Achieving the Requirements


i) Planning the correct number of I/O

Now the really hard work begins. It is imperative that the system is fully laid out with regards all signals that need to be relayed via the radio telemetry system. Using a 'matrix' method of planning works extremely well with each signal named down the left, where it originates, and where it is expected to appear. An example of such a matrix is pictured showing a simple water supply system comprising of a reservoir, pumphouse, and caretaker stations.

screenshot of System Planner

What will be noticed is if DI, DO, PI, PO, AI, and AO are used to define the signal types the spreadsheet automatically calculates the required number of inputs and outputs per site. Formulae are also in place to help calculate the number of modules required based on the number of required I/O. Although the formulae are based on the Elpro 105U range of modules they are easily modified for any make of telemetry modules. Note how in our example the same type of module is used throughout, this amounts to good system design as only one type of spare is required to maintain the entire system. If just one module in a huge system is different the size of the spares cupboard increases and therefore the costs of the entire system.

This Excel spreadsheet is available for download and may be freely used, modified, and distributed. Handy hints: Inserting more stations should be done by copying the last column across to as many columns as required before filling in any data (if you do copy data across just select the input portion and delete everything copied). Inserting more rows should be done above the "Totals" heading.

ii) Using Analogue I/O as Digital

Although frowned upon by the perfectionists there is a way of reducing the number of modules required when an enormant amount of digital I/O is required by using any spare analogues as digitals.

If you have spare analogue inputs and/or outputs, and you are prepared to use a little trickery as well, they could very well reduce the number of modules you require and the section "Interfacing Analogue and Digital" is written just for you. This may not be accepted practice among some consultants so do expect some resistance, especially from those who are not electronically adept. At least you have the option open to you.

iii) Planning Aids

Not seen very often but something that has helped me tremendously in planning sites is a "I/O Planning Card". It is nothing more than a pictorial view of the I/O available on each of the types of modules to be used in a telemetry system. As example of one drawn up for the Elpro E Series is shown here. The type of module required at a site can be decided at a glance when using such a card without having to commit to memory all the available options.

What will be noticed is the power supply arrangement is displayed too. This is as a result of modules having had some well meaning engineer try to power an instrument from the 24 volt sensor supply which exceeded the 150mA and damaging the module.

Catering for expansion  >>

| | Ask a Question |