Reason for 4-20mA current as a standard signal for transmitting data to and from PLCs

Hi,

What is the reason behind the 4-20mA current as a standard signal for transferring data? Well for starters, we choose current over voltage because there is less signal / data loss when the signal is being transferred over.

How about 4-20mA?

Well the purpose of having 4mA as a minimum is to be able to make sure that process actuators are "on" so that SCADA is able to detect any possible fault. Is that right?

how about 20mA?

just a standard that was implemented, could easily have been any other current, but 20mA was "the chosen one".

Typically these are measured as voltage across a 250R resistor, 1 to 5V dc

4-20mA can readily be sourced by devices that run off 24Vdc, and can therefore provide the current for up to 4 receiving devices in series.

daba said:

just a standard that was implemented, could easily have been any other current, but 20mA was "the chosen one".

Typically these are measured as voltage across a 250R resistor, 1 to 5V dc

4-20mA can readily be sourced by devices that run off 24Vdc, and can therefore provide the current for up to 4 receiving devices in series.

Click to expand...

Ok, was my theory about 4mA current correct though?
"Well the purpose of having 4mA as a minimum is to be able to make sure that process actuators are "on" so that SCADA is able to detect any possible fault. Is that right?"

ivanlch said:

Ok, was my theory about 4mA current correct though?
"Well the purpose of having 4mA as a minimum is to be able to make sure that process actuators are "on" so that SCADA is able to detect any possible fault. Is that right?"

Click to expand...

That is correct, if you used 0 mA, you couldn't tell the difference between Zero Output, and a broken wire. As well, many Two Wire Transducers use that 4mA to power the unit, but I doubt that was the original intention of the 4mA Standard.

Current Loops are used due to their immunity to noise over long distances, as opposed to say 1-5VDC or 0-10VDC.

Stu....

sthompson said:

That is correct, if you used 0 mA, you couldn't tell the difference between Zero Output, and a broken wire. As well, many Two Wire Transducers use that 4mA to power the unit, but I doubt that was the original intention of the 4mA Standard.

Current Loops are used due to their immunity to noise over long distances, as opposed to say 1-5VDC or 0-10VDC.

Stu....

Click to expand...

ok thanks =)

No problem!

Do a Google Search for 4-20mA History, and you will find some interesting stuff.

Stu....

sthompson said:

No problem!

Do a Google Search for 3-20mA History, and you will find some interesting stuff.

Stu....

Click to expand...

No results, can you give me some more information relating to the 3-20mA history.

Sorry, that was a Typo, I meant 4-20mA History.

Stu....

It's called a "Live Zero" for diagnostics reasons. As you suspected.

If you measured zero is it because you have a broken wire or the process is calling for zero output.

http://www.google.com/#hl=en&sugexp...94,d.aWM&fp=48362b96de64f48b&biw=1585&bih=807

ivanlch said:

ok thanks =)

Click to expand...

I take it there was a big test? How did you score?

ivanlch said:

Hi,Well the purpose of having 4mA as a minimum is to be able to make sure that process actuators are "on" so that SCADA is able to detect any possible fault. Is that right?

Click to expand...

It's nice, but it wasn't the primary consideration for an elevated zero. The purpose of the elevated zero at 4mA is to allow a 2 wire loop powered transmitter to use 3.5-3.6mA of current to power the transmitter's electronics. The 2 wire concept of power over the same 2 wires as the signal would not be doable with a true zero based low end.

Were the low end 0.0mA, there would be no current flowing to power the field instrument's electronics.

A 'live' zero is a benefit, but it isn't the primary reason for an elevated zero.

ivanlch said:

the 20mA?

Click to expand...

4-20mA followed in the wake of 3-15psi pneumatics.

3-15 psi has a working span of 4 times the zero offset.
zero offset = 3; 4 * 3 = 12; 3 + 12 =15

for 4-20mA
zero offset = 4; 4 * 4 = 16; 4 + 16 = 20.

Even Foxboro's current standard, 10-50mA, was the same ratio:
zero offset = 10; 4 * 10 = 40; 10 + 40 = 50

To have wide spread industrial signals use the same ratio is not a coincidence.

Current driven circuits will vary the supply voltage to keep current constant through a specific
load range, thus voltage drop over distance is not a factor of signal transmission degredation.
The offset from zero (4mA) represents a true analog quantity, i.e. (current flow already exists
at the value of zero). The value of current representing zero simply has to change, it does not
have to turn on, and then change.

I use 4-20 ma for 2 very straight forward reasons.
1) Very immune to noise/induction.
2) I can detect loss of signal if the 4 ma goes below about 3.6 ma indicating a problem/

Ok i have a little question so you start with 4 to 20 mA and the real world has the minimum signal going less than 4 mA or the highest signal is slightly higher than 20 mA , now the question is other than being unconventional or faulty is there a problem interpreting the signal , in as much that it may be slightly parabolic by nature and nonlinear . Now Allowing that in most applications it might be considered unrealistic to expect to use the total range of 4 to 20 mA scale . Or might the numbers be more like 8.999 mA to 13.500 mA mind you these are all throw away numbers but for the purpose of discussion only

One component of "accuracy" is linearity. There's not supposed to be any non-linearity or hyperbole. It depends on the hardware.

Some industries tend to care, like pharmaceuticals, where calibrations are mandated by regulatory agencies.

Others wouldn't know if the linearity bowed 15%, because they hook it up, scale it and run it.

Linearity from commercial chipcs pretty good, even in the low end A/D and D/A's. Otherwise they chip people couldn't stay competitive.

Here's NAMUR's (German standard) map for 4-20mA: