Crossover function in the standard.

I don't know if the wording is in the standard, but as I understand it, it is required to work. The only difference between MDI and MDI-X ports is the crossover of the wires.

In the early, and not so early, days of 100baseTX I knew of some 10baseT devices that would not connect properly to 10/100 ports.

Also, I once had a cable modem that wouldn't connect properly to a 100baseTX NIC at 100. Connecting through a 10baseT repeater it worked fine. (At the time cable wasn't that fast, anyway, though it may be now.)

It seems that there are devices that won't work together. As far as I know, that is unrelated to crossover, NIC-NIC, NIC-repeater, NIC-switch, repeater-repeater, or any other combination.

I believe so, yes. Though there were some pre-standard devices that didn't do link pulses at all.

As I understand it, there is no lower limit for UTP ethernet.

-- glen

To the OP:

Your experience is different from mine:

Well when I do this it ALWAYS works.

I have done it hundreds of times over more than 10 years and it just works. PC to PC, Router to Router, Switch to Switch.

Of course sometimes a miss-configuration results in terrible performance due to duplex missmatch.

I would worry first of all about your cables. Are the pairs wired correctly? How do you test them?

Ready-made, certified cables tested using stock standard cable tester.

With regard to my original question, I dig deeper and have found some problems with MB layout. Basically the network adapter (Intel btw) is completely integrated in the chipset, with just four lines on the outside (+ 3 lines to control LEDs). On the motherboard there is only integrated RJ45 module jack (with transformer & LEDs).

The design violates the rules regarding the length and spacing of the routes. I still have to find what exactly is wrong with it but probably too long tracks increase loses to unacceptable level and it doesn't work with some adapters.

Best regards, Trebor.

Trebor Mushroom wrote: (snip)

It shouldn't be that hard to get right before the transformer. It is a little harder to keep the signals balanced after the transformer through long PCB traces.

It has to last through 100m of cable, so a few inches of PCB trace normally won't have that much loss. It gets somewhat harder for gigabit, but you didn't say that. It is nice to impedance match, though, in any case.

-- glen

I'm not sure what do you mean by "keeping the signals balanced after the transformer" - firstly there are virtually no tracks _after_ the transformer, secondly isn't the goal here keeping the tracks equispaced and short as possible between the chip set and the transformer to to avoid crosstalk and pick-up of unwanted interference? (see below)

Same as above, I'm talking here about the signals between the PHY in the IC and the transformer, which are normal (i.e. voltage level) fast digital signals with all associated problems. After the transformer (as you rightly pointed out) it doesn't matter because they became differential pair (current) lines up to 100 meters long, inherently prone to the problems mentioned above.

The engineer who laid out the MB was thinking similarly to you because all he did was terminating the lines with resistors. As it shows :-( it is not enough under some circumstances.

Re-designig the MB is not an option for us, so we will have to live with it I'm afraid. Given some time I will try to probe the lines to learn what exactly goes wrong when using crossover cable as compared to connecting it to the switch.

Anyway thanks for your input.

Best regards, Trebor.

Minimizing radiation (RFI) depends on the signals being balanced. The currents are opposite in the two wires of the pair, such that they pretty much cancel out as far as a radiated signal.

If you run them across the PC board after the transformer, you have to be extra careful to keep them balanced. Stray capacitance differences could easily change that. That effect is not there before the transformer.

That should only be true for long cables, approaching 100m.

So, you don't have to be quite as careful before the transformer, but they are still fast digital signals. Normal precautions still apply.

I still don't see how it could depend on crossover. I could easily believe cable length, though.

Another that I thought of, some use a transformer that isn't 1:1. The voltage (and impedance) is different across the transformer. If that was wrong, you could get unexpected results. Otherwise, it is hard to guess at all the possible reasons.

-- glen

The signals coming out of an Ethernet PHY are balanced differential signals, capable of driving 100m+ of cable. They are not "normal fast digital signals" (e.g., 5V swing with nanosecond risetimes). The purpose of the transformer is to provide electrical isolation between network devices.

They were differential pairs even before the transformer. The transformer is *not* a balun.

I seriously hope he did not either: (a) include line terminators for the transmit pair (which would effectively reduce the available drive current in half), or (b) include an *extra* set of terminators on the receive pair, in addition to the terminators normally located at the transformer. This would similarly reduce the receive levels.

If the designer included incorrect terminations, you have little choice but to redesign or rework the motherboard.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

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Rich Seifert wrote: (snip)

and to absolutely positively guarantee that the signal is balanced. Maybe you could do that with a ferrite ring, but then that is somewhat related to a transformer.

-- glen

To the extent that you mean "the transformer takes out any common mode (e.g., DC offset) in the signal", then you are correct. However, if the signal has short-term imbalance (e.g., asymmetrical high/low times), the transformer will not correct the problem.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

Rich Seifert wrote: (snip, I wrote)

No common mode is what I meant. Some common mode signals will be due to differences in the two drivers.

Do they also put electrostatic shielding on the transformer to remove capacitive coupling between the windings?

-- glen

Generally not in that type of transformer. The windings are close-coupled (typically wound as a pair, and sometimes even a twisted pair); capacitive coupling is not always a detriment in a signal transformer application.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

Send replies to: usenet at richseifert dot com