Lenze PLC Designer PLC Designer (R3-1) - Manual (Page 117)

L-force | PLC Designer
Concepts and Base Components

DMS 4.1 EN 03/2011 TD29 115
The (negative) address offset describes the number of address components that a
packet has to be handed upwards in the tree before it can be handed down again from
a common parent. Since nodes can use partial addresses consisting of more than one
address component, the number of parent nodes to be passed is always = the address
offset. This means that the demarcation between parent nodes is no longer
unambiguous, which is why the common initial part of the addresses of the
communication partners is used as parent address. Each address component is
counted as an upward step, irrespective of the actual parent nodes. Any errors
introduced by these assumptions can be detected by the respective parent node and
must be handled correctly by the node.
On arrival at the common parent the relative path (an array of address components) is
then followed downwards in the normal way.
Formal: The node address of the receiver is formed by removing the last AdressOffset
components from the node address of the sender and appending the relative path to
the remaining address.
Example:
Within the example a letter will represent an address component, whereas a point will
separate the particular nodes. Since a node is allowed to have multiple address
components, it is allowed to have multiples letters within the example.
Node A: a.bc.d.ef.g
Node B: a.bc.i.j.kl.m
• Address of the lowest common parent: a.bc
• Relative address from A to B: -4/i.j.kl.m (The number -4 results from the 4
components d, e, f, and g the packet has to be raised)
To ensure the correct working of the routing, the relative address has to be adjusted
with each pass through an intermediate node. It is sufficient to adjust the address
offset. This is always done by the parent node: If a node receives a packet from one of
its subnets, the address offset is increased by the length of the address component of
this subnet:
• If the new address offset is < 0 the packet must be forwarded to the parent node.
• If the address offset >= 0 the packet must be forwarded to the child node the local
address of which is located at the position described by the address offset within
the relative address. First, the address offset must be increased by the length of the
local address of the child node to ensure that the node sees a correct address.
A special situation arises when the error described above occurs while determining the
common parent. In this case the address offset at the “real” common parent is
negative, but the magnitude is greater than the length of the partial address of the
subnet the packet originates from. The node must detect this case, calculate the local
address of the next child node based on the address of the previous node and the
length difference, and adapt the address offset such that the next node will see a
correct relative address. Also in this case the address components themselves remain
unchanged, only the address offset will be modified.