
Fig. 4-43 Build a Watch Table to Monitor Variable Status
Ch. 4 Programming the Application 26
Fig. 4-44 Starting and Stopping the Program
Troubleshooting the Allen-Bradley Processor
Access Who either from Communications or from the symbol here:
Fig. 4-45a Downloading to the CompactLogix Processor
Ch. 4 Programming the Application 27
Fig. 4-45b Downloading to the CompactLogix Processor
Fig. 4-45c Downloading to the CompactLogix Processor
Ch. 4 Programming the Application 28
Fig. 4-46 The A-B Program in Run Mode
Fig. 4-47 Changing from Run to Program or Test Mode
Ch. 4 Programming the Application 29
Fig. 4-48 Monitoring the Variables using Monitor Tags
Ch. 4 Programming the Application 30
Exercises
1. In the program of Fig. 4-49, if input Catsup were changed from a NO contact to a NC contact, how would the program have changed to not change the function of the push
button?
2. Would the program of Fig. 4-49 be significantly changed if the two N.C. contacts in the third rung were removed?
3. Where in the program of Fig. 4-49 could the copy/paste function have been used effectively?
4. Name the different processor modes for the Siemens S7-1200, the A-B L23E.
Ch. 4 Programming the Application 31
Lab 4.1 The Hot Dog Counter
Project Description:
Fred and Rudy are making hot dogs at the ballpark. Fred dispenses mustard and Rudy dispenses
catsup. A hot dog is not sold without each Fred and Rudy putting both mustard and catsup on
the dog. As each pushes the button for their ingredient, a signal is fed to the PLC for the action.
Either button may be pushed first. Design a program to count the total number of hot dogs made.
Inputs should be wired to contacts and labeled as mustard and catsup. A display is kept in the
PLC showing up-to-date counts of hot dogs made by Fred and Rudy.
To complete the lab, enter the program shown later in the lab into the PLC and wire the two
inputs.
Watch the count accumulate in the counter as the two buttons are pressed in any order.
Get a listing from the listing software on the programming software package.
The documented listing of the program may be used as the final lab report.
Wire the PLC to the inputs for this lab and to inputs or outputs for other labs per the diagram on
the next page.
The next page shows the layout of the PLC on the trainer and the PLC wiring schematic. To
wire the two inputs, wire through the two pushbuttons selected so that 24 volts is at the terminals
of I/0 and I/1 when the two buttons are pushed.
catsup mustard
Ch. 4 Programming the Application 32
Enter the following 4 rung program in both Siemens TIA Portal and A-B RSLogix 5000.
Download both and wire the inputs. Demonstrate a working counter to your instructor:
Catsup
(Input)
Hotdog
(Internal bit) Catsup Remember
(Internal bit)
Catsup Remember
(Internal bit)
Mustard
(Input)
Hotdog
(Internal bit) Mustard Remember
(Internal bit)
Mustard Remember
(Internal bit)
Hotdog
(Internal bit)
Catsup
Remember
(Internal bit)
Mustard
Remember
(Internal bit) Catsup
(Input)
Mustard
(Input)
Hotdog
(Internal bit)
Hot Dog Counter
Fig. 4-49 Program to be Entered
The count of hot dogs made is found in the accumulated value of the counter.
Both PLC platforms have instruction help features which may be used at this point to find how
the counter function above is programmed. RSLogix 5000 has this feature in its Help>Instruction
Help tab. Siemens has similar help features but has helps with the instruction to identify variable
types in the instruction itself.
Ch. 4 Programming the Application 33
Fig. 4-50 RSLogix 5000 Counter Instruction
Fig. 4-51 Siemens’ help with the PV Variable
Ch. 4 Programming the Application 34
Fig. 4-52 Siemens’ help with the CV Variable
In the example above, Figs. 4-51 and 4-52 show the type of inputs available for PV and CV. In
general, PV is short for process variable and CV is short for the controlled variable. For the up-
counter, PV is the count preset and CV is the active count. The PV may hold a constant as
shown below:
Fig. 4-53 Siemens’ CTU Instruction with PV Constant (=9999)
