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Depending on whether you are working on an S7-300 or an S7-400 PLC you will need to use
different racks. For the S7-300 racks have two different jobs.
Primarily, the rack in an S7-300 operates as a holder, or more to the point; we need
this rack to be able to install our modules. Upon further examination, you will see that
this rack has no electrical connectors; this is the reason that the S7-300 rack is also
called a Rail. The other job that an S7-300 rack performs
is to ground the components of the PLC. Grounding means that an electrical device is connected
to a path that will allow any power surges to safely exit the machine and flow into the
ground. This is why all of the metal parts of your modules should connect to the rack,
and the rack is safely grounded, allowing any surges in the PLC to be grounded safely.
By now you may be asking "If this rack has no electrical connectors, how do the modules
communicate with the CPU?". Since we know that these components have to communicate
with each other, our only option left is to install our own electrical connections between
the modules, in the form of external bus connectors. As you can see, the bus connector connects
two ways by having connections on both sides of the bus, so that two modules can communicate
with each other. Be sure to connect one side of the bus connector to a module before you
install the module on the rack. That way, once you install one module, the other connector
on the bus is open so that the next module can communicate with the first module once
it is installed and so on, until all the modules are installed.
It is important to always remember; a bus connector has two connectors, one on each
side to connect two different modules, and should be installed on a module before each
module is installed. So to re-cap, in this session, we discovered
that it is necessary to install a rail or a rack when building an S7-300 and that we
need to install bus connectors with each module to allow the components to communicate and
have our equipment run properly. See you here next time for our next installment
of PLC instruction.