31
adr
1
seC
0
Hour
00:00:00
reC
OFF
gor_r
00:00:00
date
70:01:01
:nte
00:15:00
NA5 programmable digital meters have a serial link of RS-485 standard to
communicate in computer systems and with other devices fulfilling the master
function. The MODBUS asynchronous character communication protocol has been
implemented on the serial link. The transmission protocol describes information
exchange procedures between devices through the serial link.
6.1. Procedure of the serial interface connection
The RS-485 standard enables the direct connection to 32 devices on a single serial
link up to a 1200 m distance. For the connection of a higher number of devices it is
necessary to apply additional intermediate-to-separating systems.
The exit of the interface line is presented in the service manual on the fig. 3.d. In
order to obtain a correct transmission it is necessary to connect lines
A
and
B
in
parallel to their equivalent lines in other devices.
The connection must be made with a shielded conductor. The shield must be con-
nected to the protective terminal in one point.
The
GND
line serves to the additional protection of the interface line at long distance
connections.
One must connect GND signals between devices and in one point to the protective
terminal (that is not necessary for the interface correct operation).
To obtain the connection with the computer of IBM PC class, an RS-232 into
RS-485 converter is necessary or an RS-485 interface card. The way of NA5
meter connection through the PD5 converter is shown on the fig. 3d.
The designation of transmission lines for the card in the PC computer depends on
the card producer.
6.2. Description of the MODBUS protocol implementation
The implemented protocol is compatible with the PI-MBUS-300 Rev G Modicon