PROFIBUS communicates via a serial protocol behind which complex machinery is housed to transport the data reliably through an industrial environment. The protocol defines the message design, the access to the network and fault control. There are ASICs (chips) available that coordinate the entire protocol. This way, the product developer does not have to be involved much in PROFIBUS to build a field instrument.
The following features make a PROFIBUS Protocol indispensable to your installation:
PROFIBUS has established four types of messages. Every message type has an application and fixed build-up: SD1 (contact check), SD2 (data transport), SD4 (token) and SC (short answer). Each message begins with a header, followed by the data. The message is closed with a footer. The bytes in the header and footer have a fixed function and meaning. The bytes in the messages are 11 bits large on the bus cable; 8 data bits plus startbit, stopbit and parity bit.
To create a hierarchy in the network, PROFIBUS distinguishes between two types of stations: masters and slaves. A master is the only device that can send messages on its own initiative and is often the only element that the process revolves around. Examples include: PLCs, DCS systems and PCs. The slaves are the devices that actually measure or direct something, for example remote I/O, sensors and actuators. A slave can only send data if the master instructs it to do so.
In most cases, transactions are done by the master with one device. This means that a master sends a message and the slave gives a response. After receiving the response, the master directs itself to the next slave.
A network can be made up of multiple masters. Masters give each other transmission rights by means of tokens. These are small messages used by a master to indicate it is finished and is transferring control to another master. This can be compared to a relay race. It is a wholly independently operating system with which masters are added or removed; the user does not have to set anything up. A master that has the token deals with the slaves that are programmed with it (the configuration). After handling the slaves, the master sends the token on to the next master.
During the communication process, various faults can occur. For instance, the corrosion of bits by electrical interference, a device that turns off which leads to vibrations or temporary cable breakages. The devices must be equipped to deal with these problems. This is implemented in the PROFIBUS machine at different levels; from parities to check sums and retries to time-outs.
PROCENTEC delivers high-tech ASICs to coordinate the entire protocol. These chips are simple to integrate and ensure that a PROFIBUS product fully meets the standards. PROCENTEC also offers certification to verify the implementation.