Previously we learned about hardware configuration and network arrangement of an S7-300 redundant
system. In this video we are going to see how this system works in action.
When we use the word “redundant” for a control system, we are talking about a control
system that provides us with an availability that is higher than a normal control system.
The hardware configuration and programming have been done in such a way that, under normal
circumstances, one of the CPUs is the master which controls the system and the other one
is a reserve. If the master CPU detects a problem, the reserve CPU will become the master
and take over control of the system. This makes a safer system with a higher availability
compared to a normal control system. As you can see, both CPUs are currently in
the RUN mode. You may be wondering how do we know which one is the master? This can
easily be determined by the LED light on the IMs! There is an “ACTIVE” LED light on
each one of these IMs here. The “ACTIVE” LED of the “left side” IM here in the
second rack is ON. We have the same situation in the third rack. This is indicating that
PLC A is our master. This lighting configuration means that the
data of the modules installed on the second rack is transferring to the PLC A using this
IM and the data of the modules installed on the third rack is transferring in the same
way to PLC A, using this IM. PLC B is in RUN mode however, since it serves as the reserve
system, it isn’t processing any code; it is only monitoring signals in the event it
needs to take over control of the system. As we said before, using the MPI network and
this “repeater” here, we have connected both CPUs to the HMI panel in order to see
the necessary signal data on the HMI. Let’s discuss these three LED lights on
this “RS-485” repeater here. This lower green light shows us that the 24 volt power
for the repeater has been provided and the device is ON.
This upper “orange” LED light here indicates if the connection between repeater and the
incoming devices is properly established. When this light is on and blinking, it means
that everything is working properly in the inputs and that the two CPUs are transferring
data to the repeater properly. If this light is not blinking, you need to check your connections
and make sure the CPUs are in RUN mode. When the lower “orange LED” is blinking,
it indicates that the connection between the repeater’s output and the output device,
which is the HMI in our case, is properly established.
In the normal condition the green LED should be on and the two orange LED lights should
also be on and blinking otherwise there is something wrong. In our experience, when the
CPUs and the HMI are not communicating data properly, the most common issue is an improper
cable connection. If that is the case, you need to check your cable connections or change
the “Profibus connectors”. The switch positions on the “repeater”
should be left in the default mode for proper function. Changing the switch positions should
not be attempted without proper instruction. We’ve created different graphic pages for
the HMI to be able to see the various signal values and conditions in the system.
In the upper section of all pages, we can see whether “PLC A” or “PLC B” is
the master. Here we see that “PLC A” is the currently master.
Let’s put the “PLC A” CPU in the stop mode and see what happens.
Great! As we expected, “PLC B” is now the master. Looking at “PLC A” IMs,
we’ll see that the “System Fault” LEDs on these IMs are blinking, since the “PLC A” is
in stop mode, but the “PLC B” IMs are both active.
Let’s place “PLC A” CPU back in RUN mode and we’ll see that both “System Fault”
LEDs are off. Let’s turn off the power supply to “PLC
B” and see what happens. As you can see, “PLC A” has resumed control
and is now the master. Even when the “PLC B” has no power and is completely turned
off, “PLC A” is controlling the entire system without any problems. So this is how
an S7-300 redundant control system works in action.
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