Touchpoint 4 Serial Communication Protocol Issue 1 09/07
Topology
Touchpoint4 Serial Communication system has one trunk cable, along which devices
are connected, directly (daisy chaining) or by short derivation cables.
Thetrunkcable,alsonamed“Bus”,canbelong(seehereafter).Itstwoendsshouldbe
connected to Line Terminations.
Length
Theend-to-endlengthofthetrunkcablemustbelimited.Themaximumlength
depends on the baud rate, the cable (Gauge, Capacitance or Characteristic Impedance),
and the quantity of loads on the daisy chain.
Foramaximum9600BaudRateandAWG26(orwider)gauge,themaximumlength
is1000m.
If the derivation cable is needed for the connection of the master, the derivation must
beshort,nevermorethan20m.
Grounding Arrangements
At one end of each cable its shield must be connected to protective ground.
Line Termination
To minimize the reflections from the end of the RS485-cable, it is recommended to
place a Line Termination near each of the 2 Ends of the Bus.
It is important that the line must be terminated at both ends since the propagation
isbi-directional,butitisnotallowedtoplacemorethan2LTononepassiveD0-D1
balanced pair. Never place any LT on a derivation cable.
Each line termination must be connected between the two conductors of the balanced
line:D0andD1.
Lineterminationmaybearesistorof100ohmsto150ohms(0.5W).
Pull-up and Pull-down Resistors
Line control has been implemented in Touchpoint4 gas detector controller so that the
buscontainsavalid"off"signalwhennonodeistransmitting.Noexternalpull-upor
pull-down resistors are needed.
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