Chapter 5 - Connection to a controller
Version EN-092019-1.21
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5.1.5
High availibility registers
Using a primary gateway and a backup gateway as redundant pair increases the overall availability of a
system. The redundant role of a gateway is controlled by the Modbus registers 9990 to 9993. By reading
the error state in register 9993 and the status information in register 9991, a redundancy switching
mechanism can be implemented in the PLC. For this purpose, the target state of one of the gateways is
reset by writing to register 9990. The other gateway then automatically enters into the complementary
status. This is true with one exception: If the redundancy link is interrupted, the status of both devices
must be set accordingly.
This chapter describes which registers are available specifically for redundancy (high availability), how to
read the redundancy state and how to read the error state. If an error occurs, the program running on the
PLC can decide to switch to primary, backup or fail-safe mode.
Note
The gateways do not automatically switch to redundancy mode when an error condition
occurs. By writing to register 9990, one gateway is set to a certain node state (
PRIMARY STOP,
PRIMARY RUN, BACKUP),
while the other gateway enters into the corresponding
complementary state when the redundancy link is up.
Register
Register Type
Meaning
9990
Holding Register
This register represents the redundancy state of the gateway.
HighByte: always zero
LowByte : Node State
The register content is provided in byte ordering big-endian.
NodeState:
0x01 PASSIVE (PROFIBUS mode OFFLINE)
0x02 PRIMARY STOP (PROFIBUS mode CLEAR)
0x03 PRIMARY RUN (PROFIBUS mode OPERATE)
0x04 BACKUP (PROFIBUS mode STOP)
0x08 IN PROGRESS
NOTE
: The write access controls the state of the PROFIBUS masters
of all channels. The state can switch between BACKUP, PRIMARY
STOP and PRIMARY RUN mode.
PRIMARY STOP (fail-safe mode) reads only input data from the
PROFIBUS devices.
PRIMARY RUN reads input data and writes output data from/to
the devices. By using the operation modes PRIMARY RUN and
PRIMARY STOP, the PLC can ensure that the input data is read at
least once before the output data is propagated to the device.
After the register has been written it is recommended to read it
back until the read value is the same as the requested one. This is
needed due to the latency of the change of the redundancy role.
9991
Holding Register
This register represents the redundancy state.
NOTE
: Write access for this register area is ignored. The content of
this register uses the same values as described for register Holding
register 9990.
