User Manual

Operation
RFI-9256 Radio Modem User Manual Page 22 of 96
25ms 269 28 86kbps 9kbps
30ms 338 31 90kbps 8kbps
35ms 410 31 93kbps 7kbps
Table 8: Directional bias configuration for different frame times
RETRIES
The maximum number of retries per frame can be configured between 0 to 255. When a low number of
retries is selected, the link may become unreliable in the presence of interference. When a high number of
retries is selected the link will be reliable, however it may induce substantial latency in the presence of
interference.
If an RFI-9256 is given data to transmit to a slave that is non-existent, either due to a misconfigured
destination address, the slave being out of range, or the slave unit being faulty, then it will transmit each
frame the maximum number of retries. This can dramatically slow down the throughput of a radio network.
The RFI-9256 also supports exponential back-off and retry. This mechanism is intended for a situation
where there are multiple slaves that may have data to transmit at the same time. If these slaves are
configured with exponential back-off and retry mode enabled, collisions will have a minimal impact on
system performance.
SYNCHRONISATION
When two or more RFI-9256 are located in very close proximity, such as when they are in a back-to-back
repeater configuration, the transmitter from one can interfere with the receiver of the other even though they
are on different channels, simply due to the large amount of power that is being radiated.
In order to prevent this from happening it is desirable to synchronise the radios so that they will transmit at
the same time. Only master units can be synchronised in this way, as slave units must obtain their
synchronisation from the network master.
The frame synchronisation is a TTL I/O on the DB25 connector described in Appendix A.3.
Each RFI-9256 can be configured with one of the following synchronisation options:
 No Sync Mode : Disables synchronisation.
 Output Sync Signal : The radio will output a frame synchronisation signal. This is a falling edge
on the TTL output every time a new frame starts.
 Follow Sync Signal : The master radio will follow the synchronisation signal output by another
unit. This causes the master radio to adjust the start of its frame to match the falling edge detected
on the synchronisation TTL input. This mode should be used when a master is following another
collocated master.
 Repeater Sync Mode : The master radio will follow the synchronisation signal output by another
unit. This causes the master radio to adjust the start of its frame to be 50% offset from the falling