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addressed device.
A MODBUS® message is placed by the
transmitting device into a frame that has a
known beginning and ending point. This al-
lows 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 complet-
ed. 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
:
01
03
1
2
2
CHAR
CHAR
CHAR
Start of
Quantum™
Function
message
ID
00 = H. O. Address
87 = L. O. Address
00 = H. O. # of data registers
01 = L. O. # of data registers
QUANTUM™ HD COMPRESSOR CONTROL PANEL
COMMUNICATIONS SETUP
LRC
DATA
END
CHECK
00870001
74
CRLF
8
2
2
CHAR
CHAR
CHAR
CRC Error
End of
Correction
message
Code
RTU
In RTU mode, messages start with a silent
interval of at least 3.5 character times. This
is most easily implemented as a multiple of
character times at the baud rate that is be-
ing used on the network (shown as T1–T2–
T3–T4 in the figure below). The first field
then transmitted is the device address.
The allowable characters transmitted for all
fields are hexadecimal 0–9, A–F. Networked
devices monitor the network bus continu-
ously, including during the 'silent' intervals.
When the first field (the address field) is re-
ceived, each device decodes it to find out if
it is the addressed device.
Following the last transmitted character,
a similar interval of at least 3.5 character
times marks the end of the message. A new
message can begin after this interval. The
entire message frame must be transmitted
as a continuous stream. If a silent interval
of more than 1.5 character times occurs be-
fore completion of the frame, the receiving
device flushes the incomplete message and
assumes that the next byte will be the ad-
dress field of a new message.
Similarly, if a new message begins earlier
than 3.5 character times following a previ-
ous message, the receiving device will con-
sider it a continuation of the previous mes-
sage. This will set an error, as the value in
the final CRC field will not be valid for the
combined messages. A typical message
frame is shown below:
START
ADDRESS FUNCTION
T1-T2-T3-
8 Bits
8 Bits
T4
Start of
Quantum™
Function
message
ID
00 = H. O. Address
87 = L. O. Address
00 = H. O. # of data registers
01 = L. O. # of data registers
090.040-CS (JAN 14)
Page 47
DATA
LRC CHECK
END
N x 8
T1-T2-
16 Bits
Bits
T3-T4
CRC Error
End of
Correction
message
Code
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