Siemens SPC3 Specifications download pdf (Page 8)

Introduction To ProfiBus DP

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Data Exchange State

After parameterization and configuration have been accomplished, the

slave cyclically exchanges I/O data with the master. This is a cyclic transfer

of I/O data and possible diagnostic information.

Fail Safe Operation

A ProfiBus master runs in two modes: Operate and Clear. With

respect to the master of a ProfiBus DP system, the term fail-safe simply

refers to whether the class 1 master sends 0 length data, or data set to 0,

when it is in Clear Mode. With respect to a DP slave device, the term fail-

safe refers to whether the slave will process output telegrams with zero

length data, or not. Whether the combined master/slave system is

considered fail-safe depends on the actions taken by the slaves if the master

fails, or if the master switches to Clear Mode. Ideally, the failure of a master

should not cause errors in any of its slaves and the slave outputs should go

to a predictable (defined) state. Using a fail-safe mode, the slave outputs

can automatically switch to a fail-safe state in the event of master failure, or

when the master switches to Clear Mode.

A slave may assume a fail-safe state if its watchdog time expires

without having received a message from its assigned master. Normally this

timer is reset every time the master talks to the slave. If this time expires,

this means the master has not communicated with the slave recently, and

the slave is not being controlled. The slave will then leave the data

exchange mode and its outputs will go to a pre-defined state (either their

reset state, or another user-defined state). This state is usually set via user-

defined parameters of the parameterization telegram and its GSD file, or

sometimes via hardware switches on the slave. Some slaves may provide

parameters or switches that also allow the slave outputs to retain their last

state, but this is not considered fail-safe.

A slave may also assume a fail-safe state if its master switches from

Operate Mode to Clear Mode. With normal operation in Data_Exchange

mode, a class 1 master is in Operate Mode and cyclically exchanges I/O

data with its assigned slaves. The class 1 master may use a global control

telegram to inform the slaves that it is switching from Operate Mode to Clear

Mode. A master may elect to switch to Clear Mode while it is bringing slaves

online and not all slaves have been parameterized and configured yet. It

may also switch to Clear Mode as a result of a run/stop switch on the

master. In the Clear State, the master may attempt to parameterize and

configure the remaining slaves assigned to it in an effort to reinitiate data

exchange, while it continues data exchange with the other slaves (they will

be receiving output data of 0, or output data of zero length). Operate mode

does not resume until all slaves are online and exchanging data, or until the

master is told to resume operation via a run/stop switch or under program

control. Further, some masters may go to Clear Mode if a slave is disabled,

rather than continue to control a partial system (this response may be

specified as a parameter in the master’s GSD file and parameterization

telegram, via a mechanical switch, or as part of its master program).

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