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