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10.1.2 Communication Protocol
The communication protocol requires the Host to initiate all transactions. There are two basic categories of
message types:
Interrogate
, which is used to read data from an addressed AW400, and
Change
, which is
used to alter a value in an addressed AW400. The addressed instrument decodes the message and provides
an appropriate response. The protocol definitions for the message types are provided in the following table:
Message Field Definition
Symbol Title
Definition
SOH
Start of Header
This character, 7E, denotes the beginning of a message
I.A.
AW400 Address
The address of the AW400 responding to the transaction. It must be within
a range of 00-1F (00-31 decimal).
CMD
Command
Is the operation to be performed or a description of the message that
follows the Command-I.A. byte. The Command-I.A. byte has two fields: the
Command field (3 bits), and the I.A. field (5 bits). There are the following
commands type:
Interrogate
Change
Acknowledge
Response
The command descriptions are covered in the following section.
NUM
Number
The number of data bytes transferred or requested. The NUM must be in a
range of 00-32 decimal.
LO-ADD
Lower Address Bits
The least significant 8 bits of a 16 bit address in the AW400 database (*)
HI-ADD
Higher Address Bits The most significant 8 bits of a 16 bit address in the AW400 database (*)
DATA
An 8 bit data type
XXXX
Represents a variable number of data bytes
LRC Longitudinal
Redundancy
Character
Is a character written at the end of the message that represents the byte
content of the message and is checked to ensure data was not lost in
transmission. Is the sum of all bytes Modulo 256 of the message not
including the SOH character or its own bit setting (LRC)
(*) The addresses of the single variables to be accessed in the AW400 database are listed in the
following pages.
Summary of Contents for AW400 Series
Page 1: ...Instruction Manual IM AW4TX Issue 5 Chlorine Transmitter AW400 Series ...
Page 69: ...67 Notes ...
Page 70: ...68 ...
