Output voltage problem with ST802RT1A

jinkang · ‎2012-09-18

Posted on September 18, 2012 at 17:34

Hi there

I am using the Ethernet Transceiver ST802RT1A to make my own PCB board following the design of STEVAL-PCC012V1 (http://www.st.com/internet/evalboard/product/250577.jsp) except one thing - I changed the magnetic RJ45 from Pulse J00-0086 to another one with a turn ratio of 1:1. But now a problem occurs that the computer cannot setup a communication with the board I made.

When I run the sample Hello World program, I see the transceiver is transmitting some information from the oscilloscope. But the computer cannot receive anything from the transceiver when sniffing with Wireshark. The output voltage level of TD+ and TD- is from -0.7V to +0.7V. I am wondering if it is too low for the computer to detect??? Is it necessary to use a magnetic jack with a turn ratio of 1.414:1 instead??

Thank you!

#st802rt1-ethernet

jinkang · ‎2012-09-18

Posted on September 18, 2012 at 18:32

One thing to correct, the magnetic jack I use has a turn ratio of 1CT:1.

Any clue of this problem??

emalund · ‎2012-09-19

Posted on September 19, 2012 at 17:03

well, the one you sub is 1:1 one way and 1:2.5 the other way.

you state you replace it with one that is 1:1 both ways,

if you do that you need 2.5 times the drive on the transmit (good luck)

substituting with non-identical parts requires VERY careful consideration, not just ''let us see if it works'' (which evidently it does not)

Erik

Tesla DeLorean · ‎2012-09-19

Posted on September 19, 2012 at 18:09

Is it necessary to use a magnetic jack with a turn ratio of 1.414:1 instead?

The data sheet basically states so. It indicates a single acceptable ratio, not a list/range of acceptable ones.

Try confirming if the original Pulse part works with your current circuit, or if there are additional issues.

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jinkang · ‎2012-09-19

Posted on September 20, 2012 at 00:37

Ok, I will try to use the original one, at least one with the same turn ratio. But I am wondering what is the direct effect of choosing a different turn ratio?? Is that a change of the voltage and current, so that either one is not enough to drive the other part of the communication?

emalund · ‎2012-09-20

Posted on September 20, 2012 at 15:48

Ok, I will try to use the original one, at least one with the same turn ratio. But I am wondering what is the direct effect of choosing a different turn ratio?? Is that a change of the voltage and current, so that either one is not enough to drive the other part of the communication?

get an electronics book and read about transformers

Erik

jinkang · ‎2012-09-21

Posted on September 21, 2012 at 12:16

It could be the impedance matching problem. But now the problem is the transformers and jacks

with ratio 1.414:1 recommended by STM in UM0819 User Manual are not available on the market, not even from Digikey, Mouser, Farnell. If the transceiver does necessarily require that ''uncommon'' turn ratio, I would prefer to use another one.