Communication with Modbus
2CMC48001M0201 99 A43/A44
Revision: C User Manual
9.1 Bus Description
Modbus communication in the A series meters is done on a 3-wire (A, B and
Common) polarity dependent bus according to the RS-485 standard. Maximum
number of meters connected to one physical bus is 247 (which is the same as the
individual device address range in Modbus).
The RS-485 bus uses line topology, see figure below. Stubs at the meter connec-
tions are allowed but should be kept as short as possible and no longer than 1 m.
Bus termination in both ends of the line should be used. The resistors should have
the same values as the characteristic impedance of the cable which normally is
Cable used is non shielded or shielded twisted pair cable with wire area of 0.35-
1.5 mm
2
. If shielded cable is used the shield should be connected to ground in one
end. Maximum length of the bus is 700 m.
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