Modbus Communication
‑25‑
■ Recommended cable 1: Multi‑core twisted pair cables. Use one twisted pair
to connect RS485+ and RS485– and twist the remaining pairs into one to
connect CGND.
■ Recommended cable 2: Shielded twisted pair cables. Use the twisted pair
cable to connect RS485+ and RS485– and use the shield layer to connect
CGND.
■ In occasions where shielded cables are used, connect the shield layer to
CGND only. Never connect the shield layer to the ground.
● Terminal wiring for nodes without CGND
For nodes without CGND, do not connect CGND or the shield layer to the PE
terminal of the node directly. Do as follows:
■ Method 1: Check whether another port on this node shares a common
reference ground with the RS485 circuit. If yes, connect the CGND cable (the
shield layer) of the bus directly to the pin.
■ Method 2: Find the reference ground of the RS485 circuit on the PCB with the
node and connect the drain wire to CGND or the shield layer.
■ Method 3: If the reference ground of the RS485 circuit cannot be found, keep
the CGND cable or the shield layer unconnected and use an extra grounding
cable to connect this node to the PE terminal of other nodes.
Transmission Distance
The maximum number of nodes and transmission distance of a standard RS485
circuit vary with the transmission rate, as listed in the following table.
Table 3–3 Maximum number of nodes and transmission distance
Transmission
Distance (m)
Rate (kbps) Number of Nodes Cable Diameter
100 115.2 128 AWG26
1000 19.2 128 AWG26
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