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
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.
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
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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.
On Sun, 23 Oct 2005 23:39:22 -0400, Spehro Pefhany wroth:
Which was in turn derived from the telegraph systems of the 1850's. Here's an excerpt from
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
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
I once had an old teletype that used a 60 mA loop. I actually printed on it with my 8008. :-)
Cheers! Rich
On Mon, 24 Oct 2005 05:01:19 +0000, jmeyer wrote: ...
Geez! When did they stop bottling that stuff? Wouldn't that solve all of our energy problems? ;-P
Cheers! Rich
I would have thought that tubes would be much happier at 20mA maximum-- 50mA of plate current is a fairly hefty tube, IIRC.
Yes.
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
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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 QuickScore@USSailing.net www.QuickScoreRace.com "Walter Harley" wrote in message news:DZmdnYYZL6Sp1sHeRVn-rg@speakeasy.net... > 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! >
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
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
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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