Rockwell Automation 1715-OF8I Redundant I/O System User Manual - Manual (Page 27)

Rockwell Automation Publication 1715-UM001C-EN-P - March 2014 27
Redundancy System Overview Chapter 1
Switchover Considerations Each 1715 Redundant I/O System uses a single IP address as the primary IP
address for all communication on the EtherNet/IP network. The redundant
adapter module pair consists of two active modules, a primary adapter module
and its partner, a secondary module. For the purpose of this document, the
primary module is referred to as module ‘A’ and the secondary module is referred
to as module ‘B’.
On power-up, the adapter module in the leftmost slot is considered the primary
module (when two adapter modules are present) because it uses the primary IP
address, and because it is the module responsible for receiving/transmitting to
ControlLogix controller on the EtherNet/IP network.
The module in the rightmost slot is considered the secondary module partner
and uses the primary IP address +1. For example, if the primary module in the
leftmost slot has an IP address of ‘N’, then the secondary module in the rightmost
slot, has an IP address of ‘N+1’.
Both modules are active at all times and are responsible for monitoring all inputs
and outputs, monitoring diagnostics in the system, and reading and writing data
from/to I/O at exactly the same time. When the primary module receives a write,
it tells the secondary module the data that is to be written, and to which module
it is to be written to. At a synchronized point in time, both adapter modules
physically write to the I/O.
The adapter modules negotiate which module is primary, depending on the
status of the system. If the primary module is unable to perform its role, for
example, if a fault occurs in the primary module, then the secondary module
becomes the new primary, assuming the primary module’s IP address, taking over
the role of communication. The primary module is the only module of the pair
that produces data on the EtherNet/IP network.
In the event of a fault, the IP address swapping between the primary module and
the secondary module takes no longer than 20 ms from the time of the initiating
fault. The secondary module is the new primary and handles all communication.
(Depending on the structure of your Ethernet configuration, this time can be
impacted.)
This IP address swap is transparent to the user. The user can detect which module
the primary module is by looking at the Network status indicator near the top of
each adapter module. On the primary adapter the Network status indicator is
solid green; on the secondary adapter, the Network status indicator flashes green.
Once a swap occurs, the ‘new’ primary module remains the primary unless there’s
a good reason to swap over again. If the B module becomes the primary module
because you need to replace the A module, that is not enough cause to swap IP
addresses back again. Module B remains as the primary module.