Danfoss FCM305 56 Instruction Manual page 97

Serial bus
Leader
Serial bus
Follower 1
Address 1
Telegram communication
Control and reply telegrams
The telegram communication in a leader/follower
system is controlled by the leader. A maximum of 31
followers (FCM 300 units) can be connected to one
leader, unless a repeater is used - see also the
description of the address format.
The leader continuously sends control telegrams
addressed to the followers and awaits reply
telegrams from these. The response time of the
followers is maximum 50 ms.
Only a follower that has received a faultless
telegram addressed to that follower 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.B6.2G - VLT is a registered Danfoss trademark
Follower 2
Address 2
Data BCC
FCM 300 Series
Broadcast
A leader can send the same telegram at the same
time to all followers 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
Follower 31
(126)
Address 31 (126)
4 5 6 7 Even Stop
Parity bit
15-3