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Modbus Communications and Connectivity
20
5.3 Physical Connection
A Modbus network should consist of one, and only one, host client, and
up to 255 slaves uniquely identifiable by their slave ID. The slave ID
of each OpenComms NIC is set through the service terminal (configu-
ration) port. (See section 3, System Configuration for more details.)
The OpenComms NIC supports multi-drop connection over RS485. On
the OpenComms NIC, a client should connect to the OpenComms NIC
as a multi-drop connection via the RS485 “Repeater” port (TB2), (Pin1
as minus and Pin2 as plus. J28-1 and J28-2 should be shorted.)
The OpenComms NIC card is designed to reside on a maximum com-
mon line of 32 devices with a total cable length of 4000 feet. In a “nor-
mal environment” all devices should be connected in “daisy-chain”
fashion, with “star configurations” or branches, being used at a mini-
mum. See Figure 6 for details. A “normal environment” consists of an
installation where noise or electrical interference is minimal. This is
typical of communication wiring run in properly grounded conduit or
in areas where high voltage or radio transmissions are not present.
Otherwise, these conditions are designated as “noisy.”
Figure 6 Typical maximum installation
The recommended cable characteristics are:
Normal Environments:
• 18-22 gauge, twisted-pair, stranded (non- shielded)
Typical type is: Belden 8442
Noisy Environments:
• 18-22 gauge, twisted-pair, stranded (shielded)
Typical type is: Belden 9461