Technical data

Network configuration

3.3 Frame transmission time

PROFIBUS Network Manual

System Manual, Edition 04/2009, C79000-G8976-C124-03

An unused address lower than the HSA

The value of the HSA (Highest Station Address) parameter must be set on all end devices so

that there is at least one address in the network between bus address 0 and the value of

HSA that is not used by a node; in other words, there is an address gap. You can create this

address gap simply by increasing the value of the HSA parameter to one higher than the

highest node address in the network.

Note

If this condition is not or is no longer satisfied, the optical bus will no longer close to form a

redundant optical ring following segmentation. The fault message (LED and signaling

contact) of the two OLMs affected is not canceled even after the fault has been eliminated.

Increased retry value to at least the value 3

If a fault occurs requiring a failover (for example a wire break), there is a failover time during

which correct data transmission is not possible. To ensure a "bumpless" failover for the

application, it is advisable to set the number of frame retries on the PROFIBUS master to at

least 3.

Checked and adapted 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 failback 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 polling

sequence to form the optical ring again.

The slot time must be set to approximately twice the value it would be in a non-redundant

network.

Calculate the slot time according to the following formula:

Slot time = a + (b x length of FOC) + (c x number of OLMs)

Slot time is the monitoring time in bit times

Length of FOC is the sum of all FO cables (segment lengths) in the network. The lengths

must be specified in km.

Number of OLMs is the number of PROFIBUS OLMs in the network

The factors a, b and c depend on the transmission speed and can be found in the following

tables.

Table 3- 10 Constants for calculating the slot time with the DP standard (redundant optical ring)

Transmission

speed

a b c

12 Mbps 1651 240 28

6 Mbps 951 120 24

3 Mbps 551 60 24