Emerson Autochangeover Controllers AC8 Manual de usuario descargar pdf (Pagina 9)

Revision B.33 2/25/2011

Liebert Monitoring Group – Development

Page 9 of 149

Modbus Communications

Connectivity to SiteLink modules using Modbus

This design specification describes the Modbus communications protocol as supported by the SiteLink module. It

includes information on how to pass information to and from the SiteLink module 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 SiteLink module is configured to act as a slave device on a common network. The common network can be

point-to-point over EIA-232, where one master communicates to one slave device or in a multi-drop configuration

over EIA-485, where multiple slaves reside on a common wire or loop.

Documented write points can cause read errors if you attempt to read a write-only point. For example: an LAM air

unit has separate Setpoint View and Setpoint Set registers and if you read a write-to register it can and most likely

will result in a Modbus exception response. Do not include setpoint set registers in batch read commands as one

point read error will result in an exception response for the entire batch read.

Transmission Format

The SiteLink module supports both Modbus RTU (Remote Terminal Unit) and ASCII (American Standard Code for

Information Interchange) transmission modes. The SiteLink module has a choice of transmission medium, baud

rate, character parity and number of stop bits. See chart below.

Physical

Port

Transmission

Mode

Baud Rate

Data Bits

Parity Bits

Stop Bits

Default

EIA-232 –

DB9 DCE

RTU

9600

None

Yes

EIA-232 –

DB9 DCE

ASCII

1200 –

38.4kbd

(19.2kbd omit)

Configurable

EIA-485 – 2

or 4 wire

RTU or ASCII

1200 –

38.4kbd

(19.2kbd omit)

Configurable

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