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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.
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