NORD Drivesystems BU0050 - Manual (Page 20)

NORD USS and Modbus RTU Manual
20 Subject to technical amendments BU 0050 GB-3111
1.3.3 Data Coding
STX (Start of Text): ASCII character: 02 Hex
The start character forms the first character in the telegram and together with the start pause it is used
for reliable detection of the start of the telegram.
LGE (Telegram length): 1 Byte, contains the length of the telegram.
The telegram length is located in the 2nd byte of the telegram and indicates the length of the telegram
from the 3rd byte onwards. Specification of the length enables differentiation between the various types
of telegram. The data recipient can use the length byte to check the number of characters to be
received.
ADR (Address byte): 1 byte, also contains the slave address.
Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0
0 M BC | Address (0..30) |
The USS address is located in the 3rd byte (data bits 0 to 4) of the telegram. The slave device which is
to transmit or receive data is identified via the USS address. Therefore, a maximum of one slave device
can be represented by each of the 31 possible addresses For this, the appropriate address must be set
in the slave device. Bit 5 and Bit 6 have a special meaning.
Bit 5 Broadcast: A so-called broadcast telegram can be triggered by setting this bit. In a broadcast
telegram, address bits 0 to 4 are ignored by the connected slaves, i.e. the transmitted telegram is
processed by all of the slaves. However, in contrast to standard addressing, the slaves do not transmit a
response telegram as this would result in bus conflicts.
Bit 6 Echo: By setting the 6th bit, the inverter returns a telegram which is identical to the one which it
has received (for commissioning).
N
1
... N
n
Information characters: each one byte, content depending on task
BCC 1 byte, data saving character (Block Check Character)
The check-sum BCC is formed byte-wise over the entire telegram as an Exclusive OR link. The result
after the last net character is then the BCC.
BCC = STX XOR LGE XOR ADR XOR N
1
XOR....N
N
1.3.4 Character Frame
Each transmitted character begins with a start bit (logical 0) and end with a stop bit (logical 1). 8 bits (1 byte)
are transmitted. Saving is performed by a parity bit (even parity). Therefore 11 bits are transmitted for each
character.