Ethernet Funnies

Sometimes it is very hard to connect to an embedded system, because the designers cut some corners to simplify the system and keep memory use down, or simply because the system is prehistoric and full of bugs.

One such device worked fine provided that there was a little ethernet switch between the laptop machine and the target, but a direct connection between the laptop machine and target device only worked about half the time. Even the little switch sometimes could not connect.

MAC, PHY, MAG

An ethernet interface device consists of three main parts: The Media Access Controller (MAC), the Physical Interface (PHY) and a set of transformers - the Magnetics. When you plug a cable in, the PHY sends out little pulses to figure out what is going on and then swaps the wires around internally and changes the speed and duplex settings to make the interface work.

The trouble was that the target only supports 100 Mbps, while the laptop machine wanted to run at 1 Gbps and the two just could not reach agreement.

Ethtool

The ethtool program can be used to configure the ethernet interface device manually:
# ifconfig em0 up
# ethtool em0
# ethtool -s em0 speed 100 duplex full autoneg off
# ethtool em0
# ifconfig em0 192.168.111.1 netmask 255.255.255.0

That forced the laptop machine to the correct speed and duplex settings, turned the broken auto negotiation off and then life was good.

Shortly after writing the above, I ran into a case where the embedded system works better with a 100 Mbps half duplex connection, but the auto-negotiation usually resulted in a full duplex connection.

# ethtool -s em0 speed 100 duplex half autoneg off

Problem fixed.

These weird issues are usually due to a bad board layout around the ethernet chip set.

Reference

More information here:
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Deployment_Guide/s1-ethtool.html

La voila!

Herman

Comments

Unlock CRA PDF Forms

Unlock Canada Revenue Agency PDF Forms It appears that there is a relatively new PDF feature to prevent casual copying and saving of a file and that some programs save PDF files with these foolish features active by default. Many forms from the Canada Revenue Agency are locked in this way, which makes it difficult to do one's taxes, since one can fill the form, but cannot save it. One can only print the form. It should be possible to print to a file or export it to a new PDF file, but it is far better to reset the annoying anti-taxpayer flags, since the 'printed' form cannot be edited easily any more and I always manage to make a mistake or three that need to be corrected after review. If there is a Linux (virtual) machine handy, install qpdf and use it to reset the silly flags: $ su - password # dnf update # dnf install qpdf # exit $ qpdf --decrypt lockedfile.pdf unlockedfile.pdf One doesn't need a password to unlock these flags, so the fix is instant. La voila! He...

Parasitic Quadrifilar Helical Antenna

This article was reprinted in OSCAR News, March 2018: http://www.amsat-uk.org If you want to receive Satellite Weather Pictures , then you need a decent antenna, otherwise you will receive more noise than picture. For polar orbit satellites, one needs an antenna with a mushroom shaped radiation pattern . It needs to have strong gain towards the horizon where the satellites are distant, less gain upwards where they are close and as little as possible downwards, which would be wasted and a source of noise. Most satellites are spin stabilized and therefore the antenna also needs circular polarization, otherwise the received signal will flutter as the antennas rotate through nulls. The helical antenna, first proposed by Kraus in 1948, is the natural solution to circular polarized satellite communications. It is a simple twisted wire - there seems to be nothing to it. Various papers have been published on helix antennas, so the operation is pretty well ...

Weather Satellite Turnstile Antennas for the 2 meter Band

NEC2, 2 m band, 146 MHz, Yagi Turnstile Simulation and Build This article describes a Turnstile Antenna for the 2 meter band, 146 MHz amateur satcom, 137 MHz NOAA and Russian Meteor weather satellites. Weather satellite reception is described here . A quadrifilar helical antenna is described here . Engineering, is the art of making what you need, from what you can get. Radiation Pattern of the Three Element Yagi-Uda Antenna Once one combine and cross two Yagis, the pattern becomes distinctly twisted. The right hand polarization actually becomes visible in the radiation pattern plot, which I found really cool. Radiation Pattern of Six Element Turnstile Antenna Only a true RF Geek can appreciate the twisted invisible inner beauty of a herring bone antenna... Six Element Turnstile Antenna Essentially, it is three crosses on a stick. The driven elements are broken in the middle at the drive points. The other el...

Aeronetworks sro - Airborne Linux

Search This Blog