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4-20mA history

Discussion in 'Electronic Design' started by Walter Harley, Oct 24, 2005.

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  1. 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.)

  2. 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.
  3. 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
  4. Guest

    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.

  5. Don Bowey

    Don Bowey Guest

    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.

  6. Don Foreman

    Don Foreman Guest

    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
  7. Dan Hollands

    Dan Hollands Guest

    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 Hollands
    1120 S Creek Dr
    Webster NY 14580
  8. I once had an old teletype that used a 60 mA loop. I actually printed
    on it with my 8008. :)

  9. .
    Geez! When did they stop bottling that stuff? Wouldn't that solve
    all of our energy problems? ;-P

  10. I would have thought that tubes would be much happier at 20mA
    maximum-- 50mA of plate current is a fairly hefty tube, IIRC.
    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
  11. Don Bowey

    Don Bowey Guest

    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.

  12. Peter

    Peter Guest

    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!
  13. sPoNiX

    sPoNiX Guest

    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.

  14. Rich Grise

    Rich Grise Guest

    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

    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!
  15. Peter

    Peter Guest

    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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