Telegram Structure; Telegram Length (Lge); Frequency Converter Address (Adr); Data Control Byte (Bcc) - Danfoss VLT AutomationDrive FC 300 Design Manual

RS-485 Installation and Set...

10.7.2 Telegram Structure

Each telegram has the following structure:
1. Start character (STX)=02 Hex
2. A byte denoting the telegram length (LGE)
3. A byte denoting the frequency converter address
(ADR)
A number of data bytes (variable, depending on the type
of telegram) follows.
A data control byte (BCC) completes the telegram.
STX LGE ADR
Illustration 10.5 Telegram Structure

10.7.3 Telegram Length (LGE)

The telegram length is the number of data bytes plus the
address byte ADR and 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
bytes

10.7.6 The Data Field

The structure of data blocks depends on the type of telegram. There are 3 types, and the type applies for both control
telegrams (master⇒follower) and response telegrams (follower⇒master). The 3 types of telegram are:
Process block (PCD)
The PCD is made up of a data block of 4 bytes (2 words) and contains:
•
Control word and reference value (from master to follower)
•
Status word and present output frequency (from follower to master)
STX LGE ADR
Illustration 10.6 PCD
®
VLT AutomationDrive FC 300 Design Guide 90-1200 kW
DATA BCC
1)
+n
PCD1
MG34S202 - Rev. 2013-08-19
1)
The 10 represents the fixed characters, while the "n'" is
variable (depending on the length of the text).

10.7.4 Frequency Converter Address (ADR)

Two different address formats are used.
The address range of the frequency converter is either 1–
31 or 1–126.
1. Address format 1–31:
Bit 7=0 (address format 1–31 active)
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:
Bit 7=1 (address format 1–126 active)
Bit 0–6=frequency converter address 1–126
Bit 0–6=0 Broadcast
The follower returns the address byte unchanged to the
master in the response telegram.

10.7.5 Data Control Byte (BCC)

The checksum is calculated as an XOR-function. Before the
first byte in the telegram is received, the Calculated
Checksum is 0.
PCD2
10 10
BCC
243