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Introduction To ProfiBus DP

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Acromag, Inc. Tel:248-624-1541 Fax:248-624-9234 Email:sales@acromag.com http://www.acromag.com

The length (and timing) of the I/O data to be transferred from a single

slave to a master is predefined in the slave’s device data base or

GSD file

The GSD files of each device connected via the network (slaves and class 1

masters only) are compiled into a master parameter record which contains

parameterization and configuration data, an address allocation list, and the

bus parameters for all connected stations. A master uses this information to

set up communication with each slave during startup.

After a master receives its master parameter record, it is ready to begin

exchanging data with its slaves. During startup, after a system reset, or

upon return to power, a master will attempt to re-establish contact with all the

slaves assigned to it before assuming the cyclic exchange of I/O data. Each

slave must already have a unique valid address from 0-125 in order to

communicate with the master. Any slave that has a default address of 126

will await the Set_Slave_Address command from a class 2 master before it

can be parameterized. In attempting to establish communication, the master

starts with the lowest address slave and ends with the highest address

slave. A master will send parameterization and configuration telegrams to all

of its assigned slaves (a slave may only be write-accessed by its assigned

master, the master that parameterized and configured it during startup). The

parameterization and configuration telegrams ensure that the functionality

and configuration of a slave is known to the master. If an additional slave is

added to the network bus and is not already accounted for in the master

record, a new master record must be generated and a new configuration

performed so that the master is informed of the status of the new device.

ProfiBus DP most often uses a single class 1 master device (

mono-

master

), cyclically polling many distributed slaves. However, ProfiBus also

allows for acyclic communication between class 2 masters and slaves,

making more than one active station or master possible. A class 1 master

will automatically detect the presence of a new active station connected to

the network bus (a class 2 master). When the class 1 master completes its

polling cycle, it will pass a “token” to the class 2 master granting it temporary

access to the bus. Deterministic behavior is maintained because the class 2

master can only use the time allotted to it via the

gap time

specified.

Although, mono-master operation is generally recommended, it is not

mandatory. That is, a ProfiBus system may have more than one class 1

master, but master to master communication is not permitted, except for the

granting of bus access rights via token exchange.

To illustrate the idea of communication between masters in a ProfiBus

DP system, a class 1 master cyclically exchanges data with all of the slaves

assigned to it, one at a time, according to its list of assigned slaves taken

from the master record. At the end of this data cycle, additional time (gap

time) is allotted to provide for acyclic communication between a class 2

master and the same slaves. During this time, the class 1 master will pass a

token to the class 2 master granting it bus access rights. The class 2

master which currently holds the token has the opportunity to exchange data

with all the slaves within a specific period of time called the

token half-time

token hold-time

). The class 2 master may then proceed to read data or

diagnostic information from any of the slaves, and then at the completion of

its cycle, it will pass the token back to the class 1 master.

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