Micromod MOD: 30ML Installation - Manual (Page 93)

MOD 30ML Multiloop Controller
COMMUNICATIONS CONNECTIONS
5.5.4 RS-485 4-Wire Modbus Communication
The built-in and modular circuits for RS-485 4-wire communication use a pair of transceivers
which support a 4-wire point-to-point or point-to-multipoint Modbus network. The modular
circuit is contained in a 2034N communications module. All field connection terminals are
isolated from the instrument circuitry.
Both the built-in and modular circuits support the Extended Modbus protocol which provides
full communications functionality between controllers, and between a controller and any PC
software.
! 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.
P C Master
The diagram in Figure 5-8 shows connections for a typical RS-485 4-wire Modbus network in
which a personal computer acts as the Modbus master and the controller is the slave. The
master is responsible for providing the bus stabilizing resistors. When using the modular
communications circuit, the module provides the slave function, and the TERM switch on the
module must be set at NO to disconnect the resistors inside the module. 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.
Controller Master
The diagram in Figure 5-9 shows connections for a typical RS-485 4-wire Modbus network in
which one controller acts as the master, and the other controllers on the network are slaves.
In this network, it is recommended that the modular communications circuit be used in the
master because the 2034N module provides the required bus stabilizing resistors. The
TERM switch on the master module must be set at YES to connect the resistors to the
network. The TERM switch on each slave module must be set at NO to disconnect the
resistors inside the module. Connect 120 ohm termination resistors across the transmission
line at both ends as shown. These resistors may not be required if the line length is very
short.
Communications Common
The built-in communications circuit has a communications common connection at terminal 1
to provide improved noise resistance. The modular circuits do not use a communications
common. When using the built-in circuit, connect a communications common line as follows:
 Connect to terminal 1 of all instruments communicating on the network via the built-in
circuit.
 When a personal computer or other host device is on the network, connect to the RS-485
interface board if a communications common terminal is available.
 Connect to ground at some point in the system.
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