Real Time Programming

The previous chapter showed how to write PLCnext runtime applications that do not require real-time, deterministic performance. The priority of the process executing ACF components (including worker threads) is no higher than other threads or processes on the device, and as a result the "jitter" - or the variation in the execution period - is likely to be very high for worker threads. This is fine for many applications, but some applications require a more deterministic execution period, like is available on traditional PLCs.

The Linux kernel on PLCnext Control devices includes the PREEMPT-RT patch, so it is possible to execute code with real-time priority. On its own, the use of this feature requires extensive experience of C++ and POSIX thread handling.

Luckily for us, the PLCnext runtime includes a set of PLC components, which allow users to easily implement C++ applications that have the same deterministic performance as applications on traditional PLCs. One of these PLCnext runtime components is the Execution and Synchronisation Manager (ESM), which - as the name suggests - is responsible for scheduling and executing real-time tasks.

The ESM uses terminology from the IEC 61131 standard, which PLC programmers will be familiar with. For example, a program is a piece of code that will be executed in real time. One or more named program instances are created in a task. Tasks can be executed cyclically with a fixed period, or executed when other criteria are satisfied e.g. on a specified event. When a task is executed, the program instances defined in that task will be executed one after the other, in the order specified. It is possible for a PLC application to contain multiple tasks. On PLCnext Control devices, it is possible to assign tasks to one of multiple ESM instances. Each ESM instance corresponds to a core on the device CPU, so (for example) an AXC F 2152 with two CPU cores has two ESM instances in the PLCnext runtime.

There is more information about the ESM in the PLCnext Info Center.

In the previous chapter we saw how to configure the Global Data Space using a .gds.config file in XML format. The ESM is configured in a similar way, using an .esm.config file. This configuration file:

  • Defines real-time tasks.
  • Assigns each task to an ESM instance.
  • Defines real-time program instances.
  • Assigns program instances to tasks.

This chapter will discuss C++ programs, and look at how to execute these on a PLCnext Control device.