OpenPLC can be programmed in all 5 IEC 61131-3 languages: ST, IL, LADDER, FBD and SFC. On top of that, it lowers the barrier of entry to developing this kind of industrial hardware by being compatible with all the favorites Arduino, Raspberry Pi, Windows, Linux, etc.

“The OpenPLC is the first fully functional standardized open source PLC. We believe that opening the black-box of a PLC will create opportunities for people to study its concepts, create new technologies and share resources.”

http://www.openplcproject.com/

for example, there is no need RTC in many projects, the workers only turn on the device when they are working, ….

So, for the cheap PLC, to save these costs，the market need many differents kinds PLC.

all we need to do is how to let user feels more easy, and make the PLC more strong to against the interference.

sorry for my bad(strange) english.

Some applications that the OSPLC (Open Source PLC) can be used in:

Machine Control
Automatic Test Equipment (ATE)
Computer I/O and communication expansion
Data logging to EEPROM memory
Process Control
Process Monitoring
Home Automation

Features include digital I/O, analogue inputs, RS-232 port, RS-485 port, USB device port, JTAG debug port, real-time clock (RTC) with battery backup, Flash memory, SRAM and EEPROM.

Programming of the PLC can be done in C and C++. The internal microcontroller is directly accessible to the programmer.

Open-source software C programming tools are available for programming the PLC.

The boards were designed using Altium designer. The download below is for the Altium designer source files for all three boards.

If you would just like to look at the schematic diagrams for each board and do not have Altium designer, see the schematic in PDF format for each board below.

All new projects on the Starting Electronics website will be based on the open source KiCad EDA tools for schematics and PCBs.

In industrial applications, controlling relays, servos, solenoids, and the like isn’t just a matter of wiring in an Arduino and plugging in some code. No, for reliable operation you’ll need a PLC – a programmable logic controller – to automate all your hardware. PLCs are usually pretty expensive pieces of hardware, which led [Warwick] to come up with his own. He built two versions, one large and one small that can handle just about any task thrown at them.

Both devices are powered by an ATMEL SAM7S ARM chip running at 48 MHz. The smaller of the two devices has 10 digital inputs, 4 analog inputs, and 8 digital outputs able to sink 200 mA each. The larger PLC has 22 digital ins, 6 analog ins, and 16 digital outputs. Both of these devices have a ton of connectivity with USB, RS-232 and RS-485 ports

SimpleModbus is a collection of Arduino libraries that enables you to communicate serially using the Modicon Modbus RTU protocol. Both SimpleModbusMaster & SimpleModbusSlave implements function 3 and 16 in addition SimpleModbusMaster implements function 4. Broadcasting is supported on both libraries for function 16. Both libraries share the exact same API.