Siemens SINAMICS G120 Original Instructions Manual page 159

The telegram runtime is longer than just purely adding all of the character runtimes
(=residual runtime). You must also take into consideration the character delay time between
the individual characters of the telegram.
Figure 7-15
The total telegram runtime is always less than 150% of the pure residual runtime.
Before each request telegram, the master must maintain the start delay. The start delay
must be > 2 × character runtime.
The slave only responds after the response delay has expired.
Figure 7-16
The duration of the start delay must at least be as long as the time for two characters and
depends on the baud rate.
Table 7- 16
Baud rate in bit/s
9600
19200
38400
57600
115200
Note: The character delay time must be shorter than the start delay.
Inverter with CU230P-2 Control Units
Operating Instructions, 11/2013, FW V4.6.6, A5E02430659B AG
Telegram runtime as the sum of the residual runtime and character delay times
Start delay and response delay
Duration of the start delay
Transmission time per character (= 11 bits)
1.146 ms
0.573 ms
0.286 ms
0.191 ms
0.059 ms
Configuring the fieldbus
7.6 Communication via RS485
Min. start delay
> 2.291 ms
> 1.146 ms
> 0.573 ms
> 0.382 ms
> 0.117 ms
159