Cable Characteristics; Bus Isolation And Termination Resistors - Astralpool 65321 Manual Del Usuario

3. CABLE CHARACTERISTICS

The recommended wiring for a MODBUS-RTU Communication is based in a linear structure, active bus
with termination at both ends. It is possible coupling and uncoupling of devices during operation
without affecting other devices. The wire shall be twisted and shielded according to EN 50 170.
The values of transmission rate supported for the device, allow maximum cable length of 1,200 m
without repeaters, or up to 10 km using repeaters, when installation is according to the standard.
For the balanced pairs used in an RS485-system, a Characteristic Impedance with a value higher than
100 Ohms may be preferred, especially for 19200 and higher baud rates.

4. BUS ISOLATION AND TERMINATION RESISTORS

If the communication bus is accessible for the user, it shall be double insulated. As far as in general the
accessibility of the bus to users will depend on each single installation, safety isolation has NOT been
implemented in the Compact Electric Heater physical bus layer. Moreover, for safety purposes, it is
recommended to ensure that other devices sharing this bus also implements this insulation.
Additionally, the use of bus insulated devices not only enhances the security level, furthermore
increases the equipment reliability, larger immunity to electromagnetic interference, longer life, higher
reliability, more stability over the range of temperatures.
Whenever single or multiple devices are connected sharing a bus physical connection, it is
recommended to use terminating resistors at the ends of the bus, even more when use large cable
length or high speed data rates. The terminating resistor is used to prevent an RF signal from being
reflected back from the end, causing interference. The terminating resistor must be in both ends of the
bus, connected in parallel (as shown in the image below). A typical value of this resistance is 120Ω,
0.5W. The value of the resistor must be the same in both ends. The terminating resistors are the
resistors ��
of the Image 3: terminating resistors.
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Image 2: terminating resistors
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