Data Sheet

GNM3D and GNM3T Series Communication Protocol
6
1.3 Application notes
1.3.1 RS485 general considerations
1.
To avoid errors due to the signal reflections or line coupling, it is necessary to terminate the bus at
the beginning (master side, if not already embedded, by inserting a 120 ohm 1/2W 5% resistor
between line B and A) and at the end (in GNM3D and GNM3T series interface by connecting the
terminal A-with the terminal T in the last instrument.
2. The net
work termination is necessary even in case of point-to-point connection and/or of short
distances.
3. For connections longer than 1000m or if in the network there are more than 160 instruments (with
1/5 unit load as used in GNM3D and GNM3T series interface), a signal repeater is necessary.
4. For
bus connection it is suggested to use an AWG24 balanced pair cable and to add a third wire for
GND connection. If a shielded cable is used, connect the shield to GND.
5. The GND should be connected to ground only at the host side.
6. If an instrument does not answer within the “max answering time”, it is necessary to repeat the
query. If the instrument does not answer after 2 or 3 consecutive queries, it is to be considered as
not connected, faulty or reached with a wrong address. The same consideration is valid in case of
CRC errors or incomplete response frames.
1.3.2 MODBUS timing
Fig.
1 : 2-wire timing diagram
Timing characteristics of reading function:
ms
T response: Max answering time
500 ms
T response: Typical answering time
40 ms
T delay: Minimum time before a new query
3,5 char
T null: Max interruption time during the request frame
2,5 char