Communication Specification - Delta ASDA-B3 Series User Manual

Modbus Communication
9.5 RS-485 communication specification
Compared with RS-232, the RS-485 communication can carry out one-to-many transmission
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and has better anti-interference ability. RS-485 uses a balanced transmission line for signal
reception and transmission. The transmitter converts the TTL signal into a differential signal and
then sends it to the receiver. The receiver receives the differential signal and then converts it
back to the TTL signal. Since the transmission process uses the differential signal, it has better
anti-interference ability. However, there are still restrictions on its use, so note the following
when wiring.
 Number of stations
CN3 can only support up to 32 servo drives. If your application requires more than
32 stations, install a repeater to connect more servo drives. The current maximum is
127 stations.
 Transmission distance
The longer the transmission distance, the slower the transmission speed. The cable length
can be up to 100 meters when the servo drive is installed in a quiet environment. If the
required transmission speed is over 38,400 bps, a 15-meter cable is recommended to
ensure data transmission accuracy.
 Transmission line
The quality of the transmission line affects the signal transmission process. If there is
interference during the transmission process, it may result in data loss. It is suggested that
you use a shielded twisted-pair cable as it has metal shielded cover and a grounding wire,
which ensures better anti-interference ability.
 Topology
For topology, the closer to the master station, the more stable the transmitted signal.
RS-485 supports bus topology. The transmission line must connect from the first station to
the second station, and then from the second station to the third station, and so on until
the last station. RS-485 does not support star and ring topologies.
 Terminal resistor
In the communication transmission process, if the impedance is not continuous, it causes
signal reflection and signal distortion. This usually happens to the device that is configured
at the end of the transmission line. If the impedance is small or even 0
reflected. To solve this problem, add a resistor of the same characteristic impedance as
the cable at the end of the cable, which is called a terminal resistor.
transmission line used in the RS-485 signal transmission circuit is a twisted-pair cable, and
its characteristic impedance is about 120
also
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120 Ω.
Ω, so the impedance of the terminal resistor is
ASDA-B3
Ω, the signal will be
In general, the