Parameterized Data Exchange With Mdt; Prefetch - Bosch Rexroth ID 40 Manual

It is recommended to only exchange data directly with stopped workpiece pallets,
since otherwise the dwell time of the MDT in the field is too brief to transfer the
amount of data through the field. The runtime of the host system also has to be
taken into consideration.

6.4 Parameterized data exchange with MDT

For parameterized data transmissions, the control system sends read and write
commands to the read/write head in a special parameterization mode when there is
no mobile data tag in the HF field. These commands are buffered in the read/write
head and automatically executed when an MDT enters the HF field.
Parameterized data exchange is particularly quick, since the read and write
commands are already stored in the SLK. This makes it best suited for dynamic
reading and writing.
6.4.1

Prefetch

Prefetch refers to any parameterized read access to the mobile data tag. It is
separated into the read data being transmitted immediately to the control system
(unbuffered prefetch) or held in a special data buffer in the SLK (buffered prefetch).
Setting the parameters of a prefetch means entering SLK addresses that should be
read in the prefetch. Multiple, even overlapping data areas can be set up.
The prefetch function is only executed once the parameters have been set.
If an error occurs during prefetch (e.g., the MDT is no longer in the HF field), the
invalid/unrefreshed data areas are flagged and the error information is returned in
the status byte of the fieldbus channel.
The amount of data in the prefetch has to be readable within the dwell time of
the MDT in the HF field.
6.4.1.1 Unbuffered prefetch
With unbuffered prefetch, the parameterized read commands are executed and the
read data is sent to the control system in an event-oriented data transmission. The
low protocol overhead is advantageous here. Since the number of bytes in the event-
oriented data channel is limited, unbuffered prefetch is suitable primarily for reading
small amounts of data.
Accessing data on the ID 40/MDT
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