Modular PLC hardware basics IV.

By pushing the backplane bus’ speed even further, differential signalisation must be used on the TX and the RX lines all the way ’round, and even more powerful transmission ICs are to be used (for eg. PROFIBUS capable transceivers).
Even with these modifications the transmission speeds hardly can exceed 10 Mbps.

If there is need for even more backplane speed other type of communication bus shall be chosen.

The most obvious and most widespread network nowadays is the ethernet. It is open, low cost and widely used in office and in industry areas.
There are several types of ethernet in the industry, they are more or less capable of real time transmission.
One of which is EtherCAT, which is far the most speedy and efficient network type.
Its basic operating principle is the same as shown in the previous posts (the INTERBUS principle).

Since it uses ethernet physical layer, its speed is fixed 100 Mbps, and its “data insert on the fly” method guarantees an ultimately high performance compared to other fieldbus systems.

Its physical layer IC has an internal switch that redirects the transmission back to the CPU (it receives the message at its RX1 port then transmits it to its TX2 port, but if there is no other card in the line – there is no carrier signal on RX2 port – it retransmits the message to the TX1 port).
EtherCAT PLC card schema

For space limitation and cost reduction reasons the connectors are 6 pole ones, where 4 of them are for signal transmission, the rest 2 are for the +5V backplane supply. This +5V is for the ASICs only, the more power hungry I/Os has to be fed through the sperarte +24V input connector (at the card’s front panel). This also provides the electrical isolation between the backplane bus and the I/Os (usually the I/Os are also isolated from the backplane using optocouplers).

Here is an example topology:

To put it all together:
For slow or moderate speeds UART based backplane buses are used. They are simple, cheap, and can be rugged enough for industrial purposes (in case of using RS-485 or PROFiBUS transceivers).

For high speed transmission an ethernet based approach is the best choice.


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