Wiring
24
4.4.6 Host connection: Modbus Serial, EIA/TIA-485 (RS485)
Two-Wire Modbus Definition
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 EIA/TIA-485 bus settings
• Bus biasing resistors (must be present a one point on the bus) (software selectable inside NXA822)
• Bus termination resistor (must be present a each end of the bus) (software selectable inside
NXA822)
Cable Specification
4.4.7 Host connection: Modbus TCP/IP (via System LAN port)
L00-NXA82xxx-04-00-00-yy-024
Terminal EIA/TIA-485
Modbus
Meaning Remarks
18 C Signal Common
Connector: Phoenix FKC 2,5HC/3-St-5,0817 B + signal
16 A - signal
Signal Purpose Remarks
A Data signal (-) These signals must be connected using a balanced
twisted pair cable.
B Data signal (+)
C Signal Common Must interconnect all devices on the bus.
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
Host
TCP/IP Client
Modbus
Master
NXA822
TCP/IP Server
Modbus
Slave
NXA820
Tank
Data 1.15
TCP/IP Connection
Port: #500
Modbus Request
Modbus Reply
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