1.3 USS - Protocol speciication
BU 0050 GB-3111
Subject to technical amendments
19
1.3 USS Protocol Specification
1.3.1
General information
The USS protocol defines an access procedure according to the Master/Slave principle for communication
via a serial bus. A sub-set of this also includes point-to-point connection. A master and a maximum of 31
slaves can be connected to a bus. The individual slaves are accessed by the master via an address
character in the telegram. Direct exchange of messages between the individual slaves is not possible.
Communication is carried out in semi-duplex mode. The master function cannot be transferred (single master
system).
Data transfer via the two-cable bus is carried out by individual characters in the format:
1 start bit, 8 data bits 1 even parity bit and 1 stop bit (
8E1
) - This results in a character frame of 11 bits.
The direction of the data on the bus (transmit or receive) is specified by the master.
1.3.2
Telegram Structure
Each telegram starts with the start character STX (= 02 Hex), followed by the length (LGE) and the address
byte (ADR). This is followed by the information characters. The telegram
is concluded by the data saving character BCC (Block Check Character).
STX
LGE
ADR
N
1
N
2
..
N
n
BCC
For word information (16 Bit) in the information data block (= information character block) the High Byte (first
character) is always transmitted first, followed by the Low Byte (second character). The same applies for
double-word information:
First the High Word is transmitted, followed by the Low Word.
