Protecting Notebook
RS232 Ports

Notebook Personal Computers, now affectionately known just as notebooks, have radically influenced business and work methods. How cumbersome demonstrations and presentations became when a full blown desktop had to be carried in, set-up, broken down, and carried out. Now it is a simple matter of opening a briefcase and our business life stares at us through an approximately 8 by 10 screen neatly attached to an internal keyboard, mouse, disk drives, and other required bits.

Although notebooks have made a huge impact on our lives, they also have inherent weaknesses which are not usually considered during their use. They have become an almost essential tool during configuring or setting up of a system, and it is during these times these compactly engineered bits of microelectronics are most likely to put at risk. During setting up or configuring of a system the notebook would be attached to the device being configured, either through ththe commonly used serial port or less commonly used parallel port and in some instances both.

At a small tangent for a brief moment. It must be remembered that to attain such small boxes of many tricks manufacturers use integrated circuits that each do a host of functions. Unlike normal desktop PCs which use individual ICs to do individual tasks, it is not uncommon to find all peripheral functions of a notebook being done by one enormous IC with a myriad of legs protruding from its body. Functions such as the serial ports, infrared port, parallel port, floppy driver, and miscellaneous inputs are normally handled by just one such IC. One other very seldom known aspect of notebooks is with the size of motherboard being so compact the grounding is often not as beefy as would be found on desktops.

Back to the main subject; It is not uncommon to experience ElectroMagnetic Induction during the configuration process, the level of EMI depending on the environment being operating in. If high, then damage to a notebook can be extensive. Such EMI was experienced while commissioning a system at a power plant for a remote community on an island.

In this case the equipment being configured was 'sandwiched' in a 20 foot gap between a generator and a transformer feeding one section of the community. The cables between the switch panel and transformer ran through the floor directly below the table on which the equipment and notebook rested. The feed from the transformer had developed a fault resulting in huge surges of electromagnetic energy induced from the cables/transformer into the cables between the notebook and equipment being configured.

Although the equipment being configured was not damaged, the notebook was not so lucky. In situations like this the result of these currents, which could be many amps and well in excess of the working currents of the ICs used for the ports, would cause energy to be dissipated within the notebook's components, dissipation for which the components were not designed.

This has been the first and only damage I have ever experienced on any IT equipment I have used or owned and can only attribute this to not using a cable normally used in such situations. It was always the practice to have a specially protected shielded multi-core cable from the equipment to the notebook or PC, but the complete cable is not an off-the-shelf item.

The shield is used for two purposes. The first to be the signal ground connection between the two pieces of equipment, and second to shield any extraneous energy (RF mainly) from the signal cores. The shield is, however, not taken directly to the ground pin of either piece of equipment, but rather via a 10 ohm resistor on each end. This firstly ensures there is a ground path for the signals (the resistance not being too high for this) but also to limit any currents induced in any earth loop that may have formed.

Each signal wire is taken through a 100 ohm resistor at each end of the cable, the reasons being the same as mentioned for the shield. Lastly, each signal core is bridged to the respective ground pin at each end by either two 12 volt zener diodes in a back-to-back arrangement or, better still, small AC 12 volt 'Transorb' diodes. The reason this is done at each end is to ensure protection of both the equipment being connected to and also the notebook. It is also not uncommon to use the same cable, via null modem, to connect two computers together (e.g. via LapLink) for file transfers. This would ensure the computers on both sides of the link are protected.

There will hopefully be no resistance to purchasing the parts to make such cables as the potential savings to a company are vast, especially should damages result in loss of data!

| | Ask a Question |

© 22.06.02