Modular PLC hardware basics III.

Remote I/O concept

 If someone want to use lots of I/Os with some of them located far from the PLC system, remote I/Os must be used.

The wonderful thing in the former mentioned approach is that it can easily be expanded with I/Os located far away.

A simple “expander card” must be put to the end of the line, which strengthens the transmission and makes is more immune to electrical noises. The interconnection is done via commonly used shielded or unshielded twisted pair lines (ethernet UTP cables with commonly used RJ-45 connectors).


PLC I/O expansion using UTP cables

From the programmers point of view, all the I/Os can be handled as local I/O, irrespective of their spacial placement.
The far side’s line starts with an expander, which fits the data signals to the remote I/Os’ backplane. This I/O expansion is applicable as many times as the user wants.
Inside if the Expander Transmitter and the Expander Receiver
In addition the I/O expansion can be used for “I/O splitting” as well. This I/O splitter is also acts like a mechanical switch. If there is nothing connected to any of its RJ-45 ports, then this empty port simply redirects the TX line to the other port or back to the backplane.
Various cabling architecture can be realised (line, tree, star) .


PLC architecture with I/O splitters

The inside of such a splitter card is shown on the schematic below.

HW6PLC I/O splitter card schematic

It is like doubling the Expander Transmitter card’s output section (the section related to the RJ-45 port).

As can be seen, using expansion cards only the +24V power supply is extended and the further cards’ +5V and +3,3V supply is derived from this.

 For creating huge size networks, galvanic isolation of data and power lines is necessary.

In practice each line (or in other words each rack) has their own power supply, and the signal lines are isolated from each other both sides (for eg. there is a galvanic isolation both the Expansion Transmitter and the Expansion receiver card).

This helps avoiding ground loops and makes the signal transmission more reliable.

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