4-20mA signalling

Hi John,

Usually 0 mA = fault. That's one reason for the elevated zero of 4 mA.

That 4 mA is intended to power up any active sensor at the end of the line.

Well above 20 mA (22 mA?) short circuit in the loop

That's true for a loop-powered sensor.

ChesterW

That's true for a loop-powered sensor.

ChesterW

Den fredag den 10. april 2015 kl. 14.38.52 UTC+2 skrev John Devereux:

I've never seen any standard but anything out of range signals something special like overload/underload/error/warmup/etc. of course the obvious 0 for open, 24 for short.

So I would think most PLCs should be able to measure the full range

-Lasse

look up the old RS-232 Standard for Hardware Interface.

As in ye olde ASR33? Yes that uses a current loop.

But I guess that predates RS232, I can't see anything much using that search text anyway.

I believe that's just 0-20 mA current loop serial, not 4-20mA analog intended to head in to an A/D.

Yep. It's closer to 422 or 485 but not quite.

Don't. RS-232 was for Telex-type digital communication systems (except that they transmitted "characters" rather than "digits"). In the US Telex was called TWX.

both are accepted as fault conditions, undercurrent indicating an open sensor circuit, overcurrent indicating a short circuit,

Whats the difference between a charachter and a digit?

RS 232 actually transmitted bit strings and clock bits which were reassembelled into characters or digits by a SERDES (Serializer/Deserializer). RS232 ports started to disappear from pcs some 10 years ago and have now been almost completly replced by USB for device communication and Ethernet and WiFi for network comunicatations.

A digit is a number, a character includes both digits and the letters of whichever alphabet you happen to like. Don't get me started on diacritical marks.

A start bit, seven character-defining bits and an extra length stop bit, IIRR

That isn't what it was called at the time - telex predated Turing.

So?

Thanks - but do you think it is a good idea to use these to indicate the conditions I mention? For example a sensor failure other than a failure with its 4-20mA hardware. Or a sensor where the measured parameter is not available immediately?

It's common to use below 4mA for signalling out of range or fault. 0mA would leave the unit unpowered and then you cannot present anything to the user.

So your unit must live of 24Vx4mA or rarther 18Vx3.6mA which I believe is the minimum limits

Look up other commercial loop powered units and you will get the idea

Cheers

Klaus

On a sunny day (Sat, 11 Apr 2015 09:13:31 +0100) it happened John Devereux wrote in :

Out of 4-20 mA range should always trigger an alert. I can remember a case where some construction workers cut a cable to a temperature sensor, and the PLC thought it was too cold, and kept the heat on so to speak, resulting in disaster. The operator of the process (if it is manned at all) should be alerted by a symbol on the screen, audio too, or a remote alarm should be triggered and allow manual intervention remotely. At least that was the way in the late eighties and early nineties.

You can consider automated shut down procedure of the plant too.

Please don't: RS-232 did not describe current loop and certainly not instrumentation 4-20mA signalling. RS-232 defined an electrical and mechanical standard for modem to terminal data communication. It covered things like voltage levels, impedances, slew-rates and pin allocations. Although it assumed serial data it made no assumptions about nrz or rz, asynch or synchronous, it did not attempt to cover data timing, framing or coding. The spec of a 25 pin D connector in size B shell means that "RS-232 on a DE9" is not strictly RS-232.

Current loop goes right back to the days of automatic telegraphy in the century before last.

piglet

interesting, I saw that '20mA' and the word 'loop' and those two triggered memories of the RS232 interface.

It's still ok to look up the RS232 hardware interface standard, not complicated reading, and might learn something.

I'm trying to remember the protocol we used in military type 'security system' communication loops. The signals are sent redundantly over two physical wire loops. One signal goes around the physical loop one way, the other signal goes around the physical loop the other way. A 'single' failure is easily detected, yet the system continues to operate. For example, cut the cabling and the system keeps working AND tells you where the cable is cut.

But not about 4-20mA current loops as used in process control.

Don't bother. What you remember won't have enough connection to reality to be worth posting, and even less to the OP's question.