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Draft, ne pas diffuser Wed Dec 05 2018 12:41:51 GMT+0100 (Romance Standard Time)
2.4
Termination
For best quality of data transfer, Modbus should be
terminated correctly.
The following figure shows Modbus installation according
to [3].
2.4.1
Termination resistors
A 120Ω resistor is added in parallel with TVOC-2 System
connection if it is installed as first or last device on the
network. For this purpose there are double terminals for
+(B) and -(A) connections.
2.4.2
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. This is
illustrated in Figure 20 in [2].
The following devices usually have built-in bias resistors:
•
Modbus masters
•
Gateways
•
Repeaters
ABB
ARC GUARD SYSTEM™ – T VOC-2- COM MODBUS CONFIGUR ATION MANUAL
Figure 3
General 2-Wire Topology
13
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