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what is the best bus choice for inter-PCB communication ?

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anotherbrick

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hello dear forum,

I have been asked to design modular I/O boards
for a PLC like device distributed to many small PCBs

the I/O modules will be unintelligent 4/4 digital I/O , 4/4 analog I/O
inteligent I/Os like encoder input and step puls/dir output
the max digital I/O number will be less than 32 (4 digital I/O boards)

the MCU will be ESP32

I want to ask the forum
which bus system willl be best between MCU and I/Os for this application ?
shall I design a backplane or can I count on flat cable

thank you
 

Hello,

in real PLC systems the most popular are RS485 (with current loop) as physical layer or CAN (has its own hardware layer). Very often Modbus RTU is used over RS485. If you need comunication for short distances (several meters) you should consider one from serial interfaces like SPI or I2C.

In my opinion SPI may be the best in your case.

Regards
 
I don't understand why you need any kind of interfaces on your board. Basically, GPIO is an interface also :)
 

Hi,

another "what is the best xxxx" question.

It´s impossible to give a good answer without good / enough specifications.

Best in which regard?
* cost
* part count
* pin count
* safety
* low power
* low PCB space
* speed
* isolation
* half/full duplex
* ...

Klaus
 
Boards in a tight box can get away with a lot. Once you
let in the possibility of ground offsets, DC or transient,
choices narrow quickly. SPI assumes a common ground
or close to it, a simple CMOS electrical interface where
anything more than 0.7V ground-ground will be a real
problem. RS-485 and CAN will tolerate a lot more offset
and noise voltage but are power hogs at 40mA per
channel with shunt termination (you might shave a lot
off that with series termination, if).
 
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