Functions
8.7 Communication via USS
Control Units CU240S
Operating Instructions, 11/2006, A5E00766042B AA
8-25
LGE
The LGE is a single byte field indicating the number of bytes, which follow this in the
message. According to the USS specification, the telegram length is variable, and the length
must be specified in the 2nd telegram byte (i.e. LGE). Depending on the configuration, fixed
telegram lengths can be defined (see description of user data area). Different telegram
lengths can be used for different slave nodes on the bus. The maximum total length of a
telegram is 256 bytes. The LGE is defined as the user data characters (quantity n), address
byte (ADR) and the block check character (BCC). The actual total telegram length will of
course be two bytes longer than the LGE as the STX and LGE bytes are not counted in the
LGE.
Variable length telegrams and fixed length telegrams can both be used. This can be selected
using parameters P2012 and P2013 to define the PZD and PKW lengths. Most common
fixed length applications will use a 4-word (8-byte) PKW area and a 2-word (4-byte) PZD
area giving 12 user data characters. This gives the LGE = 12 + 2 = 14.
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 number 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 number 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 number
(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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