What Happens Within the VTS Code?

The VTS software module, "VTSDriver" (the source code for which is stored in a file named, "VTSDrvr.web"), provides the link between the communication driver and the VTS application. A unique instance of VTSDriver is automatically created for every instance of every driver within an application.

Briefly, the role of VTSDriver is to:

•      Distribute data to I/O tags

•      Trigger polling

•      Distribute data to client workstations via RPC.

•      Synchronize data on startup.

Reading Data

VTSDriver contains a module called, "AddRead" that is called by an input tag to create a request to read a specific range of memory. AddRead expects the following parameters:

•      The address from the input tag.

•      The number of elements (size of the block) to read.

•      A pointer to a place to put the retrieved values.

•      The rate at which to perform the read.

AddRead calls VTSGetAddr and VTSMaxBlock in the communication driver to get the information it needs to coalesce the addresses into appropriate blocks. A ReadBlock module is then launched, which determines the best organization of blocks and launches a separate VTSRead module for each actual block of data to be read.

VTSRead, VTSGetAddr and VTSMaxBlock all reside in the communication driver, not in the VTSDriver module.  This is an important detail to note if you are planning to write your own communication driver.

Writing Data

Whenever a value is changed in an output tag, a Write module in VTSDrvr.web is called. This module expects the following parameters:

•      The address from the I/O tag.

•      The number of elements to write.

•      A pointer to the source of the written data.

•      The data type.

•      The name of the I/O tag that called this module to request a write.

The Write module launches WriteData, which in turn calls VTSGetAddr in order to obtain the information it needs to call VTSWrite. VTSWrite, a module in the communication driver, is then launched.  

To emphasize, note that VTSWrite is a module that you create in the communication driver, not in VTSDrvr.web.

 

Read Block Value Coalescing

VTS combines individual PLC/RTU addresses into a single group or block that is intended to be executed as a single communication message. This is known as "coalescing", and is an attempt to increase throughput by allowing the reading and writing of large blocks of data, where possible.

There are a number of VTS modules and variables that control how coalescing will be performed:

•      VTSGetAddr: Gets the addresses for the blocks of data and parses them. The Info1, Info2, and Info3 parameters provided to this module must be either invalid or the same type in order for coalescing to occur.

•      VTSMaxBlock: The value of the VTSMaxBlock variable sets the maximum number of values that can be coalesced into a single read. As an alternative, there may be a VTSMaxBlock subroutine defined, which returns the size of the maximum block for a given address.

The result of coalescing depends on the data type of the MemAddr parameter in VTSGetAddr:

•      If MemAddr is numeric, VTS will coalesce the message blocks into a continuous address range, even if that range includes addresses that are not identified or needed. For example, if the system is configured to read addresses 15 and 50, and the VTSMaxBlock variable is set to 100, then VTSRead is called such that addresses 15 to 50 will be read as a single read message, even though address 16 to 49 inclusive are not needed.

•      If the data type of MemAddr is a string, then the result of the coalescing algorithm includes the configured addresses. Using the same example as above, VTSRead is passed an array of strings (in the MemAddr parameter) with two elements 15 and 50.