Anyone dealing with serial and USB conversion issues knows that data signal conversion between different protocols can be tricky. Serial data communication originated many years before we came to expect the plug-and-play convenience of modern systems and interface standards. In the past, determining if a device was DTE (data terminal equipment) or DCE (data communication equipment) was an issue, you had to deal with handshaking protocols, and you needed a fairly detailed knowledge of all the various signals. Today's USB, RS232, RS485 and RS422 converters, in conjunction with much smarter software, make things a lot easier, but it's still good to know what you're dealing with. So here are some answers to commonly asked questions.
You may see converter vendors advertise SMT circuitry as a selling point. SMT stands for Surface Mount Technology, which allows for significantly smaller currents and thus less heat. Less heat means longer component life and thus higher reliability and lower total cost of ownership.
Another question that frequently comes up is whether you can unplug a connector while it is being used. The answer is yes if the converter has a data receiver protection circuit. If you need your converter(s) to be hot-pluggable, make sure whatever you buy offers data receiver protection.
Anytime you deal with converters, you may need end-of-line termination so as to prevent interference on the data line. The problem is that termination increases the load on the data circuit, so it's generally best to use a terminator only at higher data rates (over 19.2hbps or on lines over 200 meters).
While RS-232 cables are pretty standard, people often ask what kind of cable to use for RS-422 and RS-485 connections. The answer is that in most cases inexpensive CAT 5 cables will work just fine. They generally exceed RS-422/RS485 recommendations and are an excellent choice.
Technology changes all the time, and one thing that has changed is the power of COM ports on new computers. Older systems used -12VDC whereas most newer ones use -8VDC. Before you buy, check converter specs to make sure they support the lower voltage. Many new laptops, and even some new desktops, no longer have serial ports and require USB-to-RS232 cables that create a "virtual" COM port. Converters generally work with those cables, but it makes sense to inquire before you buy a converter.
Almost all electronic circuitry is sensitive to electrostatic discharge. The charged capacitance of a human body is actually enough to destroy an integrated circuit in an instant. That's why good converters have static protection circuits. Note, however, that those are not surge suppressors. If you have an outdoor installation, you want to use an opto-isolated converter to protect your computer. Opto-isolators (which, however, require additional power supplies) work by inserting an optical bridge that eliminates transient surges.
Another question that frequently comes up is whether or not to use fiber optic systems. Fiber optics, of course, use light instead of electricity. This means a number of advantages in terms of higher speed, higher bandwidth, and inherent resistance to EMI/RFI and other electrical or radio interference. Fiber optics are best used for long distance communication, such as when RS-232 data must be sent to remote locations. The overhead consists of a special transmitter, the fiber optic cable, and a receiver. Note that fiber optics systems can be single-mode or multi-mode. Single-mode means a single strand of fiber for distortion-free, high-speed long distance communication. Multi-mode cable uses multi-strand fibers and is more prone to signal distortion. Each cable type requires the proper transceivers. Cables can also use either bayonet-style or push-on connectors.
Any time you deal with USB, RS-232, RS-485, RS-422 or fiber optics converters, it pays to do some research and plan ahead. Companies specializing in converters often have downloadable data sheets for all of their products, and that can help tremendously in getting the proper gear.
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Chris Robertson is an author of Majon International, one of the worlds MOST popular internet marketing companies.
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