Emerson MONITORING OpenComms Manuel D’Utilisation

This section describes the Modbus communications protocol as sup-

ported by the OpenComms NIC card. It includes information on how 

to pass information to and from the OpenComms NIC card via Mod-

bus. It is also intended to help facilitate answering questions regard-

ing supported types, frame format, function code support etc.

Protocol controls the language structure or message format between 

devices in other words, the rules for communication. The rules for 

communication include how master and slave devices initiate commu-

nications, as well as unit identification, message handling and error 

checking. Modbus protocol simply refers to the control of the query 

and response cycles between master and slave devices.
The OpenComms NIC module is configured to act as a slave device on 

a common network. The common network is a multi-drop or “daisy 

chain” configuration over EIA-485, where multiple slaves reside on a 

common wire or loop.

The OpenComms NIC communicates with local database and there-

fore is subject to the time delay resulting from the local database 

updating policies. Use of “Preset Multiple Holding Registers” and 

“Preset a single Holding Register” functions requires accessing the 

Liebert device connected and to the OpenComms NIC card database 

service. Users should be aware that the Modbus application coexists 

with other on-board application servers (HTTP and SNMP specifi-

cally) and competes for the database service and CPU time. This may 

lead to uncertainty of responding time of the OpenComms NIC. The 

user or third party integrator may have to compensate for the delay 

time if HTTP, SNMP and Modbus are going to be utilized simulta-

neously.

The OpenComms NIC module supports Modbus RTU (Remote Termi-

nal Unit) transmission modes. The OpenComms NIC communication 

rate is fixed at 9600 baud, 8 data bits, No parity, 1 Stop bit. The com-

munication media is fixed using EIA-485 standards.