Appendix E Scada-Modbus ® System Guidelines - Hach Sigma 950 Instrument Manual

Appendix E SCADA-Modbus
E.1 Introduction to SCADA - Modbus Communications
E.2 ASCII Transmission Mode
START ADDRESS
1 Char ':'
E.3 Address Field
Use this section as a guide when using the Modbus ASCII protocol to communicate
directly with the 950 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, a description of key pieces of the protocol will be described.
This section will guide you 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 950 Flow Meter.
Modbus, an open protocol, determines how each instrument will know 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 (950). The 950 will always be considered the slave,
never a master. The master can address individual 950 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 950 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.
SCADA-Modbus
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System Guidelines
DATA
(HEX)
n Chars
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System Guidelines
LRC END
(HEX) (HEX)
2 Chars 2 Chars 'CRLF'
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