Wiring
17
4.2.6 Additional information on Modbus EIA485
Connection
As described in the “Modbus over serial line specification and implementation guide V1.02”
published by the Modbus-IDA organisation (www.modbus.org) and based upon the EIA/TIA-485-
A physical layer specification, Modbus two-wire serial requires the following four electrical
connections between each of the devices on the bus.:
Additional bus settings
• Bus biasing resistors (must be present at one point on the bus) (always enabled inside NXA820)
• Bus termination resistor (must be present at each end of the bus) (software selectable inside
NXA820)
Cable Specification
Terminal Signal Purpose Remarks
10 A Data signal (-) These signals must be connected using a balanced twisted pair cable.
11 B Data signal (+)
12 C Signal Common Must interconnect all devices on the bus.
13 Shield EMC Protection Copper braided or combined foil and braided shielding.
Characteristic impedance 135 to 165 Ω at measuring frequency of 3 to 20 MHz
Cable capacitance ≤ 30 pF/m
Core cross-section ≥ 0.34 mm
2
, corresponds to AWG 22, multi-strand cable is preferred
Cable type Single twisted pair + third conductor (for common)
or
Dual twisted pair (common uses second pair with wire joined together)
Cable resistance ≤ 110 Ω/km
Signal damping Max. 9 dB over the entire length of the cable cross-section
Shielding Copper braided shielding or combined foil and braided shielding
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