Echelon LONWORKS PLT-22, User Manual

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Dual Carrier Frequency Mode
Each PLT-22 transceiver incorporates a new dual carrier frequency capability which
allows it to communicate with other PLT-22-based nodes, even if noise is blocking its
primary communication frequency range. If impairments prevent communication in
this range, a PLT-22-based node can automatically switch to a secondary carrier
frequency to complete a transaction with other PLT-22-based nodes.
With dual carrier frequency mode enabled, the last two retries of acknowledged
service messages are sent using the secondary carrier frequency. Thus when
acknowledged service is used with three retries (four total tries), the first two tries
are sent using the 132kHz primary carrier frequency. If the last two tries are needed
to complete the transaction, they are sent (and acknowledged) using the 115kHz
secondary carrier frequency. A minimum of two retries must be used if the PLT-22
transceiver is to be able to use both carrier frequency choices. For optimum
reliability and efficiency, Echelon recommends the use of three retries when using
acknowledged service messaging with the PLT-22 transceiver.
When unacknowledged repeat message service is used, the PLT-22 transceiver
leverages the reliability of both carrier frequencies by alternating between them. In
this case an unacknowledged repeat message with three repeats results in the first
and third packets being sent using the 132kHz primary carrier frequency, while the
second and fourth packets are sent using the 115kHz secondary carrier frequency. A
minimum of one repeat must be used for the PLT-22 transceiver to use both carrier
frequency choices.
Every PLT-22 transmission at the 115kHz secondary carrier frequency is
accompanied by a simultaneous 132kHz “pilot” signal which older PLT-20 and PLT-
21-based nodes can use to recognize that the channel is busy. This pilot signal
prevents PLT-20- or PLT-21-based nodes (which cannot detect the 115kHz secondary
carrier frequency) from transmitting at the same time that a PLT-22 is transmitting
on its 115kHz secondary carrier frequency.
The user can configure a PLT-22-based node to operate with the dual carrier
frequency mode either enabled or disabled. When a PLT-22-based node is operated
with dual carrier frequency mode enabled, it will operate as described above. When
a PLT-22-based node is operated with dual carrier mode disabled, it will only
transmit using its 132kHz primary carrier frequency. Note that the selection of dual
frequency mode on a PLT-22-based node only affects its transmission characteristics.
Each PLT-22 transceiver is always able to automatically detect and properly receive
packets at either frequency, regardless of whether or not dual frequency
transmission is enabled. Activation of dual carrier frequency mode is controlled by
the revision level of standard transceiver parameters, as described later in this
chapter.
3-2 PLT-22 Transceiver Programming