Siemens SENTRON PAC1020 Manual page 38

Connection
4.4 Connecting to the RS485 bus
Grounding the cable shield
The serial Modbus data line must be shielded. The shield must be connected to protective
ground at one end of the line at least. Strive to achieve grounding of the shield on both sides.
Grounding the common line
The common line must be applied direct to protective ground, preferably at only one point
for the whole bus. It must be ensured that the common signal is routed as a dedicated line.
Polarization
The PAC1020 does not support polarization of the RS485 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 PAC1020 does not support bus termination. The bus can be terminated using an external
resistor ≥ 120 Ω. Connect the resistor to terminals −/A and +/B of the RS485.
Figure 4-10
References
You can find further information in the following specification and the guidelines on the
website of the Modbus Organization (https://www.modbus.org).
36
Bus termination using external resistor
Equipment Manual, 05/2020, L1V30610008B-01
PAC1020