UM1573

ST7540 stack firmware code

Doc ID 023619 Rev 1

7/27

The FEC is used to correct wrong data (if any), the CRC is also checked, and if valid data

are decoded, these are sent to the user application level. The PLM stack directly manages

the repetitions, the necessary timing and acknowledgements.

If the target device does not acknowledge a data frame, each neighboring node, with the

repetition feature enabled, that has sensed the transited data frame, forwards it, after the

acknowledge timeout, as long as the band is not already taken. This repetition mechanism,

managed directly by the PLM stack engine, is done once per node until either the data frame

is acknowledged or each node has repeated the data frame. The repetition mechanism is

also implemented for the acknowledgement frame, when not received by the master.

Figure 4.

Data communication flow with a case of repetition

As each firmware layer adds its own information to the original frame, 100 bytes of user data

from the application level become more than double that at the power line level. This is

mainly because of the FEC redundancy added to each byte as soon as it arrives at a target

node. Network traffic is therefore reduced if the power line is not overly noisy because the

FEC algorithm can adjust the corrupted information.

Each frame also contains the target address, the source address, and other parameters.

These may include a flag byte indicating the network model (the flag indicates if the

repetition must be ignored for this frame, or if the acknowledge is awaited), a frame ID to

avoid multiple repetitions of the same frame, the CRC (CRC16) bytes, the modem preamble,

the header and the postamble byte.

Another implemented mechanism related to repetitions is 'hopping.' The hop level is one of

the user-defined PLM parameters and is employed to assign a certain hierarchy in the

repetition. If a frame must be repeated but the hop level is lower than the one stored in the

stack parameter, the frame is not forwarded. Normally, the hierarchy is set depending on the

distance and the ambient noise condition. The closer the repetition-enabled node is to the

concentrator, the higher the hop level, and in this way, the traffic of insignificant repetitions is

reduced.

Network grouping is another feature that can be implemented by the user. If implemented,

the first two bytes of any frame address, which is 6 bytes long, are considered the group

address. Each frame with a group address different from the assigned group is ignored. In

this way it is possible that more than one network can share the same power line without

interacting with each other.

M

N3

N2

N1

DATA

bACK

NOT SENSED ACK

AM15246v1