EtherCAT__Process Data

This section shall introduce a few terms and ideas how the master handles process
data.

Process Data Image Slaves offer their inputs and outputs by presenting the mas-
ter so-called \Process Data Objects" (PDOs). The available PDOs can be either
determined by reading out the slave's TXPDO andRXPDOSII categories from the
E2PROM (in case of fixed PDOs) or by reading out the appropriate CoE objects (see
sec. 6.2), if available. The application can register the PDOs' entries for exchange
during cyclic operation. The sum of all registered PDO entries defines the \process
data image", which is exchanged via datagrams with \logical" memory access (like
LWR, LRD or LRW) introduced in [2, sec. 5.4].

Process Data Domains The process data image can be easily managed by creat-
ing so-called \domains", which allow grouped PDO exchange. They also take care
of managing the datagram structures needed to exchange the PDOs. Domains are
mandatory for process data exchange, so there has to be at least one. They were
introduced for the following reasons:
 The maximum size of a datagram is limited due to the limited size of an Eth-
ernet frame: The maximum data size is the Ethernet data eld size minus the
EtherCAT frame header, EtherCAT datagram header and EtherCAT datagram
footer: 1500

FMMU Con guration An application can register PDO entries for exchange. Every
PDO entry and its parent PDO is part of a memory area in the slave's physical
memory, that is protected by a sync manager [2, sec. 6.7] for synchronized access.
In order to make a sync manager react on a datagram accessing its memory, it is
necessary to access the last byte covered by the sync manager. Otherwise the sync
manager will not react on the datagram and no data will be exchanged. That is
why the whole synchronized memory area has to be included into the process data
image: For example, if a certain PDO entry of a slave is registered for exchange with
a certain domain, one FMMU will be con gured to map the complete sync-manager-
protected memory, the PDO entry resides in. If a second PDO entry of the same slave
is registered for process data exchange within the same domain, and it resides in the
same sync-manager-protected memory as the rst one, the FMMU con guration isnot altered, because the desired memory is already part of the domain's process data
image. If the second PDO entry would belong to another sync-manager-protected
area, this complete area would also be included into the domains process data image.
Figure 2.4 gives an overview, how FMMUs are con gured to map physical memory
to logical process data images.