DCS – Distributed Control System

A DCS is typically used to describe a system that controls a process; that is, something that involves moving fluids, like a waste water treatment plant or a refinery.

SCADA – Supervisory Control And Data Acquisition

This usually describes the control system for an automated manufacturing facility like an assembly line for a furniture maker.

PLC – Programmable Logic Controller

This is the computer that receives data from sensors and controls machinery. It can perform as the core processor for a DCS or a SCADA system.

Popular PLCs are Rockwell (Allen-Bradley), Siemens, Modicon and Delta V.

PAC – Programmable Automation Controller

This is Rockwell’s new for their ControlLogix computers.

HMI – Human Machine Interface

An HMI is a terminal that is used by plant operators to control the plant. It typically shows a graphic representation of the plant and allows the operator to start and stop motors, look at tank levels, etc. The graphics are usually very similar to the P&ID drawings of the plant.

An HMI is part of a SCADA or a DCS.

Typical HMIs are based on RSView32, FactoryView (Rockwell) or Wonderware. Other PLC manufacturers have their own HMI software.

OIT – Operator Interface Terminal

This device is a self-contained screen that is located near the equipment that it controls. All OITs are HMIs, but not necessarily vice-versa.

When you are designing an HMI or a SCADA display, you have to consider color blindness.

Approximately 5% to 8% of men and 0.5% of women are born colorblind. Usually, these people are protans (red weak) and deutans (green weak). Though there are varying degrees of color blindness, there are some that cannot differentiate red from  green, so the colors look the same to them.

These people negotiate stoplights based on the position of the light that is on, not the color.

This is a big problem in industry, where red and green are commonly used to indicate the status of a piece of equipment.

There are a couple of ways to solve this problem. We could use icons that have different shapes, depending on whether they are on or off. In addition to being a different color, a pump icon that is off could have a round hole cut out in the middle, such as is shown below.

Here is what the pumps shown above might look like to a person who is colorblind.

The black dot in the middle of the pump that is off will be appreciated.

Another option is to add text. Not only does this help the person who might be colorblind, it obviously confirms the state of the equipment to those even with true color vision.

An interesting benefit to keeping the icons discernible to the color blind is that these icons are easier to read for those of us who are not color blind.

Excerpted from How to Program RSView32

One of the basic skills required to program any HMI or SCADA is the understanding of tags and the tag database.

Here is a quick lesson on building the tag database in RSView32.

Excerpted from How to Program RSView32

To fully understand how RSView functions with a PLC, you must understand how tags work and how to manage and edit the tag database.

There is a list of the tags used in this project in Excel format included with this book called hyperclr-Tags.xls. Take a moment to look over this spreadsheet and understand the attributes of each tag.

Tags are the links that connect RSView with the PLC; the tags are common to both the PLC and RSView. The PLC and RSView have access to the values held by the tags.

The value of the tag in RSView reads or writes the value of the bit (or the word) in the PLC, and vice-versa.

When you define a tag that references a bit in the PLC, every time the bit in the PLC turns on the tag value in RSView will change from 0 to 1.

Conversely, every time the bit in the PLC turns off, the tag value in RSView will change from 1 to 0.

If you define an analog tag, such as the weight of the mixing tank in our example, as the value of the word in the PLC changes, the value of the associated tag in RSView will change.

You can also use RSView to send values to the PLC.

However, if the ladder logic is such that the value of the bit or word is determined by the ladder logic, the value you send to the PLC will be overwritten. The PLC has priority. We will cover this aspect later in the book.

Let’s see how to create a tag and tie it to a bit in the PLC.

Look at the RSLogix printout for the Batching project. Find the section called “RSLogix 500 Cross Reference Report – Sorted by Address”. This section shows all the outputs, inputs, internal bits, timers and words that we want to use to create tags in RSView.

Digital Tags

Let’s start with Bit B3:0/0, which is the System Enable bit. As shown in Rung 0001, this bit will turn on when the E-Stop is cleared and there is no System Fault.

We can create a tag that references this bit. Click on “Tag Database”   in the Project tree. You will see a screen like this:

I like to use the Address Descriptor from the PLC program as the name. By doing it that way, when you go back and forth from the PLC program to the RSView project, you see the same names.

Type “SystemEnable” in the “Name:” field.

Notice that there is no space in the tag name. RSView does not allow this, or most other special characters. To play it safe, use only alphanumeric characters, the underscore (_) and the hyphen (-). Capitalize words to make it easy to read.

Tags come in three different types; analog, digital and string.

An analog tag refers to a numerical value. This can be a value from an analog input card, a scaled value from an SCP instruction, a timer preset value or a timer accumulated value, among others.

In our program, you would use this when creating a tag for the Tank Weight (word N7:0), for example.

A digital tag refers to a binary value. This is used with any bit address, such as our  “System Enable” bit. It can also refer to an output, an input, a timer done bit or a timer enabled bit.

String tags are ASCII strings or whole words. String tags are not used frequently.

Let’s continue defining the SystemEnable tag.

Choose “Digital” as the type.

The “Security” attribute of the tag is shown as a dropdown menu. The security code lets you restrict access to the tag to only those users who have access to the code assigned to the tag. The default code (*) allows access to the tag to all users. You can leave the default security setting as is.

Type “System Enable” in the description field. Some people wouldn’t bother with adding the description for this bit; after all, the Tag Name tells you the same thing. We will, however, add descriptions for all of the tags.

The default values of “Off” and “On” are fine for this tag, as they are for most tags.

The default for the “Data Source” is “Memory”. In most cases, you will use “Device”, as this tag references an address in the PLC.

Select “Device”.

A new set of fields appears. Click on the browse button   by the “Node Name:” field. The only option that appears is Node 410. This node name appears because we defined it earlier. Select that and click “OK”.

The default “Scan Class:” of A is fine.

Type in the address of the bit in the “Address:” field. It should appear just as it does in the PLC program. For the “System Enable” bit, that address is B3:0/0.

The screen should look like this:

Click “Accept”.

The tag is now displayed as the first, and only, tag in the database.

Click “Next” and you may continue adding tags.

I would like to note that it is possible export the Documentation Database from RSLogix and import it into RSView.

However, this is a complex process and requires Excel to manipulate the file that is exported from RSLogix. It is a time consuming process that is beyond the scope of this book. Depending on the number of tags that you have to import, it could be worthwhile to go through that process.

For the time being, we will simply add the remaining tags manually. If you have an electronic copy of the Cross Reference, use copy and paste (CTRL-C and CTRL-V) to enter the information into the tag database. This will speed up the process and eliminate typographical errors.

RSView32 is a bit stubborn with tags that have already been defined. Once you have accepted a tag, you cannot go back and change its name.

If you have a tag that has the wrong name, the easiest way is to duplicate the tag, rename it using the correct name and then delete the original tag. This can only be done if you have not used the tag elsewhere in the project.

To do this, highlight the tag you would like to correct.

Click on the “Duplicate Tag”   button.

Type in the correct name and click “Accept”.

Highlight the tag with the wrong name and click on the “Delete Tag”   button. Use this button with caution – there is no “Undo”.

As you are entering tags in the database, make sure your tag names are correct before you accept the tag. Double-check the tag names for accuracy and typographical errors.

To add a new tag, click on the “Insert Row”   button. Enter all the tags for the remainder of the internal bits (B3:x/x).

Add the tags for all of the inputs (I:x/x) and outputs (O:x/x)  in the same way.