Radio Telemetry involves the relaying of voice, digital/analogue process signals, and/or serial data. Defining what is required is sometimes hidden in vaguely written specifications and has been the pitfall of many a supplied system. In this subsection we are merely concerned with determining which of the three signal types are required, once done we will then progress to planning the system itself.
Digital/analogue process signals: These are simply 'On/Off' and Level (e.g. 4-20mA) signals that require duplication at one or more remote points without the need or desire for hardwire solutions. Typical examples are levels from a reservoir on a mountain top to a pump house in a valley near a stream. The approximate quantity of signals should also be known as well as determining the delays allowed in having the signals relayed from one side to another. Not all radio telemetry systems are suited to high speed transfer requirements. Further to this there are times digital inputs are required to count pulses. Some systems require the pulse be counted at 0.5mS, not all systems are capable of this. Complications such as the requirement (or not) of fully isolated analogue signals must be established at this point. Not all systems have fully isolated analogue inputs and/or outputs.
Serial Data: Units used for this purpose are usually referred to as 'radio modems', a term derived from their telephone line counterparts. Examples of radio modem use are usually found where the link required between two sites would normally be done with telephone modems, such as the linking of PLCs to each other (typically using MODBUS protocol) or to a computer running a SCADA software package. Others examples would include any remote device usually connected to a computer's serial port but where the device is too remote to have a hardwire link. The bit rate (often referred to as Baud) is an important factor to be considered as most radio modems are only capable of transferring 4800 bits per second, although some are capable of 9600 and more. Further complications arise when the connection must be a full duplex link (the devices connected to each other must be able to send data simultaneously to one another). Radio systems cannot transmit and receive on the same frequency at the same time making the maximum spec required half-duplex (each device taking its turn transmitting). If higher transfer rates or full-duplex are required then more up-market systems will need to be employed and could impact the economic viability of the system.
Voice: Some systems require a voice link to be available between sites for use by example maintenance staff, or the link may even form part of the systems specification such as in up-market alarm systems where an ops room has voice contact with a premises they are keeping watch over. Such voice traffic must be taken into account when designing a system as it puts constraints on the radio channel availability for telemetry traffic. If the system is to reside on a voice channel this too must be taken into account as such channels offer little time for radio telemetry traffic. Where voice use is directly related to the system's use (as in the alarm system) experience has proved a maximum of 2% radio time is available to telemetry commands. Where the radio telemetry resides on a voice channel (there is no direct relation between the voice traffic and system traffic) proof of as low as 0.5% telemetry traffic starts causing conflict where the telemetry system and radio operators require the channel at the same time.
Defining the requirement also involves ascertaining whether or not the system being planned, assembled, and installed now is to be, in part or whole, incorporated into a larger system in the future. It may be that the current project can receive funding from the coffers of a larger project should the requirements of the larger system be catered for together with those of the immediate system. Imagine the points one could score for oneself with such a simple strategy, yet so few think to ask the questions.
To re-iterate; The aim of this section is to obtain the overall specification of the signals that require relaying via the radio telemetry system and not to start planning the system in detail. Once all the primary uses of the system have been defined i.e. digital/analogue, data, and/or voice we can then progress to ensuring the equipment proposed can satisfy the requirement. In simple terms, here we are interested in "What we need to relay". Later we'll deal with "How much we need to relay".
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