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Basic Concepts

Help - HMI Server Functions

Overview:

A web based client cannot operate on its own. It requires a server to load the client web page from, and to communicate with. This system has a special server built into it which provides both functions.

Server Features:

The HMI server provides the following features to an application.

A web server. The server will load web pages requested by a web browser. This includes the web page itself, external Javascript files, CSS files, graphics images such as PNG or JPEG files, and icons ("favicons").
A common data table. All server data is stored in the common system (communications) data table. This means that a client can interact with other protocols and devices through the common data table. This includes monitoring inputs, setting outputs, and viewing events and alarms. (This is in MBLogic only. In in HMIServer data is read directly from the external source.)
Events and alarms. The server detects events and alarms, time stamps them, constructs the messages, stores them in an event queue where they can be requested by the client, and responds to alarm acknoweldge messages. More details on events and alarms can be found below.
Read only tags. Some tags can be defined as "read only" to protect their contents from being inadvertantly changed.
Range limits and scaling. Numeric tags can have range limits and scale factors applied by the server. Range limits can be used to prevent invalid data from being accepted by the server. Scale factors allow server data table values to be converted to a different range (e.g. convert to engineering units) for the client.
A configuration system. Tags, data table addresses, and other features are all configurable. More details on configuration can be found below.

Events and Alarms:

Events and alarms are represented as boolean conditions. An event occurs when the boolean value changes from false to true, or from true to false. An alarm occurs when the boolean value changes from false to true.

The event and alarm manager does not directly "create" events or alarms by monitoring conditions such as analogue values. Events and alarms must be created by other logic (e.g. via the soft logic or by other systems).

The addresses used for events and alarms can be anywhere in the coils area of the data table. External protocol clients, internal clients and integrated software modules can all write to the coils area to trigger and event or alarm.

Event and Alarm Buffering:

Events and alarms are "buffered" in the server. This means that when an event or alarm occurs, a copy of each message is retained in the server so that a client can retrieve all or some of them later. This means the client does not need persistent (disk based) local storage to maintain a (recent) history of events and alarms. They can be retrieved from the server at any time.

At present, the system stores the event and alarm buffers in memory. This means they do not (at present) retain a permanent event or alarm history. If the server is restarted, the existing history is lost. Permanent event and alarm storage will be addressed in a future version.

Data Scaling and Conversion:

The HMI server offers automatic scaling of numbers as part of the configuration system. Values in the data table may be automatically scaled when read from the server, with the reverse scaling being applied on writing data to the server. Scaling is user selected parameters as part of the configuration.

The server also automatically converts data types as required. Conversion is implemented as follows:

Scaling When Receiving Data

On receiving a value which is intended to be written to a register, the server first attempts to convert it to an integer. If this fails, it attempts to convert it to a floating point number. If that in turn fails, it reports it as a 'badtype' error.
The server then checks to see if the number is within the user defined range. If not, then it reports it as an 'outofrange' error.
Next, the server attempts to apply the scale factors. The equation used is scaledvalue = (datavalue - scaleoffset)/scalespan. If an error occurs, it is reported as a 'servererror'.
Next, the scaled value is checked to see if it will fit within a data table register.
If there are no failures at this point, the value is converted to an integer and stored in the data table.

Scaling When Sending Data

When sending data, the value is first read from the data table.
Next, the server attempts to apply the scale factors. The equation used is scaledvalue = (scalespan * datavalue) + scaleoffset. If an error occurs, it is reported as a 'servererror'.
The scaled value is next converted to the configured data type. If an error occurs, it is reported as a 'badtype'.
Finally, the the number is checked to see if it is within the user defined range. If not, then it reports it as an 'outofrange' error.

Platform Data Limits:

Because all data values are mapped to data table addresses, the data must exist within the limits of that data table. These are limitations of the server platform and not HMI protocol limits.

Native integers must be within the range -32768 to 32767 (after scaling is applied).
Extended data types offer integers in the range -2147483648 to 2147483647. However, these require two registers per value.
Native floating point numbers are automatically converted to integers when stored in the data table.
Extended data types offer single and double precision floating point. However, these require 2 (single precision) or 4 (double precision) registers per value.
There are no limits on string length, but a single register may hold only 1 or 2 characters.
Discrete inputs and input registers are read-only addresses.