Emerson MONITORING OpenComms Manuel D’Utilisation

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

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