Siemens MICROMASTER 440 Operating Instructions Manual page 152

3 Functions
Table 3-15
Only an STX with a preceding start interval identifies the valid start of a telegram.
Data is always transferred in accordance with the diagram illustrated below (half-
duplex mode):
STX LGE ADR 1.
Master transmits
Start
pause
BCC
Fig. 3-43 Transmit sequence
The time interval between the last character of the task telegram (BCC) and the
start of the reply telegram (STX) is known as the reply delay time . The maximum
permissible reply delay time is 20 ms, but it must not be less than the start interval.
If node x does not respond within the maximum permissible reply delay time, an
error message is deposited in the master.
The master than sends the telegram for the next slave node.
Bus structure
The data transfer medium and the physical bus interface are essentially
determined by what the bus system is used for. The physical interface of the USS
protocol is based on the "Recommended Standard RS-485". For point-to-point
links, a sub-quantity of EIA RS-232 (CCITT V.24) or TTY (20 mA current loop) can
be used as the physical interface.
The USS bus is based on a linear topology without branches. Both ends of the line
terminate at a node. The maximum cable length (50 m) and therefore the maximum
distance between the master and the last slave is limited by the characteristics of
the cable, the ambient conditions and the data transfer rate [EIA Standard RS-422-
A Dezember 1978, Appendix, Page 14]
The number of nodes is limited to a maximum of 33 (1 master, 32 slaves).
152
Minimum start intervals for various baud rates
Baud rate in bit/s
2400
4800
9600
19200
38400
57600
76800
93750
115200
n BCC
Reply delay
Start interval in ms
Slave transmits
time
STX LGE ADR 1.
MICROMASTER 440
Issue 10/06
9,20 ms
4,60 ms
2,30 ms
1,15 ms
0,57 ms
0,38 ms
0,29 ms
0,24 ms
0,19 ms
STX
Start
pause
BCC
Operating Instructions
6SE6400-5AW00-0BP0