IDEC WindSRV - Manual (Page 68)

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driver being used. To determine how an address should be entered, use the Hints button next to the Address
Parameter. The address entered can be up to 128 characters in length. Once an address has been entered, it can
be tested by using the Check Address button, which will then attempt to validate the address with the driver. If
the driver accepts the address as entered, no message will be displayed. A popup will inform of any error. Some
errors will be related to the data type selection and not the address string.
Note: Hints provide a quick reference guide to the address format of the driver. The driver's help documentation
can also be invoked from the hints dialog.
 Description: This parameter is used to attach a comment to the tag. A string of up to 64 characters can be
entered for the description. When using an OPC client that supports Data Access 2.0 Tag Properties, the
description parameter will be accessible from the tag's Item Description properties.
 Data Type: This parameter is used to specify the format of this tag's data as it is found in the physical device. In
most cases, this is also the format of the data as it returned to the client. The data type setting is an important
part of how a communication driver Reads and Writes data to a device. For many drivers, the data type of a
particular piece of data is rigidly fixed and the driver knows what format needs to be used when reading the
device's data. In some cases, however, the interpretation of device data is largely in the user's hands. An
example would be a device that uses 16 bit data registers. Normally this would indicate that the data is either a
Short or Word. Many register-based devices also support values that span two registers. In these cases the
double register values could be a Long, DWord or Float. When the driver being used supports this level of
flexibility, users must tell it how to read data for this tag. By selecting the appropriate data type, the driver to is
being told to read either one register or two or possibly a Boolean value. The driver governs the data format
being choosen. For specific information on available data types, click Hints to access the driver's help
documentation. Available data type selections are as follows:
Default - This selection allows the driver to choose its default data type.
Boolean - Single bit data On or Off.
Char - Signed 8 bit data.
Byte - Unsigned 8 bit data.
Short - Signed 16 bit data.
Word - Unsigned 16 bit data.
Long - Signed 32 bit data.
DWord - Unsigned 32 bit data.
Float - 32 bit Real value IEEE format.
Double - 64 bit Real value IEEE format.
String - Null terminated ASCII string.
BCD - Two byte-packed BCD value range is 0-9999.
LBCD - Four byte-packed BCD value range is 0-99999999.
 Client Access: This parameter is used to specify whether the tag is Read Only or Read/Write. By selecting
Read Only, users can prevent client applications from changing the data contained in this tag. By selecting
Read/Write, users allow client applications to change this tag's value as needed. The Client access selection
also affects how the tag appears in the browse space of an OPC client. Many OPC client applications allow users
to filter tags based on their attributes. Changing the access method of this tag may change how and when the
tag will appear in the browse space of the OPC client.
 Scan Rate: This parameter is used to specify the update interval for this tag when used with a non-OPC client.
OPC clients can control the rate at which data is scanned by using the update rate that is part of all OPC groups.
Normally non-OPC clients don't have that luxury. The server is used to specify an update rate on a tag per tag
basis for non-OPC clients. Using the scan rate, users can tailor the bandwidth requirements of the server to suit
the needs of the application. If, for example, data that changes very slowly needs to be read, there is no reason
to read the value very often. Using the scan rate this tag can be forced to read at a slower rate reducing the
demand on the communications channel. The valid range is 10 to 99999990 ms., with a 10 ms. increment. The
default is 100 milliseconds.
Note: With the server's online full-time operation, these parameters can be changed at any time. Changes made to Tag
Properties will take effect immediately; however, OPC clients that have already connected to this tag will not be
affected until they release and attempt to reacquire it. Utilize the User Manager to restrict access rights to server
features and prevent operators from changing the parameters.
Dynamic Tags
Dynamic Tag addressing is a second method of defining tags. Dynamic Tags allow users to define tags solely in the
client application. Thus, instead of creating a tag item in the client that addresses another tag item created in the
server, users only need to create tag items in the client that directly accesses the device driver's addresses. On client