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N31U User manual
5.5 RS-485 interface
The N31U meters are equipped as a standard with one RS-485 port connec-
ted to the terminals of the lower connector. The interface is galvanically sepa-
rated from the other circuits of the meter.
The implemented data exchange protocol is compatible with MODBUS RTU
standard and allows to save and read all conguration parameters as well
as read all measurement data with data including alarm status or other pa-
rameters related to the meter status. The meter works in the network as a
slave device.
Standard RS-485 allows a direct connection up to 32 devices on a single
serial link. The maximum permissible cable length depends on a baud rate,
and it is 1200 m for the baud rate 9600 b/s. It is necessary to use additional
intermediate-separation circuits e.g. PD51 by LUMEL S.A. to connect more
devices or to use a longer connections.
5.5.1 Connection
The terminals A, B and GNDI terminals which location is shown in Fig. 5 allow
to connect the RS-485 interface to the N31U meter. It is required to connect
the lines A and B in parallel with their equivalents in other devices to obtain the
correct transmission.
The connection should be made using twisted pair screened cable in such a
way that the A and B lines should be one pair and are connected with their
equivalents of other devices in the network. The cable shield should be con-
nected to the protective terminal in close proximity to the N31U meter. The
cable shield of the interface cable should be connected to the protective ter-
minal only in one point.
The GNDI line, which is the reference potential for the RS-485 interface, is
used for additional protection of the interface line at long connections. Then all
GNDI lines of all devices using the same bus should be connected together.
A star connection should be avoided when connecting the devices. The con-
nection should have a bus layout which ends are connected to the termina-
tion resistors.
Method of connecting the devices is shown in Fig. 13.
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