User's Manual

Radio/Serial Telemetry Module User Manual
Page 28 © March 2000
Messages transmitted via serial link are received in less than 20 mSec, however a message sent by
radio takes approx 100 mSec.
These delays are not significant is most applications, however if your application requires faster
responses, then the above delays need to be considered.
3.2.2 Radio Channel Capacity
Messages sent on a cable link are much faster than on a radio channel, and the capacity of the radio
channel must be considered when designing a system. This becomes more important as the I/O size
of a system increases.
The modules are designed to provide “real-time” operation. When an input signal changes, a
change message is sent to change the output. The system does not require continuous messages to
provide fast operation (as in a polling system). Update messages are intended to check the integrity
of the system, not to provide fast operation. Update times should be selected based on this
principle. The default update time is 10 minutes - we recommend that you leave these times as 10
minutes unless particular inputs are very important and deserve a smaller update time.
It is important that radio paths be reliable. For large systems, we recommend a maximum radio
channel density of 100 messages per minute, including change messages and update messages. We
suggest that you do not design for an average transmission rate of greater than 50 per minute - this
will give a peak rate of approx 100 per minute. Note that this peak rate assumes that all radio paths
are reliable - poor radio paths will require re-try transmissions and will reduce the peak channel
density. If there are other users on the radio channel, then this peak figure will also decrease.
The modules will only transmit one message at a time. If re-tries are necessary, another message
cannot start. The time between re-tries is a random time between 1 and 5 seconds. The time for
five tries is between 5 and 21 seconds. Another message cannot be sent until the last one has
finished. This delay will obviously have an affect on a busy system.
3.2.3 Radio Path Reliability
Radio paths over short distances can operate reliably with a large amount of obstruction in the path.
As the path distance increases, the amount of obstruction which can be tolerated decreases. At the
maximum reliable distance, “line-of-sight” is required for reliable operation. If the path is over
several kilometres (or miles), then the curvature of the earth is also an obstacle and must be allowed
for. For example, the earth curvature over 10 km is approx 3m, requiring aerials to be elevated at
least 4m to achieve “line-of-sight” even if the path is flat.
A radio path may act reliably in good weather, but poorly in bad weather - this is called a
“marginal” radio path. If the radio path is more than 20% of the maximum reliable distance (see
Specification section for these distances), we recommend that you test the radio path before
installation. Each U module has a radio path testing feature - refer to section 6.2 of this manual.