Danfoss VLT FCD 303 Operating Instructions Manual page 78

Telegram length (LGE)
The telegram length is the number of data bytes plus
the address byte ADR plus the data control byte BCC.
The length of telegrams with 4 data bytes is:
LGE = 4 + 1 + 1 = 6 bytes
The length of telegrams with 12 data bytes is:
LGE = 12 + 1 + 1 = 14 bytes
The length of telegrams containing texts is 10+n bytes.
10 represents the fixed characters, while the 'n' is
variable (depending on the length of the text).
Frequency converter address (ADR)
Two different address formats are used, with
the frequency converter's address range being
either 1-31 or 1-126.
1. Address format 1-31
The byte for address range 1-31 has the
following profile:
Bit 7 = 0 (address format 1-31 acti v e)
Bit 6 is not used
Bit 5 = 1: Broadcast, address bits (0-4) are not used
Bit 5 = 0: No Broadcast
Bit 0-4 = Frequency converter address 1-31
2. Address format 1-126
The byte for address range 1 - 126 has
the following profile:
Bit 7 = 1 (address format 1-126 acti v e)
Bit 0-6 = Frequency converter address 1-126
Bit 0-6 = 0 Broadcast
The slave sends the address byte back unchanged
in the response telegram to the master.
Example:
writing to frequency converter address 22 (16H)
with address format 1-31:
Data control byte (BCC)
The data control byte is explained in this example:
Before the first byte in the telegram is recei v ed,
the Calculated CheckSum (BCS) is 0.
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VLT FCD Series
When the first byte (02H) has been recei v ed:
BCS = BCC EXOR "first byte"
(EXOR = exclusi v e-or)
BCS = 0 0 0 0 0 0 0 0 (00 H)
EXOR
1. byte = 0 0 0 0 0 0 1 0 (02H)
BCC = 0 0 0 0 0 0 1 0 (02H)
Each subsequent byte gates with BCS EXOR
and produces a new BCC, e.g.:
BCS = 0 0 0 0 0 0 1 0 (02H)
EXOR
2nd byte = 1 1 0 1 0 1 1 0 (D6H)
BCC = 1 1 0 1 0 1 0 0 (D4H)
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