Den tirsdag den 13. marts 2018 kl. 16.48.17 UTC+1 skrev John Larkin:
ESP8266, and you can have WiFi on anything with a serial port
a module with flash and antenna is less that an FTDI chip at digikey
Oooh, ick. If everything were wireless, the next interfering signal I'd have to track down could be... anything, anywhere
To get rid of a tangle, consider instead a multiplicity of IRDA-equipped gizmos, all with eyepatches.
I use a bespoke application written by someone else, however this page should help:
-- Mike Perkins Video Solutions Ltd www.videosolutions.ltd.uk
Instead of hubs, we'd use mirrors.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Nice. Thank you.
I doubt if there is a 48VDC passive PoE system in the wild. It's possible, but I have yet to see one. Mostly, I see 5V, 12V, and 24V passive PoE systems. Mostly, they go through adapters like this: So, the most you'll need to protect against is 24VDC.
Not exactly. I said with 802.3af and similar active PoE systems, you should not have a problem. It's the passive power sources, that have no smarts and often no protection, that is likely to cause problems.
You will anyway. There is wide range of contrivances that use RJ45 connectors that might do damage. Short list: Motorola, Kenwood, Yeasu, and Icom 2-way radio mic connectors. Video terminals and teletype machines. RJ45 RS232 devices such as terminal servers. Light pens. Many telephone systems (all with PoE). Whatever else I forgot. It will be difficult to protect against all these.
Besides users that cram a USB connector into the ethernet jack, there's also the problem of the gold wire connections being mangled inside the RJ45 jack. Three ways that I know of to do that:
The mangled RJ45 has become common enough that designers are defending themselves by making the connector an easily replaceable part. Most common is using a dual RJ45 "barrel" connector. Something like this: If the connector gets mangled, just unplug, remove and replace.
Anyway, good luck.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Yes. But if the passive poe is only used on one jack, then that jack should already have been assigned and in use. So it is not available for someone to accidentally plug another RJ-45 in that socket.
Yes, passive poe is the thing I am concerned about.
Good list. Hopefully none of these will be anywhere near the system when it is installed.
Amazing what some people can do. I had one customer who plugged DE-9 connectors in upside down. It must have taken a huge amount of force.
Looks like they have the nut on the wrong side of the gasket.
If you want isolation, you need co-ax, not twisted pair.
We're designing an RF prototyping platform that will use thin co-ax jumper leads, with U/FL (IPEX) or MMCX connectors.
Clifford Heath.
Thanks. I need bidirectional control, and would like to minimize RFI to external circuits. Ordinary unshielded CAT5 seems to have little radiation, and shielded and double shielded should be even better.
Coax is single-ended, so radiation from sharp transitions can be a problem. Twisted pair is balanced, which reduces transition noise.
The coax shield can lead to ground loop problems. RJ-45 is inherently isolated by the coupling transformers. In the case of ground loop problems, there is no need to ground the shields at both ends since they are not involved in data transmission. One end can connect to ground, and the other end to the case through a 100k resistor in parallel with a 0.1uF or 0.01uF bypass cap to reduce risk to ESD.
Recall there are many diffeent versions of coax. Some of the cheap versions are only good to perhaps -80dB. Double shielded coax is better, but it can be very stiff. Some double shielded RJ-45 cables are advertised to be very flexible.
What's that supposed to mean?
Coax is balanced too, with the current on the central conductor being accompanied by an equal and opposite current along the shield. Equal to 0.03% or so, better than twisted pair.
[...]Nothing prevents the use of coupling transformers at the end(s) of a length of coax.
Jeroen Belleman
Fast transitions are wideband. Coax shielding degrades at high frequencies.
Cite? I have a few links on coax shielding effectiveness. They range from
57dB to 115dB for solid coax. I was unable to find anything on shielded twisted pair effectiveness and would appreciate any links you can give.In any case, I'm worried about RJ-45 cable noise affecting nearby sensitive circuits. So far, I haven't seen any problems from the LAN even when unshielded, but local FM stations can be a problem.
I need to tranfer power. That is done in RJ-45 using poe. Coax would require a separate cable and connector.
I also need bidirectional data transfer. That means another coax. All these cables and connectors would take time to connect and disconnect. Two identical coax cables could be mistakenly swapped. RJ-45 puts it all in one connector.
No, that doesn't work. It allows return currents to flow on the outside of the shield. The makes the cable radiate RFI. To prevent it, the coax must be grounded to the case. This can provoke ground loops.
Oh dear.
To have any current on the outside of the shield, you need a closed loop. With a coupling transformer at the end, you've just opened that loop, ergo, no current. You're supposed to have a termination resistance at the other end of the transformer, which will absorb all incident energy.
I'll admit that some current can still flow because of the inter- winding capacitance of the transformer. It may even resonate with the common-mode inductance of the cable, which would be unlikely, but bad. A few hundred Ohms from screen to GND across the transformer possible with a series cap to prevent DC and LF current, will damp that efficiently.
Jeroen Belleman
You're never going to get the same shielding effectiveness from a twisted pair than you can get from coax. Twisted pairs radiate, because the fields of the two wires do not cancel even in idealized theory. At least in theory, coax has no external field at all. I've made some measurements on actual coax shielding effectiveness, see . I did not do so for twisted pairs, alas.
It's not hard to send bi-directional data and DC power all over one single coax.
Jeroen Belleman
You do have a loop. The transfomer must match the 50 ohm cable. The center conductor has voltage applied. The return path is through the 50 ohms to the coax shield. Current can then flow on the inside and the outside of the coax. Current that flows on the outside can radiate RFI.
The coax shield must be grounded with a circumferential ground, such as a BNC connector. This keeps the return current on the inside of the shield.
For example, see Analog Devices, "EMI, RFI, and Shielding Concepts"
See Fig. 8, page 11
See page 12:
"For high-frequency interference (>1 MHz), the preferred method is
the shield and the connector, and maintaining metal-to-metal continuity between the connectors and the enclosure."
There are about as many ways to measure shielding effectiveness as there are people making the measurements!
Nice curves, but notice the results at 100MHz. This is the frequency where the transition risetimes start, and they can extend through the GHz region.
You show UT141 with a solid copper shield has the same effectiveness as RG-58 at 100MHz. I think a solid copper shield, or hardline, is much better than that. Perhaps down to 121dB. If you take a HP network analyzer of spectrum analyzer, you find most of the internal coax connections are hardline. These instruments can go down to -130dB and below. If they used plain coax, or didn't ground both ends, they could never do as well.
To measure the sheilding effectiveness, you must ground both ends with a circumferential ground, as shown in Fig. 8 of
Here's how I measured shielding effectiveness.
This method simulates the problem of running coax cables side-by-side, such as a signal generator driving an amplifier, with the output going through another coax to a spectrum analyzer or network analyzer.
The coax should be high quality double shielded or hardline.
There's plenty of connectors out there: the usual S-video connector is dual coax with an optional overall shield. You can do power POE-style over the two braids, and high bandwidth signals on the center conductors, using the usual Ethernet-like magnetics.
It ain't high performance coax, but it's attractively off-the-shelf at low cost.
There's also something to be said for M12 on the outside of boxes.
They're harder to kill and make positive contact. You can have a throughhole pigtail like you'd have using the above object.
-- Les Cargill
Yep:
They're also substanitally more expensive than RJ45.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
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