Johnson Controls Frick QUANTUM LX, Setup

Page: 47 / 140

QUANTUM™ LX COMPRESSOR CONTROL PANEL
COMMUNICATIONS SETUP
090.020-CS (APRIL 13)
Page 47
ID #). The address fi eld of a message frame
contains two characters for ASCII, or 8 bits
for RTU. Valid Quantum™ device addresses
are in the range of 01 – 99 decimal. A
leader
addresses a Quantum™ by placing
the Quantum™ address in the address fi eld
of the message. When the Quantum™
sends its response, it places its own
address in this address fi eld of the
response to let the
Leader
know which
Quantum™ is responding.
• An 8-bit function code de fi ning the
requested action (Query).
• Function Code 3 - to read holding regis-
ters (sends a request to read data from the
Quantum™).
• Function Code 6 to load a register (to re-
quest to change a setpoint or to send a
command such as starting the compressor),
or a function code 16 (RTU only) to load
multiple registers.
• Any data to be sent (Response). The data fi
eld is constructed using sets of two hexa-
decimal digits, in the range of 00 to FF
hexadecimal. These are to be made from a
pair of ASCII characters (ASCII), or one 8-bit
for RTU. The data fi eld of messages sent
from a
Leader
to the Quantum™ devices
contains additional information which the
Quantum™ must use to take the action de-
fi 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 fi eld. If
no error occurs, the data fi eld of a
response from a Quantum™ to a
Leader
contains the data requested. If an error
occurs, the fi eld contains an exception
code that the
Leader
application can use to
determine the next action to be taken.
• An error-checking fi eld.
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 ex-
ample, function code 03 will query the Quantum™ to
read holding registers and respond with their con-
tents. The data fi eld must contain the information
telling the Quantum™ which register to start at and
how many registers to read. The error check fi eld
provides a method for the Quantum™ to validate the
integrity of the message contents.
The Response
If the Quantum™ makes a normal response, the func-
tion 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 val-
ues or status. If an error occurs, the function code
is modi fi ed to indicate that the response is an er-
ror response, and the data bytes contain a code that
describes the error. The error check fi eld allows the
leader
to con fi rm that the message contents are
valid.
Data Field
The data fi eld is constructed using sets of two hexa-
decimal 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 fi eld of messages sent from a
leader
to
the Quantum™ devices contains additional
information which the Quantum™ must use to take
the action de fi 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 fi eld.
For example, if the
leader
requests a Quantum™ to
read a group of holding registers (function code 03),
the data fi eld speci fi es the starting register and how
many registers are to be read.
If no error occurs, the data fi eld of a response from
a Quantum™ to a
Leader
contains the data
requested. If an error occurs, the fi eld contains an
exception code that the
Leader
application can use
to deter-mine 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 appended to the end
of the message. Longitudinal Redundancy Check,
or LRC, is used as the method for verifying that
the ASCII message sent from the transmitting
device was properly received by the receiving
device.
The Longitudinal Redundancy Check (LRC) fi eld
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 fi 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 fi eld preceding the CRLF
(Carriage – Line Feed) characters. Each new ad-
dition of a character that would result in a value
higher than 255 decimal simply rolls over the