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Using a Quantum 984 HSBY System
840 USE 106 00 January 2003 93
Swap Address at
Switchover
In a Hot Standby system, the Modbus ports on the Primary controller may have
MEM addresses in the range of 1 to 119. This allows an offset of 128 for comparable
ports on the Standby controller, with 247 the maximum number of addresses.
For example, if controller A is the Primary controller and its two Modbus ports have
addresses 1 and 2, then the default addresses for the comparable ports on Standby
controller B are 129 and 130. By default, this offset is maintained between port
addresses in the event of switchover. For example, if controller B becomes the
Primary controller as the result of switchover, its Modbus ports assume the
addresses of 1 and 2, and the comparable ports on controller A assume addresses
129 and 130.
The check boxes allow you to change this default condition on any or all of the
Modbus ports on the two controllers in your Hot Standby system.
Modbus ports on the two controllers in your Hot Standby system. For example: if you
deselect the parameter Modbus Port 1, then no offset is maintained at switchover
and after switchover the two ports have the same address. Thus if controller A is the
Primary controller and its Modbus port 1 address is 1, then that port address remains
1 after a switchover occurs. Likewise, if controller B becomes the Primary controller
as a result of switchover, its Modbus port 1 address is also 1.
Modbus Plus
Port Address
Swapping at
Switchover
In a Quantum Hot Standby system, the Modbus Plus port addresses on the Standby
controller are offset by 32 from the comparable ports on the Primary controller. For
example, if controller A is the Primary controller and its Modbus Plus port has
address 1, then the address for the corresponding port on Standby controller B is 33.
The numerical range for addresses for both ports is 1 through 64. Thus, if the port
on the Primary controller has address 50, then the address for the corresponding
port on the Standby cannot be 82, so it is 18 (that is, 50 minus 32).
These addresses are automatically swapped at switchover; you do not have the
option to change the offset or prevent the addresses from being swapped.
Note: If you change the selections, the port addresses are not affected until a
switchover occurs.
Note: The Quantum Hot Standby system swaps Modbus Plus addresses almost
instantaneously at switchover. This means that host devices polling the Quantum
controller can be assured that they are always talking to the Primary controller and
that the network experiences no downtime during switchover.