H
oogkamerstraat 188, 9140 Temse Oos-Vlaanderen Belgium +32 470 80 80 83
8
Maximum distances and number of slaves
The maximum length of the communication cable should be no longer than 800m [recommended].The
maximum number of devices that can be connected over this network can be 32[for Modbus RTU],
including the master device [The PC in our case]
*apart from the network, No. of slaves also depend on the data logger specification.
Terminal Resistances
In order to avoid signal reflections, it is required to turn on the 120 ohm terminal resistance at ends
of the communication cable. In our case, we are required to turn ON the terminal resister of the last
device. In some cases, it is also required to turn ON the terminal resister of the first device.
Note: The terminal resistor is assembled on the PCB and you can turn it on/off (Enable/ Disable) if
the condition of your network falls in above situation. To activate the terminal resistor please
follow the
for modifying menu on the device or use
Grounding
For MODBUS communications, properly grounding your instrument is important, especially when
working close to high voltage sources such as PV arrays. An instrument that is not appropriately
grounded can give false readings, fail completely or turn into a safety hazard
QUICK TIP-
If you are encountering noise or irregular or abnormal problems with a Modbus serial
link, the problem is likely related to grounding, incorrect shielding, or wiring power wires next to
Modbus wiring.
Type of cables to use
For MODBUS communications, a shielded and twisted pair cable is used. One example of such cable
is
Belden 3105A
. But any cable with similar characteristics can be used to connect all the devices
together. The shielding can be of 2 types: braided [like a mesh of thin conducting wires] or like a foil
[consiting of a thin sheet of metal covering the twisted wires]. Both types of shielding are OK.
