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4-20mA history

W

Walter Harley

Jan 1, 1970
0
Idle curiosity: What is the history of the 4-20mA standard for industrial
process control signalling? E.g., why those particular values; were other
values also used at some time; who developed the idea of current rather than
voltage signaling, and who first used it in industrial equipment? (All
Wikipedia says is that it dates to the 1950s.)

Thanks!
 
J

John Popelish

Jan 1, 1970
0
Walter said:
Idle curiosity: What is the history of the 4-20mA standard for industrial
process control signalling? E.g., why those particular values; were other
values also used at some time; who developed the idea of current rather than
voltage signaling, and who first used it in industrial equipment? (All
Wikipedia says is that it dates to the 1950s.)

Thanks!
You might post this question on sci.engr.control

You might find someone who was involved in developing that standard.

If this guy isn't him, he probably knew him.
http://www.driedger.ca/
 
S

Spehro Pefhany

Jan 1, 1970
0
Idle curiosity: What is the history of the 4-20mA standard for industrial
process control signalling? E.g., why those particular values; were other
values also used at some time; who developed the idea of current rather than
voltage signaling, and who first used it in industrial equipment? (All
Wikipedia says is that it dates to the 1950s.)

Thanks!

Other current loop standards: 1-5mA and 10-50mA. Of course,
offset-zero pneumatic systems (3-15 PSIG) were popular prior to that.

Possibly the origin is related to 20mA digital current loop
transmission which dates from the 1930s or so, at least in the case of
Telex, maybe earlier for other forms of such devices.

Lipták doesn't have much to say about it directly, though he gives
some references from the mid-fifties.


Best regards,
Spehro Pefhany
 
Possibly the origin is related to 20mA digital current loop
transmission which dates from the 1930s or so, at least in the case of
Telex, maybe earlier for other forms of such devices.

Which was in turn derived from the telegraph systems of the 1850's.
Here's an excerpt from http://www.du.edu/~jcalvert/tel/morse/morse.htm#HB

The solution to this problem was to drive the register or sounder from a local
battery that could supply the necessary power through a short local circuit
without the resistance and leakage of the line. The local circuit was opened and
closed in synchronism with the line currents by means of contacts operated by an
electromagnet in series with the line. This electromagnet had only to operate
the light armature bearing the contacts, and so could be made small and light.
The line current could now be reduced to 15-25 mA, while the local current was
about ten times higher. The arrangment of electromagnet and armature with
contacts was called a relay by analogy with the relaying of a message, in this
case from one circuit to another. A line relay typically had a resistance of 150
Ohms, while a sounder to operate on a local circuit had a resistance of 4 Ohms.
The local battery was normally two jars of gravity, supplying 2 V with an
internal resistance of 4 Ohms, so the local current was 2 V / 8 Ohms = 250 mA.

Jim
 
D

Don Bowey

Jan 1, 1970
0
Idle curiosity: What is the history of the 4-20mA standard for industrial
process control signalling? E.g., why those particular values; were other
values also used at some time; who developed the idea of current rather than
voltage signaling, and who first used it in industrial equipment? (All
Wikipedia says is that it dates to the 1950s.)

Thanks!
It was a Telco "standard."


The model 33KSR and 33ASR Teletype machines, and most later printer's
selector magnets were driven in 20 mA loops. A variety of simple DC:Tone
"modems" and multiplexers" drove 20 Ma loops starting in the early 60's as I
recall. The modern Telco TTY/trlrgraph multiplexer of the day was the 43A1
system. If the channel was placed in the CO, a 20 or 60 (Usually 60) Ma
loop delivered the service to the customer premises. When the channel unit
was placed on the customer's premises, the signals were transported on
4-wire VF cable, and the 43A1 terminal delivered a 20 Ma loop to the TTY or
other terminal equipment.

Earlier telegraph and Teletype channels used 60 Ma local loops.

Don
 
D

Don Foreman

Jan 1, 1970
0
Idle curiosity: What is the history of the 4-20mA standard for industrial
process control signalling? E.g., why those particular values; were other
values also used at some time; who developed the idea of current rather than
voltage signaling, and who first used it in industrial equipment? (All
Wikipedia says is that it dates to the 1950s.)

Thanks!

Partial answer which does not address why the particular values:

Current loop is preferable to transmitting with a voltage source
because line length and resistance then become irrelevant. mA in =
mA out with zero error due to line length. This accomodates
non-electronic receiving instrumentation having fairly low impedance.

The non-zero lower level is for fault detection. Xmtr failure or
power loss, or open or shorted line all result in < 4mA to the
rcvr.
 
D

Dan Hollands

Jan 1, 1970
0
Early implementations of current loop control and feedback used vacuum tube
devices that worked better on the 10 to 50 ma. When transistors came into
being 4 to 20ma was easier for the transistors to handle. 1 to 5ma provided
the offset to detect broken wires but with the advent of transistors,
sensors were developed that used the 4 ma offset to power the remote sensors
so that only 2 wires needed to run to the remote device - known as 2 wire
or self powered sensors. Another advantage of the 4 to 20ma was the
developement of "Intrinsically Safe" instrumentation that operated at such a
low voltage and current that it could not initiate an explosion if operated
in an explosive environment.

At the time these devices were being developed major instrument companies
such as Foxboro and Honeywell used these different techniques to
differentiate their products and lock in a user their designs. I believe
that Foxboro Instruments was the company that championed the 4-20ma standard
which eventually won the race.

At the time this was happening, I was working for Rochester Instrument
Systems that pioneered the developement of a lower cost implementation of
these devices that could be used as replacements for the very expensive big
company products.

Dan

--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
[email protected]
www.QuickScoreRace.com
 
R

Rich Grise, Plainclothes Hippie

Jan 1, 1970
0
Other current loop standards: 1-5mA and 10-50mA. Of course,
offset-zero pneumatic systems (3-15 PSIG) were popular prior to that.

Possibly the origin is related to 20mA digital current loop
transmission which dates from the 1930s or so, at least in the case of
Telex, maybe earlier for other forms of such devices.

Lipták doesn't have much to say about it directly, though he gives
some references from the mid-fifties.

I once had an old teletype that used a 60 mA loop. I actually printed
on it with my 8008. :)

Cheers!
Rich
 
R

Rich Grise, Plainclothes Hippie

Jan 1, 1970
0
.
The local battery was normally two jars of gravity, ...

Geez! When did they stop bottling that stuff? Wouldn't that solve
all of our energy problems? ;-P

Cheers!
Rich
 
S

Spehro Pefhany

Jan 1, 1970
0
Early implementations of current loop control and feedback used vacuum tube
devices that worked better on the 10 to 50 ma.

I would have thought that tubes would be much happier at 20mA
maximum-- 50mA of plate current is a fairly hefty tube, IIRC.
When transistors came into
being 4 to 20ma was easier for the transistors to handle. 1 to 5ma provided
the offset to detect broken wires but with the advent of transistors,
sensors were developed that used the 4 ma offset to power the remote sensors
so that only 2 wires needed to run to the remote device - known as 2 wire
or self powered sensors.
Yes.

Another advantage of the 4 to 20ma was the
developement of "Intrinsically Safe" instrumentation that operated at such a
low voltage and current that it could not initiate an explosion if operated
in an explosive environment.

At the time these devices were being developed major instrument companies
such as Foxboro and Honeywell used these different techniques to
differentiate their products and lock in a user their designs. I believe
that Foxboro Instruments was the company that championed the 4-20ma standard
which eventually won the race.

At the time this was happening, I was working for Rochester Instrument
Systems that pioneered the developement of a lower cost implementation of
these devices that could be used as replacements for the very expensive big
company products.

Dan

I think of Rochester as one of the old established companies... must
be a bit before my time. I suppose that there are whippersnappers who
thing of Action as an old established company these days.


Best regards,
Spehro Pefhany
 
D

Don Bowey

Jan 1, 1970
0
I would have thought that tubes would be much happier at 20mA
maximum-- 50mA of plate current is a fairly hefty tube, IIRC.

The Telco 43A1 channel unit had two tube sockets in the loop current path.
For a 20 Ma loop, only one tube was installed. For 60 Ma, both were
installed. I think the tube was a 429A.

Don
 
P

Peter

Jan 1, 1970
0
Walter Harley said:
Idle curiosity: What is the history of the 4-20mA standard for industrial
process control signalling? E.g., why those particular values; were other
values also used at some time; who developed the idea of current rather than
voltage signaling, and who first used it in industrial equipment? (All
Wikipedia says is that it dates to the 1950s.)

Thanks!

I think somebody is getting confused between

a) 4-20mA - an ANALOG industrial sensor interface, where the sensor
draws current in the range 4mA to 20mA according to the parameter
being mesured, and

b) 20mA loop, also known as TTY - a very old way of sending serial
data, by interrupting the current (normally around 20mA) in a loop.

The two are COMPLETELY different!
 
S

sPoNiX

Jan 1, 1970
0
I think somebody is getting confused between

a) 4-20mA - an ANALOG industrial sensor interface, where the sensor
draws current in the range 4mA to 20mA according to the parameter
being mesured, and

b) 20mA loop, also known as TTY - a very old way of sending serial
data, by interrupting the current (normally around 20mA) in a loop.

The two are COMPLETELY different!

However, I do believe that the 4-20mA loop was used for primitive
signalling as well as analogue readings (Although not together).
Depending if it were above or below a certain threshold would indicate
go/no go.

sponix
 
R

Rich Grise

Jan 1, 1970
0
I think somebody is getting confused between

a) 4-20mA
b) 20mA loop
The two are COMPLETELY different!

Yes, and in fact, if you'd bothered to read the actual post, Walter Harley
didn't ask about anything remotely resembling the 20 mA teletype loop -
just what's the history of the industry standard 4-20 mA current loop in
instrumentation?

I think that's a valid question. It's fairly easy to see the logic
for it - it's a current loop, so inherently differential (i.e.,
cancels out common-mode noise, like big motors turning on and off
and stuff), and the current that represents "0" on your measurement
is 4 mA (20 mA being 100% of whatever, of course), so if there is
a current of 0 mA, you know there's an open sensor. (or wire).

Hope This Helps!
Rich
 
P

Peter

Jan 1, 1970
0
However, I do believe that the 4-20mA loop was used for primitive
signalling as well as analogue readings (Although not together).

You may be thinking of the HART protocol; this uses Bell-type modem
signalling to send bidirectional data over the 4-20mA loop. In fact I
believe this replaces the process variable completely; the sensor can
draw whatever current it likes because the data is returned via the
digital link.
 
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