Seitenhistorie

Overview

The linux master is a PC with Linux installed. The kernel is patched with the rt preempt patch to make the system real-time capable. The Ethernet port is used to connect the PC with the EtherCAT network.

Realtime capability

Depending on the requirements, the PC must have very good real-time properties.

Very good real-time behavior (max. jitter < 30 usec) is required if:

• The Linux master must set the clock. E.g. if no distributed clock is used and the bus cannot be synchronized with a slave. See Understanding Synchronisation with Distributed Clocks for more information.

A time buffer, which is larger than the largest expected jitter, has to be used. If the time buffer is too small, then in some cases the timing cannot be met and the data from this cycle is lost.

The smaller this time buffer is, the more time is available for calculations on the master. However, the buffer must be large enough so that the system can reliably meet the timing deadlines. A good real-time behavior enables a smaller time buffer.

The minimum cycletime, and thus the maximum frequency of the control loop, is mainly given by this time buffer plus the calculation time (EEROS).

See Benchmark realtime system (TODO) to benchmark your master.

Ethernet Hardware

The ethernet hardware has to be compatible with the link layer from Acontis. To check the installed network hardware use this command:

lspci | grep Ethernet

This lists all ethernet hardware, which could look like this:

00:19.0 Ethernet controller: Intel Corporation Ethernet Connection I218-LM (rev 04)

sudo lspci -v

you get something like:

00:19.0 Ethernet controller: Intel Corporation Ethernet Connection I218-LM (rev 04)
	DeviceName:  Onboard LAN
	Subsystem: Dell Ethernet Connection I218-LM
	Flags: bus master, fast devsel, latency 0, IRQ 50
	Memory at f7400000 (32-bit, non-prefetchable) [size=128K]
	Memory at f743c000 (32-bit, non-prefetchable) [size=4K]
	I/O ports at f080 [size=32]
	Capabilities: [c8] Power Management version 2
	Capabilities: [d0] MSI: Enable+ Count=1/1 Maskable- 64bit+
	Capabilities: [e0] PCI Advanced Features
	Kernel driver in use: e1000e
	Kernel modules: e1000e

Check with your contact from Acontis if this ethernet controller is suitable for one of there link layers. Check chapter 2.3.1.3 of the Acontis stack documentation for supported network controllers.

Tested Devices

The maximum jitter of a device roughly describes how well real-time requirements can be met.. If a device has a maximum jitter of 100us, then it must be expected that an event is handled up to 100us too late.

• Low Jitter (<40us)

  • Dell Tower OPTIPLEX 9010

    • ~30us max jitter

    • 2 x i7-3770 @3.4Gz

  Medium Jitter (40us...100us)

  • IB908

    • ~100us max jitter

    • 2 x i7-4650U @ 1.7GHz

  High Jitter (>100us)

  • Laptop Dell Latitude E5530

    • ~250us max jitter

    • 2 x i7-3540M @ 3.00GHz

  • Laptop Dell Latitude E5540

    • ~230us max jitter

    • 2 x i7-4600U @ 2.10GHz