Redundant connection or device

In this section, there is some more detailed explanation concerning the redundant configurations. The figure below shows the normal (non redundant) case and the case with redundant connections to one PLC.

Redundant connections

In the normal configuration (neither redundant PLC nor redundant connection, see first case in the figure below) the S7 driver establishes two read and one write link to the PLC. If the config entry "UseOneConnection" is activated (see Possible config entries of the S7 driver for more information on this entry), only one link is used for reading and writing. The latter case might be better for old S7 300 equipment, which are very limited in available connections.

Figure: Redundant configurations part 1

The second case in the figure above shows the configuration with redundant links. For this case the several cases can be distinguished.

1. The host running the driver has only one network address and the PLC has only one CP. The case is not very useful because both connections will probably break at the same time if anything happens in the network.

2. The host running the driver has only one network address and the PLC has two CP (i.e. two IP addresses). This is a useful configuration, because a problem with one CP or on the link to one CP might be recovered.

3. The host running the driver has two network interfaces (IP address) and the PLC has one CP. This is not useful in the moment, because in that case we cannot determine in the driver which route is used to the PLC. Because it might probably be that both connection take the same route to the PLC, which means that both will fail if a failure in the network appears.

4. The host running the driver has two network interfaces (IP address) and the PLC has two CPs (i.e. two IP addresses). This is the most useful case, but it must be considered, that if the two connections are not separated into 2 subnets, a proper routing must be set up on the S7 Driver host, which makes sure, that different connections are using routes via different network cards.

Active connection

If a redundant connection has been defined the driver will establish the two connections and check the alive-status of the connection periodically. For this purpose it is important that the alive checking is not turned off via the "AliveInterval" config entry. Otherwise lost connections are not detected. The active connection is determined in accordance to the following rules:

1. The highest priority has the manual selection of the connection. If connection 1 is selected manually neither the tag nor a break of connection 1 will cause a switch to connection 2.

2. If the connection selection is defined to Automatic and no PLC Address for connection switch is defined, the driver selects the first working connection as Active connection.

3. If the connection selection is defined to Automatic and a PLC address for connection switch is defined, the value 0 selects connection 1 and 1 selects connection 2.

The following table summarizes the behavior:

C1 ... means connection 1 (or first connection).

C2 … means connection 2 (or redundant connection).

The table shows the active connection depending of working connections (columns) and selected mode (rows):

C1 not connected

C2 not connected

C1 connected

C2 not connected

C1 not connected

C2 connected

C1 connected

C2 connected

C1 manually selected

None

C1

None

C1

C2 manually selected

None

None

C2

C2

Automatic -

No tag

None

C1

C2

C1 or C2

Automatic -

Tag value 0

None

C1

None

C1

Automatic -

Tag value 1

None

None

C2

C2

Redundant PLC

In the figure below the configuration with redundant PLCs is shown. Redundant PLCs are very similar to redundant connections. One difference, concerning the network point of view, is that redundant PLCs cannot have the same IP address. So the cases 1. and 3. of section "Redundant connections" cannot happen.

The other important difference is that the PLC address triggered switching is different, because for the redundant connection the PLC address is read from the same device (memory location), and it does not matter if connection one or two is active. If the switching between PLC is concerned the address is read either from one PLC or from the other. Therefore the method is different. If there is a PLC Tag defined the active PLC is considered as active as long as the read value is 1. If the PLC sets the value to 0, the driver tries the other PLC. The other PLC must set now the value to 1 to become the active one.

Figure: Redundant configurations part 2

Active PLC

If a redundant PLC has been defined the driver will establish one connection to each and check the alive-status of the connection periodically as it is done for redundant connections. The active PLC is determined in accordance to the following rules.

1. The highest priority has again the manual selection of the PLC. If PLC 1 is selected manually neither the tag nor a break of connection to PLC 1 will cause a switch to PLC 2.

2. If the connection selection is defined to Automatic and no PLC Address for connection switch is defined, the driver selects the first PLC with a working connection as Active PLC.

3. If the connection selection is defined to Automatic and a PLC address for PLC switch is defined the value 1 means, that the current PLC is active and the value 0, that the driver should switch to the other PLC.

The following table summarizes the behavior:

P1 …. means PLC 1 (or first PLC).

P2 …. means PLC 2 (or redundant PLC).

The table shows the active PLC depending of working connections (columns) and selected mode (rows):

P1 not connected

P2 not connected

P1 connected

P2 not connected

P1 not connected

P2 connected

P1 connected

P2 connected

P1 manually selected

None

P1

None

P1

P2 manually selected

None

None

P2

P2

Automatic -

No tag

None

P1

P2

P1 or P2

Automatic -

P1 Tag value 0

P2 Tag value 0

None

None

None

None

Automatic -

P1 Tag value 1

P2 Tag value 0

None

P1

None

P1

Automatic -

P1 Tag value 0

P2 Tag value 1

None

None

P2

P2

Automatic -

P1 Tag value 1

P2 Tag value 1

None

P1

P2

P1 or P2

Redundant PLC with redundant connections

This is the second case shown in the figure "Redundant configurations part 2". It is a combination of the cases described in the sections "Redundant connections" and "Redundant PLC". The active PLC is determined in accordance to the rules described in "Redundant PLC" and the active connection is selected as described in "Redundant connections". For switching the connection the corresponding PLC address tag is only used from the active PLC.