User's Manual

905U-K.MAN Rev. 1.1 January 2002
 ELPRO Technologies Pty Ltd Page 15
The peak transmission density should be calculated for large systems. These values are
calculated by determining the number of transmissions from inputs changing value and the
number of update transmissions per hour.
The probability of success for an individual message depends on the transmission density and
the number of re-transmissions for each message. This is shown in the following graph:
Probability of successful transmission
0%
10 %
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 1000 2000 3000
Messages Per Hour
1
2
3
4
5
Transmissions
per message
This assumes that the radio path is reliable and that there is no other radio users on this radio
channel. If intermediate repeaters are used, then each repeated message should be counted as
another message.
We suggest that you use two transmissions per message - with a higher number for a small
number of high priority inputs.
In addition to the above comments, you need to consider the affect of re-transmissions on
other messages being transmitted from the same 905U-K module. Once a 905U-K starts
transmitting a message, all transmissions for this message must be complete before another
transmission can start. The time between retransmissions of the same message is a random
time between 0.5 and 4 seconds. For example, if a module is configured to transmit each
message 5 times, then each message will take up to 16 seconds. Another message cannot be
transmitted until the previous message has finished. In the above example, the maximum
number of messages which can be transmitted reliably is approx 4 per minute. If more
messages are required, then a lower number of re-transmissions should be selected.
2.4.2 How far will the radio transmit?
The 905U-K will operate reliably over large distances. The distance that may be reliably
achieved will vary with each application and is dependent on the following factors:
• Type and location of antennas (the higher an antenna is, the further it will transmit)
• Amount of radio interference from other transmitters or radio “noise”
• Obstructions such as buildings, hills or trees in the radio path
• Maximum radio transmitter power allowed in the country
The transmitter power level of each 905U-K is set in the factory to the maximum power
allowed in the country of sale. The maximum output power of the 905U-K is 1 Watt,