Micromod MOD: 30ML Installation - Manual (Page 91)

MOD 30ML Multiloop Controller
COMMUNICATIONS CONNECTIONS
5.5.3 RS-485 2-Wire Modbus Communication
The built-in and modular circuits for RS-485 2-wire communication use a transceiver which
supports a 2-wire point-to-point or point-to-multipoint Modbus network. The modular circuit is
contained in a 2032N communications module. All field connection terminals are isolated
from the instrument circuitry.
! CAUTION: If the modular communication option is used, be sure the module is
wired properly before applying power. Although this module is isolated,
it can be damaged if excessive voltage is applied across the input pins.
This module is not a drop-in replacement for the ICN module (2030N),
and will be damaged if ICN level voltages are applied to the input pins.
Connections for a typical RS-485 2-wire Modbus network are shown in Figure 5-6. In this
network, the personal computer acts as the Modbus master and the controller is the slave.
The master is responsible for providing the bus stabilizing pull-up and pull-down resistors
which keep the bus in a MARK/IDLE state when all the transmitters are tri-stated. Connect
120 ohm termination resistors across the transmission line at both ends as shown. The
termination resistors may not be required if the line length is very short.
The built-in communications circuit provides a communications common connection at
terminal 1 to provide improved noise resistance. The modular circuit does not use a
communications common. When using the built-in circuit, connect the communications
common as follows:
 Connect to terminal 1 of all instruments communicating on the network via the built-in
circuit.
 Connect to the RS-485 interface board in the personal computer if a communications
common terminal is available.
 Connect to ground at some point in the system.
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