wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

KNX 42-7
these GOs using the at-most-once multicast service Data_Group provided by the KNX TL. For this
purpose, it maintains an association table between AL service access points (SAPs), that is, GO identifiers,
and TL SAPs (which correspond one-to-one to DL group addresses). Thus, a group address describes
a group of GOs in a communication relationship with each other. Every node is aware which groups it
belongs to and treats messages accordingly.
Usually, data sources will actively publish new data via the AL GroupValue_Write service. If a user
application changes, the value of a GO and A_GroupValue_Write.req is invoked, the network stack
determines the group address this information is to be sent to. The user application is not aware of the
group addresses associated with a GO (implicit addressing). In receivers that recognize this group address
as one of their own, the AL generates an A_GroupValue_Write.ind for the AL SAPs bound to this
group address, providing the user data from the GroupValue_Write PDU (protocol data unit). The
application environment (KNX application interface layer) updates the GOs accordingly and notifies the
user application. This mechanism allows passing a piece of information to an arbitrary number of recipients
by way of a single message. For a single receiver only, it is shown in Figure 42.3. GroupValue_Write
is an unconfirmed service. Message delivery is not guaranteed. If exactly-once semantics are required,
confirmations have to be obtained by the user application.
In this mode of communication, senders do not need to know which nodes will actually be receivers
of their messages. The knowledge concerning which nodes participate in any particular communication
relationship is distributed over all nodes in the system. In KNX, it has traditionally even been
impossible (or at least infeasible) for any node to determine which other nodes will accept and process
messages to any specific group address. This pattern is sometimes referred to as the producer/consumer
1. Wall switch is
pushed
2. Device receives
signal
3. User application recognizes
change of state at input port
User application
GO 1 GO 2 GO 3
6. Network stack looks up KNX network stack
group address associated
with this group object Group obj. ID Group addr.
GO 1 1/1/2
GO 2 4/7/3
GO 3 2/8/1
4. User application updates
associated group object
5. Application environment
recognizes group object
update
7. Network stack sends L_Data
frame (with GroupValue_Write
PDU) to this address
13. Lamp lights up
12. Signal energizes
relay
11. User app. performs associated
action (activates output signal)
KNX
network
User application
10. Application environment
updates appropriate group
object with value received
and notifies user application
GO 1
GO 2
KNX network stack
Group obj. ID
GO 1
GO 2
Group addr.
2/8/1
5/1/6
8. Network stack recognizes
destination address of L_Data
frame as one of its own
9. Network stack determines which
group object is associated with this
group address
FIGURE 42.3
GroupValue_Write—end-to-end overview.
© 2011 by Taylor and Francis Group, LLC