S90-510 CS (MAR 07)
FRICK
®
QUANTUM™ LX AcuAir
®
CONTROL PANEL
Page 32
COMMUNICATIONS SETUP
The Query
The function code in the query tells the
addressed Quantum™ what kind of action to
perform. The data bytes contain any additional
information that the Quantum™ will need to
perform the function. For example, function code
03 will query the Quantum™ to read holding
registers and respond with their contents. The
data field must contain the information telling the
Quantum™ which register to start at and how
many registers to read. The error check field
provides a method for the Quantum™ to validate
the integrity of the message contents.
The Response
If the Quantum™ makes a normal response, the
function code in the response is an echo of the
function code in the query. The data bytes
contain the data collected by the Quantum™,
such as register values or status. If an error
occurs, the function code is modified to indicate
that the response is an error response, and the
data bytes contain a code that describes the
error. The error check field allows the
leader
to
confirm that the message contents are valid.
Data Field
The data field is constructed using sets of two
hexadecimal digits, in the range of 00 to FF
hexadecimal. These can be made from a pair of
ASCII characters.
The data field of messages sent from a
leader
to
the Quantum™ devices contains additional
information which the Quantum™ must use to
take the action defined by the function code. This
can include items like discrete and register
addresses, the quantity of items to be handled,
and the count of actual data bytes in the field.
For example, if the
leader
requests a Quantum™
to read a group of holding registers (function
code 03), the data field specifies the starting
register and how many registers are to be read.
If no error occurs, the data field of a response
from a Quantum™ to a
Leader
contains the
data requested. If an error occurs, the field
contains an exception code that the
Leader
application can use to determine the next action
to be taken.
Error Checking
When data is transmitted to and from the
Quantum™ Controller, each message has an
Error Checking value appended to the end of the
message. Because the Quantum™ utilizes
MODBUS
®
ASCII protocol, Longitudinal
Redundancy Check, or LRC, is used as the
method for verifying that the message sent from
the transmitting device, was properly received by
the receiving device.
The Longitudinal Redundancy Check (LRC) field
is one byte, containing an eight-bit binary value.
The LRC value is calculated by the transmitting
device, by adding together successive eight-bit
bytes of the message, discarding any carries,
and then two's complementing the result. It is
performed on the ASCII message field contents
excluding the colon character that begins the
message, and excluding the CRLF pair at the
end of the message. The LRC is appended to the
message as the last field preceding the CRLF
(Carriage Return - Line Feed) characters. Each
new addition of a character that would result in a
value higher than 255 decimal simply rolls over
the field's value through zero. Because there is
no ninth bit, the carry is discarded automatically.
The receiving device recalculates an LRC during
receipt of the message, and compares the
calculated value to the actual value it received in
the LRC field. If the two values are not equal, an
error results.
ASCII Framing
In ASCII mode, messages start with a colon ( : )
character (3A hex), and end with a carriage
return-line feed (CRLF) pair (0D and 0A hex).
The allowable characters transmitted for all other
fields are hexadecimal 0 - 9, A - F.
All Quantum™ panels connected to the network
monitor the network bus continuously for the
colon character. When one is received, each
Quantum™ decodes the next field (the address
field) to find out if it is the addressed device.
A MODBUS
®
message is placed by the
transmitting device into a frame that has a known
beginning and ending point. This allows receiving
devices to begin at the start of the message, read
the address portion and determine which device
is addressed, and to know when the message is
completed. Partial messages can be detected
and errors can be set as a result.
A typical message frame as sent by the
Leader
is shown below.
START ADDRESS FUNCTION
DATA LRC
CHECK
END
:
01
03
7D000001
7E
CRLF
1 CHAR
2 CHAR
2 CHAR
8 CHAR
2 CHAR
2 CHAR
Where
:
= Start of Message
01
= Quantum™ ID
03
= Read Function
7D
= H.O. address (hex)
00
= L.O. address (hex)
00
= H.O. # of Data Registers
01
= L.O. # of Data Registers
7E
= Error Correction Code
CRLF = Carriage Return – Line Feed