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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

A ProfiBus master forms an “active station” on the network. ProfiBus

DP defines two classes of masters. A class 1 master handles the normal

communication or exchange of data with the slaves assigned to it. A class 2

master is a special device primarily used for commissioning slaves and for

diagnostic purposes. Some masters may support both class 1 and class 2

functionality. Master-to-master communication is normally not permitted in

Profibus, except in order to grant bus access rights to another master via the

exchange of a token. However, master-to-master communication between

two mono-master systems can be facilitated using a DP-DP gateway. Note

that the exchange of bus access rights via this “token ring” only applies

between masters on the bus.

A class 1 master device is normally a central programmable controller

(PLC), or a PC running special software. The class 1 master sets the baud

rate and the slave’s auto-detect this rate. The class 1 master handles the

data exchange with the slaves assigned to it, and acts as the main controller

for the exchange of I/O information with its distributed slaves, cyclically

retrieving user I/O data according to a defined message cycle. A master can

communicate actively with its assigned slaves, but only passively (upon

request) with another class 2 master device.

The class 2 master is usually a configuration device, perhaps a laptop

or programming console, and is provided for commissioning, maintenance,

or diagnostic purposes. It acts like a “supervisory” master in that it can

actively communicate with class 1 masters and their slaves, in addition to its

own slaves, but usually only for the purpose of configuration, problem

diagnosis, and data/parameter exchange. That is, class 2 masters may only

briefly take over control of a slave. All exchanges between a class 2 master

and class 1 master originate with the class 2 master.

ProfiBus DP normally operates using a cyclic transfer of data between

master(s) and slave(s) on an RS485 network. That is, an assigned master

periodically requests (polls) each node (slave) on the network. All data

communication exchanges between a master and slave originate from the

master device. Each slave device is assigned to one master and only that

master may write output data to that slave. Other masters may read

information from any slave, but can only write output data to their own

assigned slaves.

Masters can address individual slaves, a defined group of slaves (multi-

cast), or can broadcast a telegram to all connected slaves. Slaves will return

a response to all telegrams addressed to them individually, but do not

respond to broadcast or multi-cast telegrams from a master device.

ProfiBus sends Broadcast and Multi-Cast messages as global control

telegrams using address 127 and an optional group number for a targeted

group of slaves.

Because ProfiBus uses a cyclic (periodic) polling mechanism between

masters and slaves, it is also

deterministic

. That is, the behavior of a

ProfiBus system can be reliably predicted over time. In fact, ProfiBus was

designed to guarantee a deterministic response. To contrast, CAN and

Ethernet are event-driven bus systems and consequently form non-

deterministic systems.

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