Appendix D Scada-Modbus® System Guidelines - Hach 911 User Manual

Appendix D SCADA-Modbus
D.1 Introduction to SCADA - Modbus Communications
D.2 ASCII Transmission Mode
START ADDRESS
1 Char ':'
D.3 Address Field
Use this section as a guide when using the Modbus ASCII protocol to communicate
directly with the 940 Flow Meter via an RS232 or modem connection.
This guide assumes that the user has a working knowledge of Supervisory Control and
Data Acquisition (SCADA), its components, and the different topologies used to construct
the communications network. Because a basic understanding of the Modbus ASCII
protocol is necessary, the key pieces of the protocol will be described.
This section guides the users through the setup process by describing key points that
need to be addressed for successful communication. This section will not outline specific
implementation details of any particular Man Machine Interface (MMI) or controller,
although examples may reference certain manufacturers for illustrative purposes. The
description of the Modbus ASCII protocol is provided for reference only and is not
intended as a tutorial. The scope of this section is limited to the description of Modbus
ASCII as it pertains to the 940 Flow Meter.
Modbus, an open protocol, determines how each instrument knows its device address,
recognize a message addressed to it, determine the type of action to be taken, and extract
any data or other information contained in the message. The flow meter and Man Machine
Interface (MMI) communicate using a master-slave technique in which only the master
can initiate queries to a slave (940). The 940 is always considered the slave, never a
master. The master can address individual 940 Flow Meters or can broadcast a message
to instruments within its scope. Responses are never returned to broadcast queries from
the master. The Modbus protocol establishes the format for the master's query by placing
into it the device address, a function code defining the requested action, any data to be
sent, and an error-checking field. The flow meter's response message is constructed
using the Modbus format which confirms the action to be taken, any data to be returned,
and an error checking field.
The 940 Flow Meter is designed to communicate on standard Modbus networks using
Modbus ASCII. In ASCII mode, messages start with a colon ':', and end with a 'carriage
return-line feed' pair. The allowable characters transmitted for all fields are hexadecimal
0–9, and A–F. When a message is transmitted over a Modbus ASCII communication link,
each character or byte is sent in the order of Least Significant Bit to Most Significant Bit. A
typical message frame looks like the following:
FUNCTION
(HEX) (HEX)
2 Chars 2 Chars
The address field of an ASCII message frame, ranging from 0 to 247 decimals, consists of
two characters that represent the slave address. Individual slaves are assigned values
between 1 and 247. A master addresses a slave by putting the slave's address in the
address field of the message frame. When the slave sends its response, it places its own
address in the address field of the message frame to let the master know which slave is
responding.
®
System Guidelines
DATA
(HEX)
n Chars
LRC END
(HEX) (HEX)
2 Chars 2 Chars 'CRLF'
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