Siemens SIMATIC NET PROFIBUS - Manual (Page 96)

Configuring Networks
3-30
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
Note
The slot time calculation takes into account only the optical network and the
attachment of nodes to the OLM in each case via a maximum 20 m long RS-485
bus segment. Longer RS-485 bus segments must be included by adding them to
the length
FOC
.
With the OLM/G11-1300 and OLM/G12-1300, the minimum slot times shown in the
following table must be maintained at transmission rates of 12 Mbps, 6 Mbps, 3
Mbps and 1.5 Mbps.
Table 3-11 Minimum Slot Time for
OLM/G11-1300 and OLM/G12-1300
Transmission rate Minimum slot
time
12 Mbps 3800 t
Bit
6 Mbps 2000 t
Bit
3 Mbps 1000 t
Bit
1.5 Mbps 530 t
Bit
When configuring the slot time, use the minimum slot time as shown in Table 3-11
if the calculated slot time is less than the minimum slot time.
Note
If the slot time is configured with a value that is too low, this can lead to
malfunctions and error displays on the OLM. The system LED flashes red/green.
3.3.3 Example of Configuring the Bus Parameters in STEP 7
Structure of the Network Example
The example assumes a redundant optical ring with the following structure:
S 20 OLM G12 modules in the redundant optical ring
S 20 km total ring length
S
Transmission Rate 1.5 Mbps
S
Nodes attached directly to OLMs
S
“PROFIBUS-DP” bus protocol