Glossary
S7-300, CPU 31xC and CPU 31x: Installation
Operating Instructions, Edition 08/2004, A5E00105492-05
Glossary-21
Subnet mask
The bits set in the subnet mask decides the part of the IP address that contains the address
of the subnet/network.
In general:
•
The network address is obtained by an AND operation on the IP address and subnet
mask.
•
The node address is obtained by an AND NOT operation on the IP address and subnet
mask.
Subnetwork
All the devices connected by switches are located in the same network - a subnet. All the
devices in a subnet can communicate directly with each other.
All devices in the same subnet have the same subnet mask.
A subnet is physically restricted by a router.
Substitute
See Proxy
Substitute value
Substitute values are configurable values which output modules transfer to the process when
the CPU switches to STOP mode.
In the event of an I/O access error, a substitute value can be written to the accumulator
instead of the input value which could not be read (SFC 44).
Switch
PROFIBUS is based on a bus topology. Communication nodes are connected by a passive
cable - the bus.
In contrast, Industrial Ethernet is made up of point-to-point links: Each communication node
is connected directly to one other communication node.
If a communication node needs to be connected to several other communication nodes, this
communication node is connected to the port of an active network component- a switch.
Other communications nodes (including switches) can then be connected to the other ports
of the switch. The connection between a communication node and the switch remains a
point-to-point link.
The task of a switch is therefore to regenerate and distribute received signals. The switch
"learns" the Ethernet address(es) of a connected PROFINET device or other switches and
forwards only the signals intended for the connected PROFINET device or connected switch.
A switch has a certain number of ports). At each port, connect a maximum of one
PROFINET device or a further switch.
