Functions
8.7 Communication via USS
Control Units CU240S
8-24 Operating Instructions, 11/2006, A5E00766042B AA
8.7.1 Structure of a USS telegram
Description
The following figure shows the structure of a typical USS telegram. It consists of a start
delay, 4 framing bytes (STX, LGE, ADR and BCC) and the n user data.
STX LGE ADR 1. 2. n. BCC: : :
6WDUW'HOD\
n user data
Figure 8-17 Structure of USS telegram
A typical frame consists of:
STX 1 Byte
LGE 1 Byte
ADR 1 Byte
BCC 1 Byte
PKW 8 Bytes (4 Words: PKE + IND + PWE1 + PWE2)
PZD 4 Bytes (2 Words: PZD1 + PZD2)
Sum : 16 Bytes
Start Delay
The length of the start delay is at least the time of two characters and depends on the
baudrate.
Table 8-9 Length of start delay
Baudrate in bit/s Transfer time per character (= 11 bit) Transfer time per bit Min. Start Delay
9600 1.146 ms 104.170 µs > 2.291 ms
19200 0.573 ms 52.084 µs > 1.146 ms
38400 0.286 ms 26.042 µs > 0.573 ms
57600 0.191 ms 17.361 µs > 0.382 ms
187500 0.059 ms 5.340 µs > 0.117 ms
Note: The time between characters must be smaller than the start delay.
STX
The STX field is a single byte ASCII STX character (0x02) used to indicate the start of a
message.
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