Liebert OpenComms, Installation & User Manual

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28 NIC Setup and Testing
Section 5: MODBUS COMMUNICATIONS &
CONNECTIVITY
This section describes the Modbus communications protocol as
supported by the OpenComms NIC card. It includes information
on how to pass information to and from the OpenComms NIC
card via Modbus. It is also intended to help facilitate answering
questions regarding supported types, frame format, function code
support etc.
IMPLEMENTATION BASICS
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 communications, 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.
CONSTRAINTS
The OpenComms NIC communicates with local database and
therefore 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 specifically) 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