EM58 • HS58 • HM58 POWERLINK
6.7.4 Start of Asynchronous (SoA)
Once the Isochronous Period is concluded, the MN sends a broadcast frame
called Start of Asynchronous (SoA, ID = 05h) which informs all CNs about the
start of the Asynchronous Period (see the “6.8.2 Asynchronous Period” section
on page 65). This second phase ensures the transmission of only one
asynchronous message by a selected node. The SoA frame is also used to inform
which node has been selected for the acyclic communication.
6.7.5 Asynchronous Send (Asnd)
The Asynchronous Send (Asnd, ID = 06h) frame transports asynchronous data
via the POWERLINK/ASnd protocol (e.g. NMT commands).
6.8 POWERLINK Cycle
Data exchange within a POWERLINK network is structured in fix intervals, called
POWERLINK cycles. The cycle is subdivided into the Isochronous Period, the
Asynchronous Period and the Idle Period and is managed by the MN.
6.8.1 Isochronous Period
The Isochronous Period of a POWERLINK cycle offers deterministic operation, i.e.
it is reserved for the exchange of (continuous or multiplexed) isochronous data.
Isochronous data exchange between nodes occurs cyclically. It is repeated in a
fixed interval, called the POWERLINK cycle. The POWERLINK cycle is controlled
by the MN.
At the beginning of a POWERLINK cycle, the MN sends a SoC (Start of Cycle, see
on page 64) frame to all nodes via Ethernet multicast. The send and receive time
of this frame is the basis for the common timing of all the nodes.
Only the SoC frame is generated on a periodic basis. The generation of all other
frames is event controlled (with additional time monitoring per node).
The MN starts the isochronous data exchange after the SoC frame has been
sent. A PReq frame (see on page 64) is sent to every configured and active node.
The accessed node respond by a PRes frame (see on page 64).
6.8.2 Asynchronous Period
The Asynchronous Period is the second part of the POWERLINK cycle, starting
with a Start of Asynchronous (SoA) frame (see on page 65).
In the asynchronous phase of the cycle, access to the POWERLINK network may
be granted to one CN or to the MN for the transfer of a single asynchronous
message only.
There are two types of asynchronous frames available:
•
the POWERLINK ASnd frame uses the POWERLINK addressing scheme
and is sent via unicast or broadcast to any other node (see on page 65);
•
a Legacy Ethernet message can be sent.
If no asynchronous message transmission request is pending at the MN
scheduling queues, the MN issues a SoA without assignment of the right to send
to any node. No ASnd frame follows to the SoA frame in this case.
MAN EM58_HS58_HM58 PL E 1.1.odt
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