0 modbus communications and connectivity, 1 implementation basics, 1 constraints – Emerson MONITORING OpenComms User Manual

Page 25: 2 transmission format, Odbus, Ommunications, Onnectivity, Implementation basics, Constraints, Transmission format

Modbus Communications and Connectivity

5.0

ODBUS

OMMUNICATIONS

AND

ONNECTIVITY

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.

5.1

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 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.

5.1.1 Constraints

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.

5.2

Transmission Format

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.