Access, Bus Initialization, Bus Communication

COMMUNICATION

EXCEL 50/100/500/600/800

trollers and devices to exchange data such as measured values, alarms log, and trends. This means that values from one controller or device can be sent to the entire system.

Besides allowing communication between controllers and devices, the C-bus also enables the entire system to be connected to PC front-ends.

When a controller does not have a modem directly connected, the C-bus also allows controllers to communicate with other controllers that do have a modem connected, so that data can be transmitted via the public telephone network.

Access

The C-bus supports multi-master communication using the token passing pro- cedure. A bus master is a controller governing communication between bus devices. The master asks for data and then distributes the data on the bus. Con- trollers transmit data only when asked for it by the master or when they assume the function of the master.

Multi-master communication means that all controllers in the system can function as the master, so the right to request and transmit data is not permanently assigned to a specific controller. This has the advantage that a part of the system can still continue working even if one of the controllers is defective.

The flow of data between devices can be structured hierarchically as part of the software in the user program. Structuring data exchange means defining what infor- mation can be exchanged between which bus devices. Communication is still carried out on the multi-master principle, but bus access time is reduced by con- centrating specific data in specific controllers.

Bus Initialization

When your system first starts up, the software runs a check to see what devices are connected to the C-bus. This process is called initialization. The software stores the information as the Device Type List in the controllers.

During initialization, the bus master requests information about global datapoints from each controller. At the same time, the controllers store the address of the device making the request so that the controllers can subsequently transmit the appropriate data to the appropriate device.

After initialization, each controller knows the address of the next controller with the right to transmit data. If a controller now stops communication, the bus master recognizes that this controller is no longer online and interrupts token passing. As a result, the C-bus reinitializes automatically and also updates the Device Type List, excluding controllers that are no longer online. This ensures continuous bus communication even when individual controllers go off line.

Bus Communication

The Excel 50/100/500/600/800 controller does not transmit any plant-specific data before the C-bus has been initialized. Controller communication depends on the user program stored in each controller. Bus-wide communication occurs when global datapoints have been defined in the user program.

See "Global Datapoints" on page 11 for details on defining global datapoints. During normal operation, the bus master transmits active values. The bus master transmits only values which have changed or which have been requested by another device on initialization or when a device has been offline.

This method of exchanging data reduces the load on the bus and means controllers store only the data they need for their own communication.

I/O Runtime Synchronization

XF525 After application download and after a power failure a runtime synchronization takes place which takes the complete runtime as defined in the datapoint description plus an additional 10%.

EN2B-0092GE51 R0512