Campbell Scientific RF500M Radio Modem - Manual (Page 25)

Section 4. Operation of the Radiotelemetry Network
When programming a PakBus datalogger, the SendData instruction must be
included in the datalogger to enable One Way Data. When the datalogger
executes this instruction and a new record exists in the specified table, the
datalogger sends the record out the interface declared in the instruction. In an
RF500M network, the record is sent directly to a buffer in the RF remote and
the record is collected during the next RF polling procedure. When
LoggerNet receives a One Way Data record, the record is validated and
processed.
When using the SendData instruction in an RF telemetry network, specify the
LoggerNet PakBus address as the direct neighbor address in the instruction
rather than allowing the datalogger to auto-discover the link. Specifying the
neighbor address is more effective since the RF polling process introduces
latency that may cause the messages required for auto-discovery to time out. If
my LoggerNet server PakBus address were 4094 and I wanted to send data
from a table called “Test” through the SDC7 interface, my SendData
instruction should look like this:
SendData (ComSDC7,4094,4094,Test)
The RF500M network collects and transmits One Way Data packets from the
datalogger to LoggerNet through the use of a Time Division Polling process.
On a specified RF polling interval, the RF base broadcasts a message to each
RF repeater and RF remote indicating that a polling event has started. This
message also assigns a specific time window within the polling event for each
RF remote to respond with data packets. The datalogger has already sent
records to the RF Remote using the SendData instruction and these records are
waiting in the buffer of the RF Remote when the polling process begins.
During the polling process, records in the RF Remote buffer are sent from the
RF Remote to the RF base. Since each RF Remote has an assigned polling
event time window in which the data is sent, collisions and RF interference
during data collection are avoided. The RF base stores all collected data
packets in a buffer and sends the collected packets to LoggerNet when
contacted by LoggerNet.
If, for example, an RF base has three RF remotes from which it needs to collect
data, the RF base will send a broadcast packet to all three remotes on the RF
polling interval. The broadcast packet assigns the Time Division Polling
transmission window for each RF remote to send collected data packets to the
RF Base. The default Time Division Polling transmission window allows one
second for each RF Remote to send the data to the RF Base. In this example,
the RF Remotes all hear the RF polling broadcast simultaneously. Each RF
remote takes control of the RF network during its specified transmission
window to send back any complete data packets that exist in its buffer. The
Time Division Polling process eliminates collisions and interference while
quickly transmitting data over the network.
RF remote number one uses the first second after the polling interval to send
any packets in its buffer to the RF base, RF remote number two uses the next
one second window to send any packets in its buffer to the RF base, and RF
remote number three uses the final one second window to send any packets in
its buffer to the RF base. The RF base stores the packets being sent by the RF
remotes in a buffer and waits for contact from the LoggerNet server before
transferring the collected data to LoggerNet.
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