Micromod MOD: MODCELL Multiloop Processor 2002N Model C and Associated Hardware - Manual (Page 55)

MODCELL Multiloop Processor
COMMUNICATIONS CONNECTIONS
4.2 MODBUS NETWORK CONNECTIONS
4.2.1 General
Numerous Modbus network connection arrangements are possible. Selection of a specific
arrangement depends on the requirements of the application. The connection diagrams
shown in this section provide typical examples of connection schemes which meet all the
functional requirements of the Modbus protocol and the communication modules.
The functionality of each communication module is configurable via its associated MSC
block. See IB-23G600, Section 3.26 for additional information.
Master and Slave Designations
Each communication module can function as either a Modbus master or Modbus slave.
This functionality is determined by the configuration of the MSC block.
Communications Parameters
The baud rates available are: 150, 300, 600, 1200, 2400, 4800, 9600, 19200 or 38400.
Parity can be none, even or odd, and there can be either 1 or 2 stop bits. These
parameters are configurable via the MSC block.
The transmission mode of Modbus networks using Modcell communication modules is
RTU (Remote Terminal Unit).
RS-485 Network Considerations
The RS-485 specification allows as many as 32 devices on any given network. the
number of devices can be increased by the use of repeaters. The Modbus network
supports as many as 247 slave devices.
Since the RS-485 modules (2032N and 2034N) are isolated, it is not necessary to run a
separate ground or circuit common line between the modules. If non-isolated devices are
used in conjunction with the isolated modules, it is necessary to connect grounds between
the the non-isolated units. The isolated units still do not require ground connections in this
application.
RS-485 Cable Requirements
For short runs of 10 to 25 ft (3 to 6m), virtually any 2-wire shielded or twisted pair is
suitable. For runs up to 1000 ft (305 m), Belden 9502 Cable or an equivalent cable is
recommended. This cable is a dual 24 AWG (0.5 mm) twisted pair with an overall foil
shield. A drain wire is provided for grounding the shield. For runs up to 4000 ft
(1219.2 m), Belden 9729 Cable or an equivalent cable is recommended. This cable is a
dual 24 AWG (0.5 mm) twisted pair with a foil shield for each pair. The cable insulation is
low dissipation (polypropylene). Two separate drain wires are provided for grounding the
shields
NOTE: Heavy braid shield cable may be required for certain noisy environments.
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