Appendix B: Communications Guide; Installation Guide For Rs485 Equipment - GE FHSD720C Installation And User Manual

FHSD700C Series Product Guide
Appendix B:
FHSD700C detectors can connect to a PC with GE Security Configuration and Management software
via Ethernet (optional), through a RS232 direct serial connection or with RS485 via a RS485/RS232
converter.
RS485 also provides the option of connecting up to 30 detectors together in a multidrop RS485
network. Some guidelines on installing RS485 connections are shown in the following sections.
B.1 Installation Guide for RS485 Equipment
Communications to RS485 standard are designed to reject a degree of interference, but care must still
be taken when designing and implementing the installation to minimize 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 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. The installer shall be more qualified to take the relevant technical, safety
and legal decisions.
B.1.1 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 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 twice the nominal RMS voltage.
The equipment metal case must be connected to an earth. To protect personnel this earth should be
free from voltages greater than 10 V peak.
It is recommended that galvanically isolated power supplies (in accordance with UL1481) are used
since the RS485 inputs/outputs are not usually isolated from the equipment internal zero volt line.
The RS232/RS485 converter should also be galvanically isolated.
B.1.2 Cable Material
RS485 communication is by current switching between a pair of wires at data rates of up to 250 kbps.
The cable construction is specified as:

Twisted pair of nominal 10 turns per meter or yard

40 pF to 60 pF meter between conductors
 100  to 120  characteristic impedance

Maximum signal attenuation over total length 16 dB at 0.25 MHz

Overall screen (to be connected at one end only)
36
Communications Guide
GE Security
www.gesecurity.net