Siemens SIMATIC NET PROFIBUS manual Checking and Adapting the Slot Time

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 FOC) + (c x numberOLM)

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.

3-28	PROFIBUS Networks SIMATIC NET
	6GK1970-5CA20-0AA1 Release 2 05/2000