Operation
RFI-9256 Radio Modem User Manual
Page 31 of 109
Reliability:
By reducing the maximum number of retries the latency performance of the RFI-9256
can be improved in harsh RF environments. Given that each retry will add an extra frame time to
the latency induced by the radio, calculate the maximum acceptable latency induced by retries, and
divide by the frame time to give the maximum retries that should be configured. Note that by
reducing the maximum number of retries, data may be lost so the underlying system should be
robust enough to handle this situation.
TIME BASED PACKETISATION OF DATA
Many protocols delimit packets of data by silence on the communications line for a set period of time. A
common example of such a protocol is Modbus.
Due to the TDD nature of the RFI-9256, simply passing this data into the radio can cause it to become
disrupted as there is no guarantee that timing will be maintained over the air link (see section Data Timing
on page 25). In order to correct this problem, two configuration items should be used.
Set the Rx packetisation timer to the number of milliseconds that will appear between each packet.
Set the frame time to the smallest number that will provide enough bytes in a single frame for the
largest protocol message.
The packetisation time is used to detect the end of each packet. Only once the end of the packet has been
detected will the RFI-9256 transmit any of the packet.
The frame time must be configured to allow a full packet in a single frame. If the frame time is too small
then the packet will be split across multiple frames and inter-character delay could occur due to retries.
When packets are larger than the maximum frame time, yet must still be kept together the Tx packetisation
timer can be used. This timer operates in the same way as the RX packetisation timer, except that it will
attempt to bunch received data frames together. When the TX packetisation timer is enabled the radio will
not transmit data out of its serial port until an amount of time equal to the packetisation timer has passed
with it receiving no data over the air.
For more information on configuring the packetiser timer, see section B.3.1 on page 80.
5.2.5 Diagnostics
RSSI AND NOISE
The RFI-9256 can report the RSSI and noise detected on each individual channel in the 900MHz ISM band.
This is accessed through the diagnostics menu (see Appendix B.6 on page 83).
There are two factors to be aware of when using the RSSI / Noise report of the RFI-9256.
1.
The master is transmitting all the time, while slaves only transmit when they have data or a
connection has been established to them. Thus, a slave will always be able to report the RSSI of the
master, while the master can only report slave RSSI when communications is occurring.
Main -> Serial Port
