Modbus Communication
‑18‑
Figure 2‑2 Modbus communication interface
Transmission Distance
The maximum allowable number of nodes and transmission distance of a standard
RS485 circuit vary with transmission rates, as listed in the following table.
Table 2–1 Maximum number of nodes and transmission distance
Transmission
Distance (m)
Rate (kbps) Number of Nodes Cable Diameter
100 115.2 128 AWG 26
1000 19.2 128 AWG 26
Settings of Termination Resistor
Table 2–2 Settings of termination resistor
Port Definition
Pin Description
Pin
Network Name
1
DIP switch for enabling/
disabling RS485 termination
resistor
When pins 1 and 2 are switched
to ON, the RS485 termination
resistor is enabled. This RS485
termination resistor is disabled
by default.
2
2.3 Communication Transmission Mode
In an RS485 communication network, data is transmitted in asynchronous serial and
half‑duplex transmission mode. Data is sent frame by frame in the message format
specified by the Modbus‑RTU protocol. An idle time longer than 3.5‑byte transmission
time marks the end of the previous message.
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