Single and Dual Data Highway Configurations
Publication PUB088-005-00_1017
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4.3.2
System Redundancy Slave to Master Connection
With SR (System Redundancy) there are two data highways and the aim of the redundancy is to
secure communication with the actuator even if one of the highways fails. Although two masters are
shown they will normally be a Primary and Backup pair and communication will occur directly between
them. SR mode is the default for all dual Profibus cards.
The Profibus card has two communication channels and both have the same slave address. As with
the FR mode, when the card is powered ‘on’, Channel 1 will be the Primary channel. If the card does
not enter Configuration Mode within 1 second (because there is no Master present) then Channel 2
will take over as Primary. Once again, if there is no communication within 1 second, then it will revert
to Channel 1 and now it will wait 2 seconds before Channel 2 assumes Primary status. The switch
over time will increase by a factor of 2 each time until it reaches its maximum of 32 seconds. It will
continue to switch channels using a 32 second switch over time until one channel receives PLC
messages.
In the event of a failure of the Primary channel the Backup automatically changes to become Primary
and communication will be established with the Second master on the second highway. The Backup
channel can be used for exchanging data but any commands to move the actuator directed to the
backup channel will be ignored. If a configuration message is sent to the Backup channel that is
different to the one sent to the Primary it will be ignored. The data exchanged between the Master and
the Primary includes information on the status of the device and hence the availability of a Backup
should the Primary fail.
Two Data Highways, two communication ports
Channel 1 is Primary on power up
Default communication method is SR mode
Both channels have the same address
V0 cyclic Commands to Backup channel are ignored
Reports the status of Primary and Backup to the Master
Configuration and Parameterisation changes only over the Primary channel
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Slave
Second Master
First Master
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Fig 16: Profibus DP (Mk2) System Redundancy Connection
