Delta ASDA-A3 Series, Руководство пользователя

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MODBUS Communication ASDA-A3
9-16
9
9.5 RS-485 communication specification
Compared with RS-232, the RS-485 communication can carry out one-to-many transmission
and has better anti-interference ability. RS-485 mainly uses a balanced transmission line for
signal reception and transmission. After the transmitter converts the TTL signal into a differential
signal, 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 drive stations. 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 rate. 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 to
use a shielded twisted-pair cable as the outer layer has a shield layer of metal and
grounding wire, which has better anti-interference ability.
 Bus topology
For topology, the closer to the master station, the more stable the transmitted signal.
The bus structure of RS-485 is the 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 0Ω, the signal will be
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. In general, the
transmission line used in the RS-485 signal transmission circuit is a twisted-pair cable,
and its characteristic impedance is about 120Ω, so the impedance of the terminal resistor
is also 120Ω.