The PLCopen basis is provided by the world wide standard IEC 61131, and especially Part 3 - Programming Languages which is providing the Logic, as developed by the International Electrotechnical Committee. Since IEC holds the copyright, there are no downloadable versions of the standard available.

The International Standard IEC 61131 provides a complete collection of standards on programmable controllers and their associated peripherals. It consists of the following parts:

IEC 61131-1: General information

Contains the definitions and identifies the principal characteristics relevant to the selection and application of programmable controllers and their associated peripherals. Current edition is 2.0 from 2003. Read more on IEC 61131-3-1

IEC 61131-2: Equipment requirements and tests

Specifies equipment requirements and related tests for programmable controllers (PLC's) and their associated peripherals. Current edition is 4.0 from 2017. Read more on IEC 61131-2

IEC 61131-3: Programming Languages - providing the basis for PLCopen

The third part provides the only global standard for industrial control programming. It harmonizes the way people design and operate industrial controls by standardizing the programming interface. A standard programming interface allows people with different backgrounds and skills to create different elements of a program during different stages of the software life-cycle: specification, design, implementation, testing, installation and maintenance. Yet all pieces adhere to a common structure and work together harmoniously.
IEC 61131-3 defines, as a minimum set, the basic programming elements, syntactic and semantic rules for the most commonly used programming languages. This includes the graphical languages Ladder Diagram (LD) and Functional Block Diagram (FBD), and the textual languages Instruction List (IL) and Structured Text (ST), as well as the Sequential Function Chart (SFC) language, used to structure the internal organization of a program. Via decomposition into logical elements, modularization and modern software techniques, each program is structured, increasing its re-usability, reducing errors and increasing programming and user efficiency.
This website is mostly dedicated to IEC 61131-3 as the Logic part, and contains a lot of information on it as well as extensions based on it like Motion, Safety, Communication and Exchange. The current version is 3.0 from 2013. Read more on IEC 61131-3

IEC 61131-4: User Guidelines

A technical report providing general overview information and application guidelines of the standard for the end user of programmable controllers. Current edition is 2.0 from 2004. Read more on IEC 61131-4

IEC 61131-5: Communication

Defines the data communication between programmable controllers and other electronic systems using the Manufacturing Message Specification (MMS), according to International Standard ISO/IEC 9506. Current edition is 1.0 from 2000. Read more on IEC 61131-5

IEC 61131-6: Functional Safety

This Part of the IEC 61131 series specifies requirements for programmable controllers (PLC's) and their associated peripherals, as defined in Part 1, which are intended to be used as the logic subsystem of an electrical/electronic/programmable electronic (E/E/PE) safety-related system. Current edition is 1.0 from 2012. Read more on IEC 61131-6

IEC 61131-7: Fuzzy control programming

Defines basic programming elements for fuzzy logic control as used in programmable controllers. The current edition is 1.0 from 2000. Read more on IEC 61131-7

IEC 61131-8: Guidelines for the application and implementation of programming languages

The Technical Report (TR) provides a software developers guide for the programming languages defined in part 3. The current edition is 3.0 from 2017. Read more on IEC 61131-8

IEC 61131-9: Single-drop digital communication interface for small sensors and actuators (SDCI)

This standard specifies a single-drop digital communication interface technology for small sensors and actuators SDCI (commonly known as IO-Link), which extends the traditional digital input and digital output interfaces as defined in IEC 61131-2 towards a point-to-point communication link. Current editon is 1.0 from 2013. Read more on IEC 61131-9

IEC 61131-10 PLC open XML Exchange Format

This new IEC standard is based on the original PLCopen XML specification. With the release of the 3rd edition of IEC 61131-3 in 2013, a major overhaul was needed to include the changes and extensions like object oriented features. This new version is not compatible to previous versions of PLCopen XML. IEC 61131-10 will be released in 2019. Read more on IEC 61131-10