User manual

905U Wireless I/O Module User Manual
105S Serial I/O Module
Page 30 © October 2001
after the first repeater, then the source module CF status will not set. To indicate comms status on this
type of link, the “Reset Output” function should be used.
3.1.6 Resetting Outputs
Each digital and analog output may be individually configured to reset if that output has not received a
change-of-state or an update message within a certain time period. Generally this time is set to twice
the update period, so at least one update can be missed before an output is reset.
In most cases it is desirable to reset outputs which are controlling equipment if there is a system failure,
however alarm or indication outputs are not reset so the last valid indication remains shown. See
Chapter 4 Configuration for further details.
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 system. Digital inputs have a minimum “debounce”
delay of 0.5 sec - that is, a change message will not be sent for 0.5 sec after a change has occurred.
Analog 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 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 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