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090.040-CS (JAN 14)
Page 46
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 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) 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 then appended to
the message as the last field 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
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 cal-
culated value to the actual value it received in
the LRC field. If the two values are not equal, an
error results.
RTU
In RTU mode, messages include an error–check-
ing field that is based on a Cyclical Redundancy
Check (CRC) method. The CRC field checks the
contents of the entire message. It is applied re-
gardless of any parity check method used for the
individual characters of the message.
QUANTUM™ HD COMPRESSOR CONTROL PANEL
COMMUNICATIONS SETUP
The CRC field is two bytes, containing a 16–bit
binary value. The CRC value is calculated by the
transmitting device, which appends the CRC to
the message. The receiving device recalculates
a CRC during receipt of the message, and com-
pares the calculated value to the actual value it
received in the CRC field. If the two values are
not equal an error results.
The CRC is started by first preloading a 16–bit
register to all 1's. Then a process begins of ap-
plying successive 8–bit bytes of the message to
the current contents of the register. Only the
eight bits of data in each character are used for
generating the CRC. Start and stop bits, and the
parity bit, do not apply to the CRC.
During generation of the CRC, each 8–bit char-
acter is exclusive ORed with the register con-
tents. Then the result is shifted in the direction
of the least significant bit (LSB), with a zero filled
into the most significant bit (MSB) position. The
LSB is extracted and examined. If the LSB was a
1, the register is then exclusive ORed with a pre-
set, fixed value. If the LSB was a 0, no exclusive
OR takes place.
This process is repeated until eight shifts have
been performed. After the last (eighth) shift, the
next 8–bit byte is exclusive ORed with the reg-
ister's current value, and the process repeats for
eight more shifts as described above. The final
contents of the register, after all the bytes of the
message have been applied, is the CRC value.
When the CRC is appended to the message, the
low-order byte is appended first, followed by the
high-order byte.
Framing
A message frame is used to mark the beginning and
ending point of a message allowing the receiving de-
vice to determine which device is being addressed
and to know when the message is completed. It also
allows partial messages to be detected and errors
flagged as a result.
ASCII
In ASCII mode, messages start with a colon
( : ) character (3A hex), and end with a car-
riage 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 continu-
ously for the colon character. When one is
received, each Quantum™ decodes the next
field (the address field) to find out if it is the
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