RSView Supervisory Edition Installation Guide

8Working with tagsThis chapter describes:• tags and the HMI tag database.• using data server tags.• using HMI tags.• browsing for tags.• working with tags in the Tag Browser.• using tag references.• how to log tag values.Tags and the HMI tag databaseA tag is a logical name for a variable in a device or in local memory (RAM). Forexample, a tag can represent a process variable in a network device.Data server tags, HMI tags, and their attributesRSView ® conforms to the OPC ® Data Access (DA) 2.0 specification for informationexchange among automation or control applications, field systems or devices, andbusiness or office applications.The OPC-DA specification calls tags ‘data items’. RSView provides direct access to thebasic attributes of all OPC-DA-compliant tags through a data server. In the RSViewdocumentation, the tags you access through a data server are called data server tags.RSView also provides tags with additional properties for alarms, security, and datamanipulation. These are called HMI tags and are created using the Tags editor in RSView.An application can use a mix of HMI tags and data server tags. To use data server tagssuch as those found in ControlLogix ® and other OPC-compliant devices, provide a directreference to the tag’s location wherever the application is to use the data.Tag-related limitsEach graphic display can contain up to 3000 references to expressions or tags (HMI tagsand data server tags). This limit includes the tags contained in embedded variables. Thelimit also includes duplicate references. For example, one display can contain only 3000numeric inputs that refer to tags, even if all 3000 objects refer to the same tag.8 • Placeholder8-1