Frick QUANTUM LX - The Query; The Response; Data Field; Error Checking

QUANTUM
™
LX EVAPORATOR CONTROL PANEL
COMMUNICATIONS SETUP
090.610-CS (MAY 2016)
Page 37
the Master know which Quantum™ is
responding.
• An 8-bit function code dening the re-
quested action (Query):
• Function Code 3 - to read holding reg-
isters (sends a request to read data
from the Quantum™).
• Function Code 6 to load a register (to
request to change a setpoint or to send
a command, or a function code 16 (RTU
only) to load multiple registers.
• Any data to be sent (Response). The
data eld is constructed using sets of
two hexadecimal digits, in the range
of 00 to FF hexadecimal. These are to
be made from a pair of ASCII charac-
ters (ASCII), or one 8-bit for RTU. The
data eld of messages sent from a
Master to the Quantum™ devices con-
tains additional information which the
Quantum™ must use to take the action
dened 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 eld. If no error occurs, the
data eld of a response from a Quan-
tum™ to a Master contains the data
requested. If an error occurs, the eld
contains an exception code that the
Master application can use to deter-
mine the next action to be taken.
• An error-checking eld.
The Query
The function code in the query tells the ad-
dressed Quantum™ what kind of action to per-
form. The data bytes contain any additional infor-
mation 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 eld
must contain the information telling the Quan-
tum™ which register to start at and how many
registers to read. The error check eld provides
a method for the Quantum™ to validate the in-
tegrity 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 con-
tain the data collected by the Quantum™, such
as register values or status. If an error occurs,
the function code is modied to indicate that the
response is an error response, and the data bytes
contain a code that describes the error. The er-
ror check eld allows the master to conrm that
the message contents are valid.
Data Field
The data eld is constructed using sets of two
hexadecimal digits, in the range of 00 to FF
hexadecimal. For ASCII, these can be made from
a pair of ASCII characters. For RTU, this is one
8-bit number.
The data eld of messages sent from a master
to the Quantum™ devices contains additional in-
formation which the Quantum™ must use to take
the action dened by the function code. This can
include items like discrete and register address-
es, the quantity of items to be handled, and the
count of actual data bytes in the eld.
For example, if the master requests a Quan-
tum™ to read a group of holding registers (func-
tion code 03), the data eld species the starting
register and how many registers are to be read.
If no error occurs, the data eld of a response
from a Quantum™ to a Master contains the data
requested. If an error occurs, the eld contains
an exception code that the Master application
can use to determine the next action to be taken.
Error Checking
ASCII
In ASCII mode, when data is transmitted to
and from the Quantum™ Controller, each
message has an Error Checking value ap-
pended to the end of the message. Longitu-
dinal Redundancy Check, or LRC, is used as
the method for verifying that the ASCII mes-
sage sent from the transmitting device was
properly received by the receiving device.
The Longitudinal Redundancy Check (LRC)
eld is one byte, containing an eight-bit bi-
nary 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 com-
plementing the result. It is performed on the
ASCII message eld contents excluding the
colon character that begins the message,
and excluding the CRLF pair at the end of the
message. The LRC is then appended to the
message as the last eld preceding the CRLF
(Carriage – Line Feed) characters. Each new
addition of a character that would result in
a value higher than 255 decimal simply rolls
over the eld’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 com-
pares the calculated value to the actual
value it received in the LRC eld. If the two
values are not equal, an error results.