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network, the Ethernet module will perform the translation of message at
the reverse order. First it will replace station number in message to the
original station number and then re-calculate and update the check sum
of message then send it to serial port. A standard client mode network
application is shown as follows.
In the above figure, there are two groups of PLC. The PLCs in each group
are linked together by RS485 interface and then attach to an Ethernet
module for network accessibility. The station number 1 of group 1 is a
master PLC, which not only can access the other PLCs of same group but
also can access the PLCs of group 2 with the help of two Ethernet
modules bridging. The Ethernet module attached to group 2 PLC is
configured as server mode, which means all the PLCs covered by this
module are work as slave PLC and wait for the command passively. It
must emphasis that the PLCs under standard client mode Ethernet module
can’t access by other master devices thru network communication. The
role of this kind of Ethernet module is very similar to Fire Wall. Only the
messages from inside or the corresponding reply messages can be
accepted by Ethernet module, other messages will be blocked. The
security of client mode operation is very high. Based on the reason
described above, the workstation A can only access the PLCs of group 2
.
It’s noted that, from the view point of master PLC, the station number of
station #2 of group 2 is not 2, otherwise it can’t distinguish it from the
local station of #2. This can be overcame by the introduction of
translation table.
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