Allen-Bradley ControlNet PLC-5 User Manual page 57

Since the peer-to-peer handshake connection is one NUT, it only
takes up to one NUT for the primary system to indicate to the
secondary system that a switchover must occur. There is up to
5ms of processing on each processor to get the switchover
indication from the primary system to the secondary system.
Thus the formula for the Communicated Switchover Time is:
Communicated Switchover Time = 10ms + NUT
So, if the NUT is 5ms, the Communicated Switchover Time is
equal to 10ms plus 5ms, or 15ms.
• Timeout Switchover - when the secondary processor loses
communication with the primary processor for a certain length of
time, it times out the primary processor, and then performs a
switchover. The following events can cause a Timeout
Switchover:
• Primary PLC-5 system failure.
• Loss of power to the primary PLC-5 system
• Problems with the ControlNet network which causes loss of
the handshake connection with the primary processor.
The formula that determines the length of time the secondary
system waits before timing out the primary processor is as
follows:
Primary Timeout = 5 + (3 * NUT) or 25ms, whichever is
greater.
So, if the NUT is 5ms, the Primary Timeout is 20ms. If the NUT
is 10ms, the Primary Timeout is 35ms.
The Primary Timeout is not the only factor in the Timeout
Switchover Time. There is also processing which must occur on
the secondary system after a Primary Timeout occurs. This
processing can take up to 5ms. The formula for the Timeout
Switchover Time is:
Timeout Switchover Time = Primary Timeout + 5ms.
So, if the NUT is 5ms, the Timeout Switchover Time is equal to
25ms plus 5ms, or 30ms.
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