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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

When a master switches to Clear Mode, it sends a global control

telegram to all slaves with the first data byte (octet 1) = 2 and the second

data byte (octet 2) = 0. In the next data cycle, the master sends data

telegrams to all stations with either the output data equal to 0, or the output

data length equal to 0 (i.e. only the telegram header and no data). If the

slave GSD file contains “Fail_Safe = 0”, the master sends output telegrams

with the data set to 0 in Clear Mode. However, if the slave GSD file contains

“Fail_Safe = 1” (supports Fail-Safe Mode), the master will send output

telegrams with a data length of 0 in Clear Mode. . Slaves that do not support

fail-safe mode do not process data telegrams with no data. Some older

masters do not make this distinction of fail-safe mode and will send data

telegrams with the output data set to 0 in Clear Mode. This will force all

slave outputs to go to 0 in Clear Mode and this may not be desirable for

some critical control applications.

By the master sending output telegrams with no data in Clear Mode, a

fail-safe slave has the option of either setting all outputs to 0, retaining the

last output state (though this response is not fail-safe), or going to a pre-

defined default “fail-safe” state. This state may be defined in the GSD file

and included in the user-parameters portion of the parameterization

telegram, or it may be set via switches at the slave. The slave will stay in

this state until it receives a global control broadcast telegram from the

master telling it the master is returning to Operate Mode and it receives an

output telegram with the correct output data length, whereupon it updates its

outputs normally as part of data exchange mode.

To summarize, fail-safe mode in ProfiBus DP simply refers to whether

the master sends output data messages of zero length in Clear Mode or not,

and whether a slave is able to process output messages of zero length. The

actions taken by the slave in response to these messages is optional and

specific to the slave implementation. Note that a “fail-safe” slave may not

actually act in a fail-safe manner (for example, it may retain the last state

prior to Clear Mode). In any case, for PTO compliance, a fail-safe slave

must at least give the option of clearing the outputs if the master fails or

switches to Clear Mode.

Watchdog

A slave may assume a defined state if its watchdog time expires without

having received a message from its assigned master. Normally this timer is

reset by the slave 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. A communication error is

detected by the slave and reported with a diagnostic telegram (see function

codes). The slave will then leave the data exchange mode and its outputs

will go to a predefined state (either the reset/clear state, or a user-defined

state) and await reparameterization and reconfiguration by the master. The

timeout state is clear by default, or it may be set via user-defined parameters

of the parameterization telegram and 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.

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