Profibus Installation Manual
16
4.3.2 System Redundancy Slave to Master Connection
Primary Master Backup Master
FR
First Master Second Master
Primary
n
64+n
Slave
Backup
Primary
Slave
Backup
Primary
Slave
Backup
Primary
Slave
Backup
SR
n n
m m
Fig. 9: Profibus DP System Redundancy 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 2 seconds, then it will revert to Channel 1 and now it
will wait 4 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
Single and Dual Data Highway Configurations
A4US
US
A4
US A4
US
A4
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