User Guide

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Frequency Hopping Spread Spectrum
The module uses Frequency Hopping Spread Spectrum to allow operation
at higher power levels per regulations and to reduce interference with other
transmitters. The module is configured for operation in one of 6 different
hopping sequences. Each sequence uses 26 channels for the high RF data
rate or 50 channels for the low RF data rate. Modules must use the same
RF data rate and hopping sequence to communicate. Assigning different
hopping sequences to multiple networks in the same area minimizes the
interference.
When the module is awake and not transmitting, it rapidly scans all
channels for a packet preamble. When a module starts transmitting at the
beginning of a new channel, it transmits a packet with a long preamble of
alternating 0 and 1 bits. This long preamble is sufficient to allow receiving
modules to scan through all of the channels in the hopping sequence and
find it. Modules that are scanning detect the preamble and pause on that
channel, waiting for a valid packet.
If a packet is received with a valid CRC (unencrypted) or authentication
(encrypted), the header is examined to determine whether the module
should synchronize to the transmitter. Synchronization requires that the hop
sequence matches and that the message is addressed to the receiver.
When synchronized, the receiver stays on the current channel to either
transmit a packet or to receive an additional packet. Additional packets
transmitted on the same channel within the time slot use short preambles
since the receivers are already listening to the current channel.
At the end of the time slot for the current channel, all modules which locked
to the original transmission switch to the next channel in the hop sequence.
The first transmission on each new channel has a long preamble.
A receiver that has synchronized to a transmitter continues to stay in
synchronism by staying on the received channel until the expiration of the
time slot, then waiting on the next hop channel for the duration of the time
slot. If no further packets are received, the receiver loses lock and reverts
to scanning. This allows the receiver to stay synchronized for a short while
if a packet is not received correctly.
Addressing Modes
The module has very flexible addressing methods selected with the
ADDMODE register. It can be changed during operation. The transmitting
module addresses packets according to the addressing mode
configuration. The receiving module processes all addressing types
regardless of the ADDMODE configuration. If the received message
matches the addressing criteria, it is output on the UART. Otherwise it is
discarded. The ADDMODE configuration also enables assured delivery.
There are three addressing modes: DSN, User and Extended User. Each
mode offers different communications methods, but all use source and
destination addressing. The source address is for the transmitting unit,
the destination address is the intended receiver. Each mode uses different
registers for the source and destination addresses.
Extended User Addressing mode uses the customer ID bytes
(CUSTID[1-0]) for unencrypted messages and the four user destination
address bytes (UDESTID[3-0]) as a destination address. The module’s local
address is contained in the four user source ID registers (USRCID[3-0]).
In normal operation, each module has a user ID mask (UMASK[3-0]) that
splits the 32 address bits into up to three fields to provide a network
address and address fields for sub-networks, supporting both individual
addressing and broadcast addressing within the user’s network.
The HumPRC
TM
Series is normally configured using the Join Process,
which sets the addressing mode to Extended User mode. The other modes
would normally only be used if the HumPRC
TM
Series is being implemented
in a mixed system that also uses the HumPRO
TM
Series modules.
Please see the HumPRO
TM
Series data guide for a description of the
other addressing modes. A detailed explanation and examples for each
addressing mode are given in Reference Guide RG-00105.