DeviceNet Master Protocol API

DeviceNet-Master Dual-Port Memory • 31
Source Queue Handle
The ulSrc field identifies the sender of the packet. In the context of the netX firmware (inter-task
communication) this field holds the identifier of the sending task. Usually, a driver uses this field for its
own handle, but it can hold any handle of the sending process. Using this field is mandatory. The
receiving task does not evaluate this field and passes it back unchanged to the originator of the
packet.
Destination Identifier
The ulDestId field identifies the destination of an unsolicited packet from the netX firmware to the host
system. It can hold any handle that helps to identify the receiver. Therefore, its use is mandatory for
unsolicited packets. The receiver of unsolicited packets has to register for this.
Source Identifier
The ulSrcId field identifies the originator of a packet. This field is used by a host application, which
passes a packet from an external process to an internal netX task. The ulSrcId field holds the handle
of the external process. When netX operating system returns the packet, the application can identify
the packet and returns it to the originating process. The receiving task on the netX does not evaluate
this field and passes it back unchanged. For inter-task communication, this field is not used.
Length of Data Field
The ulLen field holds the size of the data field in bytes. It defines the total size of the packet’s payload
that follows the packet’s header. The size of the header is not included in ulLen. So the total size of a
packet is the size from ulLen plus the size of packet’s header. Depending on the command, a data
field may or may not be present in a packet. If no data field is included, the length field is set to zero.
Identifier
The ulId field is used to identify a specific packet among others of the same kind. That way the
application or driver can match a specific reply or confirmation packet to a previous request packet.
The receiving task does not change this field and passes it back to the originator of the packet. Its use
is optional in most of the cases. However, it is mandatory for sequenced packets.
Example: Downloading big amounts of data that does not fit into a single packet. For a sequence of
packets the identifier field is incremented by one for every new packet.
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Edition 3 – API:DeviceNet-Master#EN– 2009/01