Supervisor Circuit

It is often needed to check if a unit is operating, and if that unit is working, another devices can be operated.

On the other hand, if the unit is switched off, it can still operate for a while, even if its power supply is switched off. For safety reasons, it is a must to prove that none of the unit’s parts are moving, so nobody can hurt.

The examples above illustrate the necessity of the supervising circuits.

A simple but very useful circuit is to be presented in the followings.
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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.

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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

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Modular PLC hardware basics II.


Staying with the INTERBUS example, the simplest way is to use UART interface (a so called serial line), because it is a common interface in processors and microcontrollers and easy to handle.

For the followings, the CPU card makes use of a microcontroller (MCU) just for the sake of simplicity. The other simplification is that the PSU card has no data interface, just supplies power to the backplane.

So far the modular PLC system used a backplane. This backplane distributed the signal and power lines along the PLC cards, making the connection between them.


At least these backplanes can be viewed as a passive PCB panels which just hold the connectors and interconnect them. (In practice there are some active elements on the backplane, but usually they can be viewed as a big PCB with a bunch of connectors soldered on it).

Modular PLC hardware basics I.

Mainly there are two types of PLC hardware:


– Compact PLCs,

– Modular PLCs.


Compact PLC:

The compact PLC consists of only one housing, and a fix number of on-board I/Os. They are the cheapest category, but this also comes with the most limited functionality.

They usually have less than a 100 (or mostly even less than 30) I/Os, which is mainly digital ones with few (or none) analogue I/Os.
For example: one with 10 pieces of 24V rated digital inputs, 6 pieces of normally open relay contact outputs, one 0-10V analog input and no analog output.
These PLCs usually have a communication interface and in some cases they have a display as well, through which they can be programmed, and this way a communication line is not needed.
The communication takes place via RS-485, CAN or an ethernet based interface, but usually only one of them.
Its program memory is little, enough only for simple tasks. Its internal processor usually is a microcontroller (MCU).

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