User Manual

Operation
RFI-9256 Radio Modem User Manual Page 25 of 96
 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, as 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.
4.2.4 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 (Appendix B.6).
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.
2. Each individual slave can only hear the master unit, while the master can hear all the slaves. This
means that when a master is communicating with multiple slaves there is no guarantee which slave
the RSSI value refers two, in fact it is most likely to be an average of all slaves. In order to counteract
this effect a special link margin test is provided on the master unit. The link margin test will take
control of the communications link and determine the exact RSSI that the master is receiving from a
particular slave. This is covered in Appendix B.6.
RF COMMUNICATIONS STATISTICS
The RFI-9256 provides a number of communications statistics that can be used to debug a RFI-9256 system.
These parameters are shown in Table 9.
Statistic Description
Frame
count
The total number of frames that have passed while the radio has been operating.
Empty
frames
The number of frames that have been received but contained no user data.
Good
packets
The number of good packets received.