I'm designing a PCB to host an Applanix APX15 GPS/inertial navigation device. The hardware integration manual (which seems not to be publicly available) shows that a 10/100 BaseT Ethernet is supported via four pins on the board and explains that the magnetics are on the APX15, but little else.


Ethernet note

Is it really just as simple as connecting these four lines to the RJ45 jack? Are other components required? How about trace widths/lengths and impedance? Grounding? Are there any gotchas with respect to e.g., routing TD to RD and vice versa? (I'm having difficulty finding schematics or explanations of this topic suitable to a novice like me.)

Note: I have tried naively connecting these pins to an RJ45 jack with no magnetics, but it doesn't work. Ethernet works fine with the test board (which has its own magnetics identical to those on the APX15) that came with the device.

Edit: There is a Pulse HX1188FNL chip (magnetics) on the board. Link goes to a datasheet with a schematic that reflects the APX documentation, with the addition of connections of the remaining pins to chassis ground via a resistor and capacitor.

Edit 2: Found layout guidelines for the magnetics on the APX. For now I'll assume that this 4-lead connection will work, but my impedances are messed up.

Edit 3: It has been suggested that this is an MDI-MDIX problem. If I'm connecting a straight-through (not cross-over) ethernet cable from my computer to the device (which works on the test board), the device should behave as the Hub MDI-X device in this image:


I had connected pin 1 to TD+ but perhaps I should be connecting pin 1 to RD+, etc.

Edit 4: It turns out the four leads do have to be reversed, which is equivalent to using a cross-over cable. So RD+ goes to TD+, and RD- to TD-, etc. Then, everything works as expected.

  • \$\begingroup\$ @jsotola I'm almost certain I didn't (not for want of checking and re-checking!) However, if I knew for certain that it should be enough to simply connect the lines as described, I'd know where the trouble lies. \$\endgroup\$ – Rob Skelly Aug 25 '20 at 1:08
  • \$\begingroup\$ Seems like you need to look at the schematic of the test board and maybe its parts list. \$\endgroup\$ – Chris Stratton Aug 25 '20 at 2:14
  • \$\begingroup\$ @ChrisStratton That information is not available. \$\endgroup\$ – Rob Skelly Aug 25 '20 at 2:28
  • \$\begingroup\$ Could it be a simple MDI/MDIX problem? \$\endgroup\$ – fraxinus Aug 25 '20 at 11:36
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    \$\begingroup\$ @RobSkelly I would try both ways. Most modern NICs and switches can sense MDI/MDIX connection, but may as well fail to sense anything or sense something wrong. \$\endgroup\$ – fraxinus Aug 25 '20 at 18:58

Like the note says, the magnetics are implemented on board. So a simple 8P8C (RJ45) connector should work. Also, you need specific impedance which can be achieved with correctly sized traces and grounding. There are plenty of application notes from different manufacturers how to design an Ethernet interface, and how to calculate trace width and spacing to get a certain impedance. Ethernet uses 100 ohms differential impedance.

  • \$\begingroup\$ Thanks for the answer, but the note says magnetics are on the board. I've added an edit with information about the chip and a link to the schematic. There are plenty of application notes but for some reason at the moment I can't find anything that details this specific circumstance. \$\endgroup\$ – Rob Skelly Aug 25 '20 at 4:55

Ethernet transmission lines are very sensitive -- low level analog signals, impedance matching required, high speeds. This article provides some guidelines. If you can't see the design of the test board, look at some open source designs that support Ethernet, from Arduino, Sparkfun, or Adafruit. They publish their schematics and PCB design files. Good luck!

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    \$\begingroup\$ 10 and even 100 BaseT Ethernet work pretty well on breadboards (at least, for small distances). Gigabit is another matter. \$\endgroup\$ – fraxinus Aug 25 '20 at 11:39

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