Siemens SIMATIC NET PROFIBUS Owner's Manual page 94

Configuring Networks
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
Slot time
Length
FOC
Number
OLM
The factors a, b and c depend on the transmission rate and can be found in Tables
3–9 and 3–10.
3-28
) + (c x number
FOC
is the monitoring time in bit times
is the sum of all FO cables (segment lengths) in the network.
The lengths must be specified in km.
is the number of PROFIBUS OLMs in the network
)
OLM
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000