Figure 6-13 Bus Termination Using External Resistor - Siemens SENTRON PAC3100 Manual

Connection
6.5 Connecting to the RS 485 bus
Grounding of the cable shielding
The serial Modbus data line must be shielded. The shielding must be connected to protective
ground at one end of the cable. The shielding is grounded at one end only.
Grounding of the common line
The common line must be applied direct to protective ground, preferably at only one point for
the whole bus.
Polarization
The PAC3100 does not support polarization of the RS 485 data lines. Polarization must be
implemented at another point on the bus. The master device usually performs the
polarization.
We recommend polarization with supply of 5 V DC, pull-up resistor with 560 Ω, pull-down
resistor with 560 Ω.
Bus terminator
The first and last node in the bus segment must terminate the bus with a terminating resistor.
The PAC3100 does not support bus termination. The bus can be terminated using an
external resistor ≥ 60 Ω. The resistor must be connected to terminals -/A and -/B of the
RS 485 terminal block.
Figure 6-13
50
Bus termination using external resistor
SENTRON PAC3100
Manual, 07/2009, A5E02385159B-01