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.
The trunk cable, also named “Bus”, can be long (see hereafter). Its two ends should be
connected to Line Terminations.
Length
The end-to-end length of the trunk cable must be limited. The maximum length
depends on the baud rate, the cable (Gauge, Capacitance or Characteristic Impedance),
and the quantity of loads on the daisy chain.
For a maximum 9600 Baud Rate and AWG26 (or wider) gauge, the maximum length
is 1000m.
If the derivation cable is needed for the connection of the master, the derivation must
be short, never more than 20m.
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
is bi-directional, but it is not allowed to place more than 2 LT on one passive D0-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: D0 and D1.
Line termination may be a resistor of 100 ohms to 150 ohms (0.5 W).
Pull-up and Pull-down Resistors
Line control has been implemented in Touchpoint4 gas detector controller so that the
bus contains a valid "off" signal when no node is transmitting. No external pull-up or
pull-down resistors are needed.
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