Presentation
- PROcess FIeld BUS.
- German project: Bosch, Siemens,
- Standardization: DIN 19245 (1991), EN50170 (1996), IEC 61158 (1999).
Variants
- Profibus FMS Fieldbus Message Specification (Industrial messaging between PLCs), the first to have been used.
- Profibus PA Process Automation ( Power supply and signal on the same cable, up to 31.25 kbits / s.)
- Profibus DP Decentralized Periphery (Up to 12 Mbps.)
Cable
Shielded twisted pair, carefully insulated by conductive foil and braid. With 2 drivers named A and B.
- Green wire: A, carry RxD / TxD-N signal
- Red wire: B, carry RxD / TxD + N signal
Line termination
Termination resistors equivalent to the cable impedance to bias the line in the absence of a signal. In general, integrated into the connector and activated by a switch.
Profibus flow and distance
The maximum distance and the speed are linked, the bus accepts up to 32 devices without repeater and 126 devices with repeater. The use of a repeater regenerating the signal allows the segments to be cascaded. There should not be more than 9 repeaters between a device and the master.
Signal coding
NRZ coding
- 1 start bit
- 8 data bits
- 1 even parity bit
- 1 stop bit
Bit order: LSB first.
When transmitting data consisting of several bytes (16-bit Word, 32-bit DWord). Most significant bytes are transmitted first.
Common structure of telegrams
Example:
Standard
The various types of telegrams, identified by their SD and the possible functions for each type, identified by FC.
Example:
Writing of outputs: SD = 68h, FC = 6h, no SSAP or DSAP and DU: contains the values of the outputs.
Addressing
Address range 0 to 127
- 0: generally used by diagnostic tools.
- 1 to 125: freely usable addresses for masters and slaves.
- 126: reserved for equipment with a defined address by bus.
- 127: broadcast address (message received by all slaves)
The configuration of the address of a slave is usually done by switches on the box.
Master slave operation
- The master sends a request to a slave.
- The slave sends a response back.
- Thus, no risk of collision.
- Successive polling of the different slaves
- Cyclic operation
- The cycle time depends on the number of slaves and the size of the data exchanged
Multi master operation
The different masters take turns on the bus and they exchange a “token”. The owner of this token has the right to use the bus. When he has finished his cycle, he sends a telegram to the next master. The following rules apply
- Only one master can write to a slave.
- All masters can read all slaves.
Slave State Machine
On startup, the master requests diagnostic information, sets the slave and defines the operating mode.
Master configuration
The profibus master can be a programmable logic controller equipped with the appropriate interface or a computer (PC) fitted with a dedicated card.
The master must know the list of slaves to manage. He must also know the size of the data to be exchanged. This information is communicated during a configuration phase. For each slave, the supplier delivers a file describing its possibilities.
The file format is standardized:
- GSD file: General Station Description
Profibus masters come with configuration software capable of interpreting GSD files, a collection of known hardware GSD files. The GSD file is a text file containing the possibilities of the slave: Supported transmission rates, version number, … and description of the slave data
Example:
Asynchronous cycles
The Profibus standard does not impose synchronization between PLC and fieldbus cycles.
Result
- The cycles are generally asynchronous.
- The different cycle times generate
- Cumulative latencies
- A significant jitter.
Comparison between Profibus and traditional cabling
advantage
- All the advantages of a fieldbus.
- Well suited to the management of inputs and outputs.
Disadvantages
- Latency, jitter.
- Unsuitable for controlling synchronized movements.
- Complexity of implementation.
- Higher risk of faults.
- Much higher required tools and skills.
