User manual

Modbus Communication and Connectivity

5.0 MODBUS COMMUNICATION AND CONNECTIVITY

This section describes the Modbus communication protocol as sup-

ported by the OpenComms NIC-485. It includes information on how to

pass information to and from the OpenComms NIC-485 via Modbus. It

is also intended to help facilitate answering questions regarding sup-

ported 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-

nication, 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-485 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.

5.2 Transmission Format

The OpenComms NIC-485 module supports Modbus RTU (Remote

Terminal Unit) transmission modes. The OpenComms NIC-485 com-

munication rate is fixed at 9600 baud, 8 data bits, No parity, 1 Stop

bit. The communication media is fixed using EIA-485 standards.

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-485 is set through the service terminal (configu-

ration) port. (See 3.0 - System Configuration for details.) The Open-

Comms NIC-485 supports multi-drop connection over RS485. On the

OpenComms NIC-485, a client should connect to the OpenComms

NIC584 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 NIC485 card is designed to reside on a maximum

common line of 32 devices with a total cable length of 4000 feet. In a

“normal environment” all devices should be connected in “daisy-chain”

fashion, with “star configurations” or branches, being used at a mini-

mum. See Figure 5 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.”