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4 INSTALLATION GUIDE FOR RS 485 EQUIPMENT
Communications to RS 485 standard are designed to reject a degree of interference, but care must still be taken when designing and
implementing the installation to minimise the level of interference imposed on the equipment. To prevent damage due to tropical storms
and other sources of large external currents and voltages, additional surge absorbing components may be required as part of the
installation.
It is the responsibility of the installer to provide a suitable environment to prevent damage to the equipment, and to reduce external
interference to a level within which the equipment can operate as defined by its specification. The provision of warning, caution and
danger information to the end user and his personnel is within the task of the installation provider, and not the instrument manufacturer.
The purpose of the guide is to assist the installation provider; but it is not intended to be a full and complete set of instructions as he is
more qualified to take the relevant technical, safety and legal decisions.
4.1 Electrical Supply
The supply voltage and frequency must be maintained within the equipment requirement specification. Any breaks in supply or reductions
in voltage below the specified minimum will result in equipment shutdown. When this problem exists, an uninterruptible power supply
should be used to supply the equipment.
If the electrical supply is known, or suspected, to be subject to be subject to noise pulses produced by motors, fluorescent lamps, storms
and lightning, or other causes then mains filtering must be fitted to reduce the peak amplitude to less than 2 x nominal R.M.S. voltage.
The equipment metal case must be connected to an earth. To protect personnel this earth should be free from voltages greater than 10
volts peak.
4.2 Cable Material
RS485 communication is by current switching between a pair of wires at data rates of up to 250kbps.
The cable construction is specified as:
Twisted pair of nominal 10 turns per meter/yard
40pF to 60pF meter between conductors
100 ohm to 120 ohm characteristic impedance
Maximum signal attenuation over total length 16 dB at 0.25 MHz
Overall screen (to be connected at one end only)
The cable materials are not specified, but in order to meet the low capacity specification polyethylene/polyolefin/polypropylene is used as
the insulator, with PVC/flame retardant PVC as the overall sheath. Other materials can be used if the impedance and capacity requirements
are met.
Installation requirements may add other features, such as fire resistance, flame retardance, UL approval, low smoke and fume etc.
4.2.1 Common Mode
The RS485 inputs/outputs are not usually isolated from the equipment internal zero volt line. The maximum potential difference between
the zero volt lines of all instruments connected to an RS485 communication cable is limited by the specification to between -7 volts and +
12 volts. If this limit is exceeded, communications will be lost.
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