Modbus Communication
‑22‑
Table 3–2 Jumper on the MD38TX1 expansion card
Terminal
ID
Terminal Name
Function
Jumper/DIP Switch
Position
J3 RS485 communication
terminal resistor setting
jumper
Connect the terminal
resistor.
Disconnect the terminal
resistor.
The jumper setting is based on the top view of the expansion card with the main wir‑
ing terminal as the bottom side. For the position of the jumper, see the PCB
silkscreen.
Communication Networking
1. RS485 topology
The following figure shows the RS485 bus topology. You are advised to use shielded
twisted pairs for the RS485 bus and use twisted pair cables to connect RS485+ and
RS485–. A 120 Ω terminal matching resistor is connected at both ends of the bus to
prevent signal reflection. The reference grounds of RS485 signals on all nodes are
connected together. A maximum of 128 nodes are supported and the distance
between each node and the bus must be less than 3 m.
Figure 3‑2 RS485 bus topology
2. Multi‑node connection mode
When the number of nodes is large, the RS485 bus must be daisy‑chained. If branch
connection is required, it is recommended that the length of the cable between the
bus and the node do not exceed 3 m. The shorter, the better. Never use the star
topology. The following figure shows the frequently used bus structure.
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