Micromod MOD: Modcell 2050R MODBUS Communications - Manual (Page 9)

IB-23C650M
INSTALLATION
4-Wire Modbus Network Connections
Connections for typical Modbus networks using a 4-wire configuration are
shown in Figures 2 and 3. The host device functions as the master and the
controllers function as slaves. It is recommended that no more than 32 devices
be connected on a single network. The number of devices can be increased
by the use of repeaters.
When the host is a device such as a personal computer, the instruments and
host must have a common ground as shown in Figure 2. When the host is a
MODCELL Multiloop Processor, Figure 3, connection of the processor to the
instrument common line is not required because the processor connections
are optically isolated.
The master is responsible for providing the 560 ohm pull-up and pull-down
bus stabilizing resistors. In the MODCELL processor, these resistors are
provided in the RS-485 communications module, and the TERM switch on the
module must be set at YES to connect the resistors to the network (see
IB-23C600 MODCELL Multiloop Processor Installation Instructions for
more information). 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.
Cable requirements depend on the length of the run. 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 (305m), Belden 9502 cable or equivalent is recommended. This
cable is a dual 24 AWG twisted pair with an overall foil shield. A drain wire is
provided for grounding the shield. For runs up to 4000 ft (1219m) Belden
9729 or equivalent is recommended. This cable is a dual 24 AWG 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.
5