Micromod MOD: MODCELL Multiloop Processor 2002N Model C and Associated Hardware - Manual (Page 61)

MODCELL Multiloop Processor
COMMUNICATIONS CONNECTIONS
4.2.4 2034N RS-485 4-Wire Communication for Modbus
The 2034N RS-485 Communication module, Figure 4-5, is a pair of 75LBC176D
transceivers that can be used for a 4-wire point-to-point or point-to-multipoint Modbus
network. Its field connection terminals are optically isolated from the instrument circuitry.
This module supports the Extended Modbus protocol which provides full communications
functionality between Modcell instruments, and between the instruments and the
Application Builder software.
! 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 4-6. In this network, the personal computer acts as the Modbus master. The
master is responsible for providing the bus stabilizing resistors. 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. 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 4-7. In this network, one module acts as the master and all other modules on the
network function as slaves. The TERM switch on the master must be set at YES to
connect the bus stabilizing 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.
When several slaves on a network require only read and write capability, a mixture of 2-
wire and 4-wire modules can be used to save slot space and transmission cable as shown
in Figure 4-8. This arrangement allows a series of 2-wire slaves (2032N modules) to be
connected to the 4-wire master (2034N). The two-slot master provides the required bus
stabilizing resistors, and the one-slot slaves are adequate for the read/write function.
The address switches on the 2034N module permit assignment of an address to each
module on the network. Addresses 1 through 247 ($01 through $F7 in hexadecimal) are
supported by the Modbus protocol.
The COMM DEFAULTS switch provides for communication with the module when its
configuration is unknown. When the switch is set at YES, a set of default parameters is
invoked. The parameters are: 9600 Baud, no parity, one stop bit, eight data bits, and the
port has slave functionality.
5 5