Micromod MOD: Modcell 2050R MODBUS Communications - Manual (Page 7)

IB-23C650M
INSTALLATION
INSTALLATION
This section provides instructions for making Modbus network connections
using either a 2-wire or 4-wire configuration. This information assumes that
physical installation and all other electrical connections are being made in
accordance with the instructions in IB-23C650.
Before making any connections, be sure the controller can support Modbus
communications; see the option requirements below.
Controller Option Requirements
The controller must be equipped with an option card and firmware Version 2 to
run MODBUS RS-485 serial communications. The option card (Catalog No.
2050NZ10100A) and Version 2 firmware may have already been installed in
the controller during manufacture. If necessary these items can be installed in
the field. Instruments manufactured with the option card and Version 2
firmware are identified by digits in the instrument catalog number as shown
below:
Sample Catalog Number: 2050RZ10 1 02 A
RS-485 Communications Option Card
RS-485 Firmware Version
2-Wire Modbus Network Connections
Connections for a typical Modbus network using a 2-wire configuration are
shown in Figure 1. 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 devices and host must have a
common ground.
The master is responsible for providing the 560 ohm pull-up and pull-down
bus stabilizing resistors. Connect 120 ohm termination resistors across the
transmission line at both ends as shown. The number of devices can be
increased by the use of repeaters. 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 4000 ft (1219m) Belden 9841 cable or equivalent is recommended. This
cable is a 24 AWG twisted pair with a foil shield. The insulation is low
dissipation (polypropylene). A drain wire is provided for grounding the shield.
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