Micromod MOD: MODCELL Multiloop Processor 2002N Model C and Associated Hardware - Manual (Page 58)

MODCELL Multiloop Processor
COMMUNICATIONS CONNECTIONS
4.2.3 2032N RS-485 2-Wire Communication for Modbus
The 2032N, Model C RS-485 Communication module is a 75LBC176D transceiver that
can be used for a 2-wire point-to-point or point-to-multipoint Modbus network. Its field
connection terminals are optically isolated from the instrument circuitry.
! CAUTION Be sure this 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 Modbus network using this module as a slave are shown in
Figure 4-3. In this network, the personal computer acts as the Modbus master. 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.
Connections for a typical Modbus network using this module as the master are shown in
Figure 4-4. In this network, one module acts as the master and all other modules on the
network function as slaves. Since the master module does not contain the required bus
stabilizing resistors, the resistors and a 5-volt power supply must be connected across the
transmission line as shown in Figure 4-4. 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.
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