# Choose a Linux master

## Overview

The linux master is a PC with Linux installed. The kernel is patch 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:

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).

graphic time management on master

See Benchmark realtime system to benchmark your master.

This list contains some tested systems. There are also some tips on how to find a PC with good real-time performance.

## 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)
00:19.0 is the ID of the ethernet controller. List all information with:
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+
Kernel modules: e1000e