Siemens SIMATIC NET PROFIBUS - Manual (Page 94)

Configuring Networks
3-28
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
Checking and Adapting the Slot Time
To allow a “bumpless” return from the optical bus to the optical ring after the fault
has been eliminated, there must be no frame on the network at the switch-back
time. The network is briefly free of frames when a master addresses a device
whose address is configured but does not actually exist.
The master waits for a response until the configured slot time has elapsed. The
OLM recognizes this frame–free state and closes the optical bus in the middle of
this query sequence to form the optical ring again.
The slot time must be set to approximately twice the value you would use in a
non-redundant network.
Calculate the slot time according to the following formula:
Slot time = a + (b x length
FOC
) + (c x number
OLM
)
Slot time is the monitoring time in bit times
Length
FOC
is the sum of all FO cables (segment lengths) in the network.
The lengths must be specified in km.
Number
OLM
is the number of PROFIBUS OLMs in the network
The factors a, b and c depend on the transmission rate and can be found in Tables
3–9 and 3–10.