20mA current loop serial data communications

- a summary or tutorial about the current loop technique used for serial data communications including the basic 20mA current loop circuit.

The current loop interface for serial data communications is used in a number of applications. As the name implies, the current loop serial data communications scheme uses the level of current instead of voltage to provide the signalling method. Often the scheme is called a 20mA current loop interface in view of the standard level of current that is often used.

The 20 mA current loop scheme is has been used for many years for sending digital data. Although not a formal standard, it is a de-facto standard that that was widely used for many serial data communications applications. It was been incorporated into the old teleprinters or teletypes for sending data between two equipments along with being used in a variety of other applications. In fact many older machines (prior to the 1960s) used a 60 mA current loop system, although later machines adopted a 20mA current loop standard - the first one being the Model 33 teletype.

Nowadays the current loop system is not as widely used, but it still finds applications in a number of areas resulting from its advantages. With the advent of RS-422 (first introduced in 1978) and RS-485 (first introduced in 1983), the popularity of 20mA current loop soon waned and it is now only used in niche areas.


Advantages and disadvantages of current loop

The 20mA current loop format for serial data communications has a number of advantages that mean it is still used in a variety of applications. Some of the main advantages are:

  • Line losses are not usually significant:   The fact that a current source is used means that voltage losses caused by line resistance are unlikely to cause a problem.
  • Can be used for long distances:   As voltage losses are not normally significant, 20mA current loop systems can be sued for carrying data over long distances, sometimes up to several kilometres.
  • Can be isolated from ground:   By using opto-isolators it is possible to isolate the signalling system from ground.
  • Provided a simple form of networking:   As the system uses a current loop, it is possible to run several teleprinters receiving data from one source by placing each teleprinter in the loop. This meant that 20 mA current loop provided an early form of networking.

While 20mA current loop provides a number of advantages, it also has some disadvantages that must be considered.

  • No official standard:   No recognised standards bodies have ever published a standard for the 20mA current loop system. This has meant that there are areas of uncertainty. For example it is necessary to know some technical details of the interface circuits to ensure that they interface correctly.
  • Slow speed:   The speed at which current loop systems are able to transmit data is generally much less than voltage based systems. However for short distances speeds up to 19.2k baud are possible, although for longer distances it will be necessary tor educe the speed, possibly as low as 300 baud.
  • Convenience:   The circuits used for voltage based signalling systems are generally more convenient than those used for 20mA current loop.
  • Signalling:   Many voltage based systems use multiple lines for handshaking which speeds the operation. 20 mA current loop traditionally only uses two lines and therefore any handshaking signals need to be carried within the messaging making it less flexible.

Analogue current loop

While the current loop system described here focuses on a digital format used for data signalling, other systems use an analogue approach. These schemes generally use a 4 - 20 mA current loop system and can be used to control transducers.

Although a little crude by today's standards, analogue 4-20mA current loop systems allow control over a single pair of wires and again the resistive losses are less significant, enabling more accurate control that that provided by a voltage based system over a distance.

Additionally, analogue systems such as this current loop system are easier to troubleshoot that any digital systems that could be used. However they may be much less flexible as only one parameter can be controlled at any one time.

While the 20mA current loop system is not as widely used as it used to be, it is still found in some niche areas. It still has advantages to offer in terms of distances that can be covered and the noise immunity it can offer. However it was never adopted as a formal standard and this means that when using equipment with 20mA current loop, it is necessary to check the specifications of the interfaces of both transmitter and receiver.

By Ian Poole


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