ABB CSU-2LV, Installation And Maintenance Manual, Page: 41 / 84

ABB
ARC GUARD SYSTEM™ – CSU-2LV/2MV INSTALLATION AND MAINTENANCE GUIDE 41
6.4.4 Termination
For the best quality of data transfer, Modbus should be
terminated correctly.
The following figure shows the Modbus installation:
6.4.4.1
Termination resistors
A 120Ω resistor is added in parallel with the CSU-2 B+ and
A- terminals if it is installed as the first or last device on
the network. For this purpose, there are double terminals
for +(B) and -(A) connections.
6.4.4.2
Pull-up / pull-down resistors
When the Modbus network is not being actively driven
by a device, the bus line is in an undefined state. Bias
resistors should then be used to obtain a defined voltage
potential on the data lines. The bias resistors act for data
line B (D1) as pull-up resistors against 5V and for data line
A (D0) as pull-down resistors against GND.
The following devices usually have built-in bias resistors:
• Modbus masters
• Gateways
• Repeaters
Figure 29
Modbus termination
1SFC170020M0201
Slave 1
D
R
Master
D
R
Slave n
D
R
LT LT Balanced Pair
D1
5 V
Pull Up
Pull Down
D0
Common
06