4-20mA signalling

Teletype machines - all mechanical - originally ran directly off the phone company's pair, at 20 or sometimes 80 mA on/off keying. The quiescent state was "on". RS232 came along later.

What the f*ck does RS-232 gave to do with 20 mA current loops ?

RS-232 was intended for interfacing computers with phone line modems in adjacent racks, nothing else !!.

Some idiots tried to use it as a general connections between dissimilar computers.

Serial comms from the Dark Ages:

It's a system that was used before RS485 and RS422.

Tried and succeeded wildly. :)

Unless you have some means* of providing out-of-band signalling for offline, open or other failure, that'll have to do. 0 mA is an excellent indication of fault.

The computer connected to the sensor can interpret the 0 mA in any way it likes.

There are some standards for adding sideband data on top of 4-20 mA, like HART. Seems sort of silly to me.

Are industrial users using PoE to transducers? That seems like a good idea to me. You can slap cheap hubs anywhere you need them.

Face it, Ethernet has won.

The nice feature about HART is that sensors can be parameterized on-line (no need for DIP switches, which do not work well in dirty industrial environment). Also you get much better diagnostics, not just good/not good.

PoE would nicely solve the UPS requirement so you do not have to have a huge number of UPS all over the factory floor, but instead a single big UPS close to the control room.

However, I haven't seen many PoE sensors in industrial applications.

On a sunny day (Sun, 12 Apr 2015 09:46:13 +0300) it happened snipped-for-privacy@downunder.com wrote in :

Wanted to make a joke: The trend is towards self powered sensors from environmental RF. Then I started thinking, and I think I can power a sensor from a speaker that picks up noise in some industrial environments. There usually is a large amount of low frequency vibration, an electrodynamic speaker with voice coil would give a large signal. Of course there is a vicious circle there, process stops, sensors stop too, no way to start again, need backup battery... Anyways Usenet patent prior art sensors powered from environmental audio, do not forget 0.00001 promille caps.

On a sunny day (Sun, 12 Apr 2015 07:54:52 GMT) it happened Jan Panteltje wrote in :

Append to patent: Cellphone charged by environmental audio / vibration. Should be order of magnitude more than just RF. Use cellphone SPEAKER as generator, not the electret.

I'm not sure what HART is really trying to be. Yet another MODBUS, I guess.

There are all kinds of heresies. Surprisingly, PoE may be at issue because of the voltages. One thing that comes up now and again is PROFINET, which makes little sense to me.

And you say "cheap hub"; there will still be vibration and temp constraints that a cheap hub may not meet. One of our "cheap hub" solutions went off the market recently.

Yep. I still find cultural resistance to it. Kinda crazy; there are highly integrated PIC32 that work just fine with Ethernet that are stupid cheap. Put those on a network with some RasPi class ARM stuff and you have a real system. Latency, schmatency - think it though and it's not a problem ( unless you need microseconds in the tens or hundreds ). You mostly don't.

The PLC business is ripe for disruption. It's just not clear that innovation would actually cause any topline growth, though. The incumbents have pretty entrenched positions. That industry is now using "standards" to close ranks.

I'm a bit flabbergasted that high school kids get better toys than the grownups.

Their girlfriends are better looking, too. ;)

Cheers

The kids start with a blank slate. The grown-ups have to deal with backward compatibility.

It took Intel a long time to get over the success of the 4040.