Datalogic GWY-01-PBS-01, Reference Manual

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PROFIBUS INTERFACE
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5
Resynchronization Protocol
Resynchronization may be used at the master startup, both to detect if a slave is on line or
not, or to restart the messaging protocol from a predefined state. It is also used during
normal operations in case of errors requiring a protocol reset procedure to be started.
Figure 25: Resynchronization State Machine
5.4.3 SAP Field
SAP (Service Access Point) is an identifier that is used to share the same communication
channel between processes of two remote stations. This allows splitting the single service
into different services.
SAP = 0 is actually used by the slave to transfer acquisition information; it should also
be used to transfer application data from Master to Slave.
SAP = 2 is currently reserved.
SAP = 255 is currently reserved.
Only SAP 255 and 2 are reserved. All other SAPs are free and may be used by new
application programs.
5.4.4 Length Field
The Application layer uses all or a part of the remaining bytes of the Exchange Area buffers
that are not used by the Datalogic AnyBus Driver. The Length Field is introduced to keep the
information of how many bytes are really used by the Application Layer. A fragment that is
not the last one of a fragmentation sequence must fill this field with Max(In/Out)Bytes-3,
depending on whether it is an INPUT/OUTPUT fragment. Otherwise this field is filled with a
number that is less than or equal to Max(In/Out)Bytes-3.