How to protect RS-232 serial connections

How to protect RS-232 serial connections article pointed me to a recent white paper from APC-Schneider Electric that explains specific power protection issues related to RS-232 cabling. The Protection of RS-232 Serial Connections document contends that the RS-232 port often represents the “Achilles heel” of computer installations which are otherwise well protected by UPS systems and other power protection equipment.

The RS-232 communication standard was developed to allow computers and terminals to interface to communication equipment (modems) at short distance (few meters). In modern business environment, RS-232 links are routinely used for many things over considerable distance (tens of meters). And many industrial automation systems also widely use RS-232 links.

The RS-232 port on any device also represents a path into the device for damaging noise and transients because the safety grounding arrangement leads to a conflict; every RS-232 workstation has two connections which are attempting to establish the common reference voltage for RS-232 communications: the common wire which connects the workstation to the CPU in the RS-232 cable, and the workstation safety grounding wire. This situation is sometimes known as a “ground loop”, something I have written quite much on my blog.

The types of transients which can enter via an RS-232 port originate from a variety of sources. These sources include inter-system ground noise, telephone line surges, and wiring faults.
The Protection of RS-232 Serial Connections document explains in detail the special vulnerabilities of RS-232 cabling, and describes appropriate protection and mitigation techniques.

It is important to follow a few simple rules when installing RS-232 data connections:

  • Avoid RS-232 wiring runs of more than a few meters in length if possible
  • Install high performance surge suppressors on modem connections
  • Optical fiber or isolators are required for long RS-232 runs in order to prevent risk exposure
  • Use of properly constructed high quality RS-232 cables can prevent equipment damage

The problems associated with RS-232 wiring cannot be solved by typical power protection equipment such as UPS systems or AC surge suppressors.


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  4. Tomi Engdahl says:

    How Isolation and IoT Play a Role in Industrial Automation

    Industrial automation arguably dates back to Henry Ford’s installation of a moving assembly line for the Model T in 1913; it is the use of various control systems to operate industrial equipment such as machinery, manufacturing processes and material handling equipment, with little or no human intervention.

    Automating industrial processes has a number of benefits: it saves energy and materials; it improves the quality, accuracy and precision of industrial processes; it allows operation in hazardous environments (in nuclear plants, for example); and it vastly saves on labor.

    The results are impressive

    The Connected Factory and the Internet Of Things

    The next stage after automating individual industrial processes is to make sure that they all work together smoothly – and provide data to their human masters, of course! The modern automated factory therefore relies on an industrial network using one of the numerous automation protocols such as Ethernet, Fieldbus, or HART Protocol to provide connectivity at the factory level.

    Galvanic Isolation and Industrial Automation

    Adding electronic control and connecting multiple systems together via a network has many benefits, but there are issues and challenges, too. One of these is the challenge of combining high-voltage, high-current machines such as industrial robots and CNC machines with low-voltage, low-current data acquisition systems and networked communications. We’re going to talk about an important technique used to battle this problem –galvanic isolation.

    Galvanic isolation is the technique of isolating functional sections of electrical systems to prevent current flow between them; no direct (i.e., resistive) conduction path is permitted. Although there’s no resistive path between sections, power or information is still transferred by capacitive, inductive, optical, or other techniques.

    Why is galvanic isolation needed in industrial automation?

    Safety – Protecting users of electrical equipment from potentially lethal voltages and currents is a key requirement in any electrical design

    Ground Differences & Ground Loops- Unlike the simple schematics we drew in school, as practicing engineers we soon learn that ground is most certainly not the same at different points in a system, especially when those systems are widely separated – between different parts of an industrial plant, say. This can lead to errors or even failure in a digital network because any difference in the ground reference

    Common-Mode Voltages – in many cases we need to extract a small signal riding on top of a larger common-mode voltage: an in-phase signal or voltage that appears simultaneously on both input terminals. In some cases, this can offset the signal being measured

    A number of regulatory standards govern isolation for industrial applications, including IEC 60204; UL508; UL60947, and CSA 14-10. In addition, IEC 61010-1 and VDE 410/411 cover industrial control.


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