Danfoss FCM 300 Series Design Manual page 75

Serial bus
Master
Serial bus
Slave 1
Address 1
Telegram communication
Control and reply telegrams
The telegram communication in a master/slave
system is controlled by the master. A maximum of 31
slaves (FC motors) can be connected to one master,
unless a repeater is used - see also the description of
the address format.
The master continuously sends control telegrams
addressed to the slaves and awaits reply telegrams
from these. The response time of the slaves is
maximum 50 ms.
Only a slave that has received a faultless telegram
addressed to that slave will respond by sending back
the telegram unchanged.
Telegram build-up
Each telegram begins with a startbyte (STX) = 02
Hex, followed by a byte that gives the telegram length
(LGE) and a byte that gives the address (ADR).
Then follows a number of databytes (variable,
depending on telegram type). The telegram ends with
a data control byte (BCC).
STX LGE ADR
MG.03.B6.02 - VLT is a registered Danfoss trademark
Slave 2
Address 2
Data BCC
FCM 300 Series
Broadcast
A master can send the same telegram at the same
time to all slaves connected to the bus. In such
broadcast communication, the control telegram
broadcast bit of the address bit has a value of 1 (see
VLT address). Address bits 0-4 are not used.
Contents of a byte
Each transmitted sign begins with a start bit.
Subsequently, 8 databits are transmitted. Each sign is
given via a parity bit set at "1" when there is an even
parity (i.e. an even number of binary 1's in the 8
databits and parity bits together). The sign ends with
a stop bit and thus consists of a total of 11 bits.
Start 0 1 2 3
bit
Telegram length (LGE)
The telegram length is the number of databytes plus
address byte ADR plus data control byte BCC.
Telegrams with 4 databytes have a length of:
LGE = 4 + 1 + 1 = 6 bytes
Telegrams with 12 databytes have a length of:
LGE = 12 + 1 + 1 = 14 bytes
Slave 31 (126)
Address 31 (126)
4 5 6 7 Even Stop
Parity bit
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