Communication
6.6 Communication via USS
Control Units CU240S
Operating Instructions, 07/2008 - FW 3.2, A5E00766042B AD
179
STX
The STX field is a single byte ASCII STX character (0x02) used to indicate the start of a
message.
LGE
The LGE is a single byte field, indicating the number of bytes which follow this in the
telegram. It is defined as the sum of
● use data characters (quantity n)
● address byte (ADR)
● block check character (BCC)
The actual total telegram length will of course be two bytes longer as STX and LGE itself are
not counted in the LGE.
ADR
The ADR field is a single byte containing the address of the slave node (e.g. inverter). The
individual bits in the address byte are addressed as follows:
7
65432
1
0
Special
Mirror Broadcast
5 address bits
● Bit 5 is the broadcast bit.
If it is set to 1, the message is a broadcast message and will be acted upon by all
inverters on the serial link. The node adress is not evaluated. The USS protocol
specification requires certain settings in the PKW area, refer to the later example on
using USS broadcast mode.
● Bit 6 = 1 indicates a mirror telegram.
The node address is evaluated and the addressed slave returns the telegram unchanged
to the master.
Bit 5 = 0 and bit 6 = 0 and bit 7 = 0 means normal data exchange for units. The node
address (bit 0 … bit 4) is evaluated.
BCC
BCC means block check character. It is an exclusive OR (XOR) checksum over all telegram
bytes except the BCC itself.
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