Interference and USB connections

Why interference should cause problems with USB connections doesn’t seem to be well understood, but it does definitely seem to be the case that USB connections are more sensitive to external interference than, for example, traditional RS232 connections. Whether this is perhaps because of the much higher design speeds of USB connections, lower voltages used or whatever else isn’t generally explained, except perhaps in the professional electronics literature. It is known that Davis expended considerable time and effort in extensive investigations of this problem with the USB logger connections, including looking at alternative USB chipsets such as FTDI, but were unable to identify any specific cause or definitive fix other than the general issue of external interference. And it is a fact that taking steps to minimise the effects of external interference often results in mitigation and – if you’re lucky – disappearance of the USB dropout issue. So the issue does seem to be strongly linked to local interference even if the precise nature of the link is unclear.

There seem to be two main sources of interference, which will need to be investigated separately: through-air wireless interference and noise or interference propagated through the AC mains (ie via the mains adapter powering the console/logger unit). And there is a range of possible fixes for this interference problem suggested below, the diversity of the suggestions reflecting the rather ill-defined nature of the problem and the difficulty of knowing which one is most likely to work in any given instance. All we can suggest is that you review the various fixes and, if necessary, work through them in turn to see which might be the best option in your specific case, starting obviously with the simpler and cheaper options. The starting point is to review the three simple fixes and see whether any of these helps, before moving on to the other options:

Simple fixes

  • If you use the Davis Weatherlink software (as supplied with the USB logger) then ensure that you have updated this to the latest available version (v6.0.3 at the time of writing). The USB driver which is packaged with Weatherlink is updated from time to time with specific fixes to minimise the USB dropout problem. (Users of non-Davis software can also install the latest Weatherlink version even if they don’t use it regularly. This will ensure that the latest USB driver is also installed on the PC.)
  • Look around the immediate environment of your USB connection and see if there is anything there that could credibly be generating interference, eg cordless or mobile phones, baby alarms, any computers or other devices runing with the covers off, fluorescent lights etc and if possible take steps to move any such items well away from the logger-PC connection – the further the better. Also, route the USB cable well away from any other cables that could conceivably be carrying or transmitting interference;
  • At least as an experiment, try disconnecting the console completely from the mains and running the console on batteries only – the wireless console should run for 6-9 months on batteries alone and the cabled one for 3-4 months. (This test will help rule out interference that is being injected into the console via the AC mains. If this successfully cures the dropout problem then it may be worth considering buying a small PC UPS and plugging the VP2/Vue adapter into that – these units usually have excellent interference suppression.);

Further suggestions

  • Choose the best quality USB cable you can find/afford and choose a length no longer than necessary – the shorter the better. Try to avoid passing the USB connection through any intermediate USB hub, especially if it’s an unpowered one;
  • Ferrite cores are an excellent means of minimising interference pickup by the USB cable itself. You can buy individual cores through which to wind the cable or to clamp over it (one at each end is best) or USB cables can be bought with prefitted ferrite cores, which often look simply like cylindrical bulges near each end of the cable. Sometimes this measure alone – ie fitting a good short new USB cable with a ferrite core at each end is sufficient to cure the dropout problem;
  • Try wrapping the complete plug and socket connection between the USB pigtail lead from the logger and the main USB cable with metal foil (eg cooking foil). It has been suggested that this physical connection is not as well screened as it could be and hence provides an easy entry point for interference. Wrapping the joint in foil provides extra screening and is a quick and simple fix to try;

Other solutions

Besides trying to minimise the entry of interference into the VP system, there are other approaches that may be worth trying if the USB port dropout problem persists, for example:

  • If you’re currently using Serial/VCP mode for the USB logger (ie ‘Serial’ selected as the Port option in the Weatherlink ‘Communications Port’ dialogue) then try using the USBXpress option instead – see our Interfaces Overview topic for further details. (Note: This option probably won’t work if you’re using software other than the Davis Weatherlink program, because other software programs don’t yet support USBX mode. Hopefully this situation will improve because Davis have signalled their intention to make USB the default mode of the logger.)
  • Try, if possible, connecting the logger to a different PC. Sometimes PCs vary in the degree to which their USB ports are affected by this dropout issue;
  • Look at using a USB-to-Ethernet adapter. This is a fundamentally different way of connecting the USB logger to a PC and is only relevant if your PC is already connected to a local network. The idea is that you connect the USB logger to an interface box, which in turn connects into your network. The USB data then passes to and from the PC over the network rather than over a direct USB cable link and the network routing might well be less susceptible to interference. This is obviously a more costly option, although not hugely so. We’ve successfully used the Silex SX-2000WG wireless adapter in this role, but there are other cheaper wired-only Silex boxes that might work just as well, in addition to other makes of serial-to-USB adapter;
  • See if a cost-effective USB optical isolator might be available. Optical isolators have long been available for eg RS232 connections and can be invaluable in helping prevent the effects of ground loops and interference. Unfortunately, they still seem to be relatively new in the USB field and hence more complex (eg providing isolation for 4 USB ports rather than one port) and more costly than might be acceptable. But this situation is likely to improve and, someday soon, relatively inexpensive single-port USB optical isolators might be marketed. Note that you would need to find an isolator that takes port power from the PC end of the connection – the USB logger cannot provide sufficient power to run the isolator;

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