FairNET Operators

This section describes FairNET networking operators that enable FairNET-capable controllers to communicate with one another (and with personal computers). The FairNET networking operators fall into two main classes: broadcasters and receivers. Broadcast operators transmit their information (signal values) over the network and receiver operators capture this information.

FairNET also includes operators that synchronize the automatic/manual state of A/M function blocks across a network of devices.

Broadcast Operators
Broadcast operators encode their input signal(s) into a communication message that they broadcast over a controller network. All controllers that are part of the network will receive the broadcasted message. But, only those controllers with a receiver operator "tuned" to that broadcast will actually decode the message.

Analog broadcast operators transmit analog signals. They have two inputs---one analog and one digital. The analog input (the red pin) is the signal that the operator is to broadcast. The digital input (the blue pin) enables (high) or disables broadcasting (low). If the digital input is not connected, broadcasting is always enabled. Digital broadcast operators transmit digital signals. They have two digital inputs: the top digital input is the broadcast signal and bottom digital input is the enable pin.

Note: If a FairNET device contains multiple digital broadcast operators in its schemas, Design Pad automatically encodes them into a single communication message, thereby reducing network traffic.

Broadcast operators have a number of properties that you must configure for proper network operation. The properties dialog for a broadcast operator is shown in Figure 17 (to view the dialog, double-click on the operator graphic). You must first specify a signal name for the broadcast to distinguish it from other broadcast signals. The name you assign the operator must be unique; no other broadcast operator in any schema on the same network may have the same signal name. Design Pad uses the broadcast signal names to associate receiver operators to the broadcast operators.

Each device attached to a FairNET network will periodically broadcast its information over the network. Some of the broadcasted messages; those that are deemed most critical can be configured to occur at user defined intervals. The FairNET protocol guarantees that these critical messages are broadcasted within the specified interval. FairNET devices will attempt to broadcast the remaining signals, those that are deemed non-critical, at regular user-defined intervals, but there is no guaranteeing that it will do so. In other words, critical signals are deterministic but non critical messages are not.

You can specify if a broadcast signal is critical or non-critical in the operator's property menu. Since critical messages are deterministic, you may be tempted to setup every broadcast operator as a critical broadcaster. When deciding between the critical and non-critical designation for broadcast messages, it is important to consider how the FairNET protocol schedules and prioritizes network traffic. All critical messages in a controller network are assigned top priority---they are broadcasted first. After all critical broadcasts from all devices have been sent, some time remains before the process must be repeated and all critical broadcasts must be sent again. It is during this time window, that the other communication messages are processed. These other communication messages (including non-critical broadcasts and network status/control messages) are placed in a first-in first-out (FIFO) queue maintained by the master controller. During the time window between critical broadcasts, the master station will process as many messages in the queue as possible. If most or all broadcasts are set up as critical broadcasts, then there may be little time left to handle all other network traffic.

Receiver Operators
FairNET receiver operators decode messages that were broadcasted over a FairNET network. Analog receiver operators decode analog signals and digital receivers decode digital signals. Receivers have two outputs: one output is the broadcast signal value that the operator receives, and the other output indicates if the operator is actively receiving the broadcasted. The status output signal is low (0) while transmissions are regularly received; the output is high (0) if the operator has not received a broadcast in a user-specified time interval (called the Maximum Broadcast Dead-Time). You specify the Maximum Broadcast Dead-Time parameter in the operator properties dialog of the receiver.

In order to associate a receiver operator to a particular network broadcast, you must set the signal name property of the receiver to the broadcast signal name. The signal name in the receiver must match the broadcast signal name exactly. (Signal names are case sensitive. Design Pad considers the names OilPressure, oilPressure, and OILPRESSURE to refer to different signals).

Network-Enabled A/M Button and Remote A/M Button Operators
The A/M Button operator and the Remote A/M Button operator are special networking operators that work together to allow synchronized auto/manual operation of multiple control stations attached to a FairNET network. Suppose a design effort required that users be able to take manual control over the same process loop at any one of four control stations. To satisfy the design requirements, you could include an A/M Button operator in the schema for one control station (the "local" station) and associate a Remote A/M Button operator in the schemas of each remaining control station (the "remote" stations). FairNET takes care of synchronizing them. If a user switches any one of the four stations from automatic mode to manual mode, the other three stations will also switch to manual operation. If the user then adjusts the manual output from any one of the four stations, the other three will match that manual output. If another user at a different control station switched the process back into automatic mode, the other three stations will also switch into automatic mode.

Both A/M Button and Remote A/M Button operators have built-in network broadcasting and receiving capabilities. The A/M Button operator arbitrates communication between itself and associated Remote A/M Button operators. It is continuously broadcasting the output state that the Remote A/M Button operators should assume. When process adjustments are made at a remote station, the Remote A/M Button operator sends a message to the A/M Button. If the A/M Button accepts the remote station command it modifies its output state and broadcasts it over the network. The A/M Button operator broadcast of the new output state serves to confirm that the local station has accepted the remote station command and to alert all other Remote A/M Button operators at the other remote stations that they should update their output states.

In the context of a FairNET communication network, the A/M Button operator acts primarily as a broadcaster, continuously transmitting its output state. It therefore includes the pertinent networking properties of broadcast operators, namely the broadcast priority (critical or non-critical). The Remote A/M Button operator acts primarily as a receiver "tuned" to the continuous broadcasts of the A/M Button operator. It includes the pertinent networking properties of receiver operators, namely the maximum broadcast "deadtime".