Tags and Their Properties

The unique and flexible nature of Visual Tag System (VTS) software is due to the custom building blocks upon which it is based. These building blocks are called "tags". The presence of the term "tag" in the VTS name indicates the central role that they play in the design and use of the VTS software.

A tag is a software component that can communicate with objects in the physical world. For flexibility, each tag type is a specialized component that can be combined with other tag types to build diverse systems.  For example:

•    Port tags handle communication between the VTS server and the outside world, whether through a serial port or a TCP/IP port.

•    A specialized driver tag is available for nearly any brand of PLC hardware. Driver tags rely on port tags in order to communicate with the hardware.

•    Input and output tag types, both analog and digital, monitor and communicate with individual addresses on the remote PLC.  I/O tags rely on driver tags to ensure that communications are properly formatted for the remote hardware.

•    Alarm tags and logger tags can be linked to any I/O tag to monitor and record values.

The linkage of VTS to remote hardware through the I/O tag to the driver tag, to the port tag to the hardware is referred to as the “communications chain”.  By choosing appropriate tags for each link in the chain, you can build VTS applications for nearly any situation.

 

Standard versus Custom Tag Types.

All of the tag types that come with VTS & VTScada are described in this chapter. These are referred to as the standard tag types.

VTS programmers have the ability to create custom tag types for specialized purposes. These custom tag types will each have their own unique configuration and are not described in this guide.

The preceding figure illustrates the communication chain. The equipment that makes up the physical system is connected to a PLC. This PLC is connected via a serial port to a PC running a VTS application.

Data collected from the equipment is transmitted from the PLC through the PC's serial port to a serial port tag. The serial port tag passes the data to the driver tag representing the I/O device (in this example, a Modicon driver tag). This communication driver tag accepts the data and passes it to the analog and digital tags representing the equipment processes within the physical system. Finally, each tag in the system has its own visual representation, showing the system operators the status, mode and data of the associated equipment. The visual representations may include such items as animated bitmaps and fluctuating numeric values.

Control is achieved by sending signals back through the chain. The user clicks a button or enters a numeric value in a field, either of which is a visual representation of an output tag.  The button click or numeric value is sent to the communication driver for translation. The communication driver translates the signal to the correct format for the PLC in use, and sends it to the serial port tag to transmit out to the PLC. The equipment process receives the new value and then performs the required action (e.g. turns the pump off).

It is typical to begin each application you create by configuring a port tag to accept the data that is flowing through your PC, and a device driver tag to provide an interface to physical I/O devices such as programmable logic controllers (PLC), remote terminal units (RTU), I/O boards, or to Windows system features such as Dynamic Data Exchange (DDE). Once you have these essential tags in place, you can create analog, digital and other tag other types to make your application communicate with the outside world.

Each tag added to an application includes properties that uniquely identify it, and link it with the other tags in the system to allow communication to occur. VTS stores all these attributes in a database. Information on the tag properties database can be found in the topic Tag Properties Database (Points.mdb).

 

Whenever you create a new tag, VTS generates a unique tag instance, based on the structure of the tag type you selected.  

For example, when you add a new analog input tag to an application, a new copy or instance is made of the analog input tag type. This copy has all of the properties and the drawing methods inherent to the analog input tag type; however, each instance is unique from the other instances of tags of the same type. Every analog input tag created from the analog input tag type requires a unique name since no two tags can have the same name. Each analog input tag you create will normally be linked to its own address on the PLC and can display data from that address using one or more of the drawing methods available to the analog input tag type. The following figure illustrates how three instances of analog input tags are generated from the analog input tag type.

 

Although each tag type has an individual set of properties and drawing methods inherent to the tag type, many of the standard tag types share similar properties and drawing methods.