Modbusappendix A - Modbus Implementation; Introduction; Implementation; Configuration - Varec 2920 Installation And Operation Manual

MODBUSAppendix A — MODBUS Implementa-
tion

Introduction

Implementation

Configuration

Varec, Inc.
This protocol guide explains the operation of the MODBUS as it pertains to the 2920 Float &
Tape Transmitter.
The implementation of the MODBUS protocol for the 2920 FTT provides a standard form of
digital communications. An effort has been made to parallel current implementations to the
greatest extent possible, so that the 2920 FTT communicates with existing MODBUS masters.
Check compatibility carefully to ensure that the 2920 FTT is properly configured for the data
format expected by the host computer. Exceptions made because of the unique requirements
of the 2920 FTT application have been noted. This is no guarantee, however, that the
interpretation made here will be the same as that followed by the MODBUS master.
The 2920 FTT implementation of the MODBUS protocol provides for the passing of measured
and calculated variables, configuration information, and diagnostics in data registers. Data is
sent in these registers as floating-point values, integer values, numeric codes related to
configuration lists, status summary words (packed bits), or individual status flags (single bits).
One master and up to 31 2920 FTTs may be multi-dropped on a single EIA485 communication
bus.
The MODBUS functions implemented in the 2920 FTT are listed in the table below.
The 2920 FTT address provides a unique identification for the host. The 2920 FTT address is set
using the configuration interface explained in "Basic Setup" on page 58. This address may
range from 1 to 254 and must be unique. Each 2920 FTT only responds when a query has been
sent to its unique address by the host.
Advanced MODBUS settings are configured in the "MODBUS Data" on page 81. The 2920 FTT
only supports the MODBUS RTU mode of communications.
MODBUSAppendix A — MODBUS Implementation
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