105 Radio/Serial Telemetry Module
User Manual
Page
30
©
January 2004
3.2
System Design Tips
The following tips will help to ensure that your system operates reliably.
3.2.1
System Dynamics
It is important to be aware of the dynamics of the 105 system. Digital inputs have a minimum
“debounce” delay (default 0.5 sec) - that is, a change message will not be sent for 0.5 sec
after a change has occurred. Analogue inputs and outputs have time delays of 1 to 2 seconds.
Messages transmitted via serial link are received in less than 20 mSec, however a message
sent by radio takes approx 60 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 105 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
average 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 40 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 105 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