>> The center taps are for Power-over-Ethernet. Apparently this device
>> does not support that. Instead they terminate the line with resistors.
>>
> more specifically, the center tap terminations, terminate the COMMON MODE mode of the line. The DIFFERENTIAL MODE of the line is terminated by the secondary.
Yes; I did a lot of reading on this.
The common mode termination is really for EMC and ESD performance. It
does nothing for the data transfer. There are some cheap RJ45
integrated-mag jacks which don't have any centre taps; you can get
them for USD 0.30. Presumably they are used on products which fake the
EMC compliance i.e. most stuff made in China :)
On the assumption that the 75R is the right value (which many
question, but others say it is a reasonable compromise, especially as
you don't know if cat5 v/ cat6 cable will be used) you cannot draw
power from the taps.
Well, you could draw a tiny amount of power, but probably not enough
to power an ethernet device :)
The other approach is to draw the required/available power while
emulating the 75R resistance over the required frequency range. This
is possible but not trivial. I haven't read up on it but probably some
chips exist.
Reply by ●November 20, 20192019-11-20
On Tuesday, November 19, 2019 at 9:22:41 AM UTC-5, Rob wrote:
> Peter <nospam@nospam9876.com> wrote:
> > This is a common type of jack with integrated magnetics
> >
> > http://www.kosmodrom.com.ua/pdf/HR911105A.pdf
> >
> > It connects the txf centre taps via 75R resistors to a 1000pf cap
> > which then (pin 8) goes to the circuit ground.
> >
> > What does this do?
>
> The center taps are for Power-over-Ethernet. Apparently this device
> does not support that. Instead they terminate the line with resistors.
>
more specifically, the center tap terminations, terminate the COMMON MODE mode of the line. The DIFFERENTIAL MODE of the line is terminated by the secondary.
m
A very useful and interesting reply.
One can achieve the BST while protecting the 75R resistors getting
blown up by cheap POE injectors, by putting capacitors in series with
them (22nF 100V in one jack I have seen, by Hanrun). You can't draw
power from the centre taps then but at least your product won't get
blown up, so if not using POE this seems to be a no-brainer.
Hanrun jacks are not cheap (though nowhere near as madly priced as the
"western brands" but are popular, and are widely counterfeited.
One thing I wonder about is whether, on a 10/100 product (not
gigabit), there is any issue with having no connection to pins 4,5 and
7,8. These are used on gigabit networks only. However it may be that
the consequent lack of the BST on these two wire pairs might
compromise the EMC angle. I have not seen this mentioned. Probably it
*is* a problem otherwise one could get power via these pins *alone*,
if the product is only 10/100.
This is one attempt
http://peter-ftp.co.uk/screenshots/20191119551282618.jpg
but the diodes mess up the BST on the 10/100 mode, and there is no
gigabit support. You get POE via both modes.
This one seems to cover all bases, again only for 10/100. No POE of
course. It is quite cheap
http://peter-ftp.co.uk/screenshots/20191119201293018.jpg
OTOH there is criticism of the BST patent. The reasoning appears to be
poor. It just seems better than nothing, so everybody is using it.
Reply by Arie de Muynck●November 19, 20192019-11-19
>Peter <nospam@nospam9876.com> wrote:
>> This is a common type of jack with integrated magnetics
>>
>> http://www.kosmodrom.com.ua/pdf/HR911105A.pdf
>>
>> It connects the txf centre taps via 75R resistors to a 1000pf cap
>> which then (pin 8) goes to the circuit ground.
>>
>> What does this do?
>
>The center taps are for Power-over-Ethernet. Apparently this device
>does not support that. Instead they terminate the line with resistors.
>
>Likely another version exists that does not do that but feeds the center
>taps to connector pins.
Indeed it does but it has more pins and isn't compatible with the more
standard integrated-magnetics jack.
http://peter-ftp.co.uk/screenshots/20191119591275514.jpg
However, it cannot be quite that since the HR911105A doesn't give you
access to the centre taps. So why are the centre taps there?
Reply by Rob●November 19, 20192019-11-19
Peter <nospam@nospam9876.com> wrote:
> This is a common type of jack with integrated magnetics
>
> http://www.kosmodrom.com.ua/pdf/HR911105A.pdf
>
> It connects the txf centre taps via 75R resistors to a 1000pf cap
> which then (pin 8) goes to the circuit ground.
>
> What does this do?
The center taps are for Power-over-Ethernet. Apparently this device
does not support that. Instead they terminate the line with resistors.
Likely another version exists that does not do that but feeds the center
taps to connector pins.