ABB EQmatic, Product Manual

Page: 103 / 107

ABB EQmatic
Planning and application
102 2CDC512090D0202 | QA/S x.yy.1
Cables and lines
Cable types
A twisted pair, screened cable is recommended as the bus cable. The cable type J-Y(St)Y n x 2 x 0.8
is suitable, for instance. The screen is to be connected to PE at one end.
The bus cable must be terminated with resistors (120 Ω, 0.25 W) at both ends so that only minimal
reflections are produced. The serial communication on the RS485 interface operates most efficiently
if the source and load impedance are matched at 120 Ohm. The EOL resistors are connected in
parallel with terminals A and B and are included in the scope of delivery.
Cable length
The RS-485 specification limits the cable length to 1200 m, the number of devices in the bus to 32
and stipulates a linear topology (daisy chain).
Number of devices
The number of Modbus devices depends on the unit load (UL) of the RS-485 transceivers.
In the worst case a transceiver has 1 UL. An RS485 segment is specified for 32 UL. If more devices
are to be connected, a repeater must be used.
Modern RS485 transceivers have 1/4 or 1/8 UL. If only such devices are used, 128 or 256 users are
possible without repeaters.
Note
The ABB A and B series energy meters have a unit load of 1/8 UL. Therefore 64 ABB meters can be
connected to the QA/S 4.64.1 Energy Analyzer without repeaters.
Caution
Modbus RTU devices must be installed and put into operation in accordance with the applicable
standards and regulations, and this task is only allowed to be performed by trained specialists (e.g.
electricians).