Designing a Custom PLC System with 8 Pinput and 8 Output

Thread Starter

Dragonoid

Joined Nov 27, 2017
64
Hello all,
I am designing a PLC System with 8 inputs and 8 Output With the following Specs-
1) 8 Digital Input (Isolated) 24v in
2) 8 Digital Output (Isolated) 24v Out
3) 2 Analog Input (0-10v and /or 4-20 mA) Isolated or Non Isolated
4) 2 Analog Out (0-10v and /or 4-20 mA) Isolated or Non Isolated
5) Isolated RS232 Port (+/- 15V Levels)
6) USB Firmware Update

So I have Finalized some Circuits and components for the above-specified system

1) INPUT SECTION
For the Input Stage, I will use ACPL 214 Opto Because it is an AC Input OPTO that will protect the internal LED In case of reverse polarity.

PLC INPUT STAGE.png
Indication LED is used on the Secondary side to know if the OPTO is healthy and serves as a normal indicator for the corresponding input.
2) OUTPUT SECTION
The output section comprises VOD205T OPTO which is Dual OPTO in a single package with I\( Icmax \) of 50mA.
Which in turns drive s an Dual Package MOSFET DMN6040SSD
PLC OUTPUT STAGE.png


Now the I need to select the circuits for -
3) 2 Analog Input (0-10v and /or 4-20 mA) Isolated or Non Isolated
4) 2 Analog Out (0-10v and /or 4-20 mA) Isolated or Non Isolated
5) Isolated RS232 Port (+/- 15V Levels)
Let me know your feedback and suggestions to bring this to reality.
Thanks
 

sghioto

Joined Dec 31, 2017
5,430
Starting with the input section. Isn't R1 at 520 ohms rather low for a 24 volt input? That's appx 44ma, getting close to the max input of 50ma. Seems like about 10ma should suffice with R1 at 2.2K. I could be wrong. Have you tested the input section measuring the collector -emitter saturation voltage of the opto with different values of R1?
 

Thread Starter

Dragonoid

Joined Nov 27, 2017
64
Starting with the input section. Isn't R1 at 520 ohms rather low for a 24 volt input? That's appx 44ma, getting close to the max input of 50ma. Seems like about 10ma should suffice with R1 at 2.2K. I could be wrong. Have you tested the input section measuring the collector -emitter saturation voltage of the opto with different values of R1?
Yes You are right, This was the error in the schematic. Thanks.
 

jiggermole

Joined Jul 29, 2016
161
Curious why you'd roll your own plc. Automation direct has a very decent selection of cheapo plcs that meet all your requirements of inputs and outputs. At reasonable prices as well. The software is a bit crunchy but its free for most of the brands.
If its for your own entertainment, for the a/d and d/a i might look into a small microcontroller to handle the detection and re-configuring of the analog information. Some i/o cards I've seen use an fpga in them to be able to re-configure the inputs and outputs on the fly. Using a small micro you could have it handle the circuit protection and use i2c or spi to connect them to the brain. Instead of trying to design discreet circuitry to do the same.
 

PaulNewf

Joined Mar 24, 2020
17
Verify whether power will be 24VDC, or wider. If this 24VDC is from a battery, truck, or boat system voltages typically hit 28VDC during charging, and under 24VDC when draining. In those cases better to design for wider range like 20-30VDC.
 

MisterBill2

Joined Jan 23, 2018
19,422
"Designing your own PLS system" iis actually a monster undertaking. The big deal of a PLC is the hardware package and the software package.
So you might be able to assemble some I/O to use with a cheap micro board that already has analog inputs and digital I/O and run some sort of code, but if you are wanting a package that is durable and ready to use, the money is indeed far better spent on a system from Automation Direct. For many of their products the software is free, and that saves you HUNDREDS OF DOLLARS. Plus they do have free customer support.
One caution is to be sure to pay the invoice. They are inexpensive but they must be paid.
 

MaxHeadRoom

Joined Jul 18, 2013
28,827
For me the definition of a PLC is not just any logic controller, but should also have the ability to monitor and visibly display the various Boolean arrangements and the consequent output result.
This was the original concept of what a so named PLC was desired to do.
For the desired 8 I/O, the Smart Relay already fulfills this requirement.
 
Top