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).


If the backplane is just for this simple task, it can be omitted, to make the overall costs smaller, and the PLC system more flexible. For this to be done, the active elements should be relocated to the PLC cards’ PCB and the connectors must be redesigned, but these modification are just little ones.
However the overall performance is not affected with either solution. Systems with backplanes look more robust and noise immune, and systems without backplanes are smaller and can be placed directly to a DIN rail.


By the way, nowadays modular PLC manufacturers use either systems with backplane (like Schneider electric, Allen Bradley…) or without it (Beckhoff, Wago…). Its just a question of decision which is to be used.


So let’s look at a simplifed modular system where the PLC cards are just connected to each orther, not using backplane.



Simplified modular PLC internal electronics


There is a PSU card which supplies the +5V for the microcontrollers, and the +24V for the higher power elements.

The CPU card has an I/O interface which can be any type of industrial communication (Modbus TCP, DeviceNet, IEC 60870-5-101, etc.).

The MCU start the I/O interrogation by sending a message on its UART’s TX line. This arrives to the first card MCU’s RX line, which processes it,  inserts its own data and sends it towards the next I/O card etc. This process continues to the end of the PLC system, where the last card should bring the whole meaasge back to the CPU card. For this task a so called “Closing card” must be inserted to the end of the line. This card’s function is to make a short circuit between the RX and TX line enabling the signal return.


The PLC cards’ I/O interface can be any type (digital input, digital output, analog intput, counter input, serial line output…), only I/O shown for clearness. Every card uses a MCU inside regardless of the function, since it can easily serve all the functions (configuration, status generation, error handling and backplane communication) needed for a given card. (In real the MCU can be a microcontroller, an ASIC, DSP, FPGA or a processor which best fits in the desired operation)


The connectors are 6 pole ones, incorporating the signal and the power lines.
This example configuration is based on some simple rules:

– the first card from the left must be the power supply card,

– the second card must be the CPU card,

– the rest of the cards can be any type in any order,

– the last card in the line must be the closing card.


This type of operation can be seen on older Beckhoff systems which used K-Bus as a “backplane” communication.

At first it seems unnecessary to use a Closing card at the end of the line, because the last card’s electronics could feed back the message to the CPU card.
At the same time one can notice that MCU’s RX and TX line is not so powerful for high speed communcation, so it would be practical to implement buffers to the transmitting lines.
These two functions can be implemented using a dual buffer on each card.



PLC card’s internal electronics with automatic backplane signal transmission


Here a SN74LVC2G241 type IC is used. Half of this dual buffer simply repeats the outgoing signal giving it more power, the other half is controlled by the 4. pin of the right hand side connector (named CTL). If the PLC card is in the middle of the line, this CTL pin is connected to the ground potential (through the following card) thus disabling the 2. buffer, but if the card is at the end of the line, this pin is left floating, enabling it to redirect the signal back to the CPU card.

In other words the SN74LVC2G241 IC act as a mechanical switch, that can redirect the last TX line to the CPU.


Creating the signal feedback path is automated this way, so the customer don’t have to deal with closing the line.


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