97 750-265
changed. If the Modbus master writes the correct OEM
password, the status register indicates this result and all data
with OEM access level and below can be changed.
03 (0x03) Read Holding Registers
This function is used to read one or more consecutive data
registers in the Falcon. The register address of the first
register (see Table 33) in the range is included in the request
along with the number of registers to read. Falcon returns a
response with the starting register address, the number of
bytes returned, followed by the register data contents in
register address order (lowest register address first, etc.).
Normally, the number of bytes returned is 2 times the number
of registers requested, because each register usually contains
a 16-bit value. An exception to this rule is that registers
representing variable length text data return the length of the
text data, which can exceed 2 bytes.
06 (0x06) Write Single Register
This function is used to write data to a single register in the
Falcon. The Falcon register address and 16-bit data value
to write into the register are sent to the Falcon, and the Falcon
returns an acknowledged response.
NOTE: This function (command) cannot be used for variable
length text data registers.
16 (0x10) Write Multiple Registers
This function is used to write data into multiple CB Falcon
registers with a single request. The Falcon registers must be
located consecutively in the register map because only a base
address is provided. The Modbus master provides the starting
register address, the number of registers to write, the total
number of bytes, followed by the actual data itself. The
R7910A writes the data into each register and acknowledges
the completion with a response echoing the number of
registers written.
When writing text data to a register representing variable
length text, the number of registers should be specified as one
and the byte count be the number of bytes in the text data.
17 (0x11) Report Slave ID
This function is used to locate and identify the CB Falcons
connected on the Global Modbus network. The Modbus
master issues a Report Slave ID request for a specific
Modbus address onto the Global Modbus network, and if a
CB Falcon exists with the requested Modbus address, it
responds to the request. If no Falcon exists, the Modbus
master times out and concludes that no Falcon is present
with that Modbus address.
Included in the CB Falcon response is the following data to
further identify it:
• OS number
• Burner name
Format of the Falcon response message is depicted in
Table 42.
The OS number (up to 16 characters) and burner name (up to
20 characters) fields are NULL filled text strings. They have a
fixed field length so that the boundaries of each field are
known. These same Falcon parameters can be obtained
with the Read Holding Register function.
The Run Indicator status contains an OFF status when the
Falcon is in a lockout or unconfigured state. In any other
case the status indicates an ON condition.
NOTE: A slave ID of 0x79 is reserved for all CB Falcon
hydronic boiler control models at this time.
Exception Codes
The Modbus exception codes in Table 43 may be given by the
Falcon in response to function code requests.
Table 42. Report Slave ID Response
Byte: 012345-2021-4041-42
Slave
Address
Function
Code Byte Count Slave ID Run Indicator OS Number Burner Name CRC
0x01-0xF0 0x11 0x30 0x79 0x00=OFF
0xFF=ON
Table 43. Modbus Exception Codes.
Code Name Comment
0x01 ILLEGAL_FUNCTION llegal function code or action requested
0x02 ILLEGAL_DATA_ADDRESS Register address out of bounds
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