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10-2 ADS Manual
Establishing a Modbus Connection
Note: Modbus connection is only possible when using a
public, static IP address or the customer’s interna l, private
network. Modbus communication is not
supported through ADS VPN.
The SCADA or process control system connects to the
either through a customer-supplied serial cable connection using the
ADS ExPAC (which may be housed within the ADS XBUS ™ or a
customer-supplied enclosure) or through a cellular wireless modem
in the monitor using Modbus TCP.
Note: The XIO does not support Modbus
communications. Only the ExPAC and XBUS support
Modbus.
Setting Up a Serial Connection
Two Modbus ports are available on the ExPAC through which to
communicate: RS-232 and RS-485.
Note: When both connections are available, ADS
recommends using the RS-485 connection. It has a greater
immunity to noise, supports longer cable runs without
signal degradation, and is easier to wire.
The required port settings are 19200 baud rate, 8 data bits, no
parity, 1 stop bit, and no flow control.
Connecting through the RS-485 Port
When using an RS-485 connection for Modbus communications,
choose a cable that is no longer than 4000 feet (1219 m) in length
with 24-gauge wiring. The cable must include at least one twisted
pair and one ground wire. If the cable extends beyond 1000 feet
(305 m), ADS recommends using 120 ohm termination resistors at
each end of the cable.