The direct-wired distance between data logger and PC is officially limited to around 5m (16′) for the USB logger and 12m (40′) for the serial logger. But there are many techniques that can be used to carry the data further, both locally and – if required – over long distances. With the capabilities of modern electronics there are so many options in fact that it can be confusing to decide on the most cost-effective one for your particular circumstances. We’ve summarised below a selection of the many options to give you a flavour of the possibilities, but for anyone planning to buy from us we’re happy to advise on the most appropriate option for your application so please don’t hesitate to contact us for further discussion.
Before getting into the detailed summary there are a few important points of background:
- We’re focusing here on ways in which the distance specifically between logger and the single primary PC to which the logger is linked may be extended. Options for extending the link between outside sensors and weather station console is a totally separate issue, dealt with elsewhere.
- Remember that most data loggers, including the Davis Weatherlink logger, can typically interact (directly) with just one PC at a time. Even where the link is long-distance and even though the link may only be active at certain times it is still best regarded as a direct one-to-one link between logger and one specific primary PC. Note especially that the loggers are not themselves web servers in the sense of being able to deal with requests for data from more than one PC at any one time. If you need this web server facility then it must be provided by the primary PC linked to the logger running appropriate software such as Virtual VP – multiple real-time data feeds cannot be provided by the logger itself. Of course, once the logged data has been processed on the primary PC, one or more of the resulting PC data files could be passed on to other computers via a long distance data link – web pages transmitted via the Internet are one obvious example.
- What is the planned use for the weather station data? All of the options below – whether short distance or long distance – should allow the successful periodic downloading of archive data from the logger because this occurs in a single one-way batch process. But displaying live (ie real-time) data requires the more-or-less continuous transfer of individual data records – a process which is much more vulnerable to timing and corruption errors en-route. For example, using the Bulletin mode of the Weatherlink software over a long distance or complex link may therefore throw more errors than over a short-distance link. So long distance links are most easily used for periodic archive downloads and not for continuous operation.
- Most of the methods summarised below are based on the fact that data passes between data logger and primary PC via a standard RS232 serial connection and that any of the many available technologies for moving serial data around can in principle be applied to the linkage between weather station logger and PC.
- Provision of electrical power: The Weatherlink data loggers are a low power design which is not intended to be able to deliver additional electrical power to any other device that may be connected to its serial or USB port. For reliable operation it is therefore essential that any signal booster or similar device is self-powered (ie has its own mains adapter or battery power) and not port-powered (drawing its power from the logger). Even various Davis parts designed for the previous generation of Weather Monitor II stations such as the Short Range Modems are not compatible with the VP logger for this reason.
Local dedicated-cable options
Up to 30m
The quoted limit of 12-15m (40-50 feet) between logger and PC is actually a limit of the official specification for RS232 serial links and not a Davis limitation. In practice it is not a hard maximum and modest extensions to lengths up to 15-20m should succeed without problems. There are in fact reports of using simple cable extensions successfully for up to 30m and more, but inevitably the longer the cable run the less reliable is likely to be the result as the serial signal becomes weaker and more vulnerable to interference. Some people recommend using Cat 5 and/or screened cables for longer extensions, which seems a sensible precaution (although not objectively tested). Probably the most important precaution is to ensure that long cable extensions are routed well away from likely causes of electrical interference, such as fluorescent light fittings, mains timer switches and the like.
A distance of 25-30m is realistically the maximum for a simple unboosted cable connection. Beyond this distance there are three approaches for dedicated direct-wired links that can in principle extend to 1000m or even to 10km. Do remember that direct-wired links of >100m start to involve various practical considerations: a long run of cable will add very significantly to the cost of the link; long runs will typically require custom cabling and hence the user will need to terminate the cable ends; and that long outdoor runs of cable need protection, eg laying in a simple conduit.
- RS232 Signal Booster: This is the simplest option and covers recommended distances of 15-40m. As its name implies, this is simply a serial signal booster and is an inexpensive option for roughly doubling the nominal serial range;
- RS422 Converters: RS422 is a serial data standard related to RS232, but one which is designed to give reliable data transmission over longer distances. RS422 Converters perform the RS232-RS422 conversion and vice versa. They can carry data over 1200m and are obviously required in pairs – one at each end of the link, though their cost remains relatively low;
- Short Range (SR) Modems are a relative of the familiar PC modems and again are used in pairs, one device at each end of the link. They work by transforming the data signal into an audio signal that can travel long distances without serious degradation, with the reverse transformation of audio back to data signal occurring at the far end of the data link. SR Modems can in principle cover a distance up to 10km. Although the modems themselves are relatively inexpensive (though typically priced singly whereas two units are essential) the installation cost for long-distance cabling is likely to be prohibitive unless a suitable link was previously installed for other purposes, eg for a private telephone link on a large site. Please remember that the Davis SR Modems are not suitable for VP data loggers, but we can advise on other suitable makes. [NB You may be wondering whether standard PC modems could be used in this sort of modem link. The answer is yes in principle, but there may well be severe practical problems unless you understand in detail how a particular modem design works. PC modems are designed to be connected to the public phone network and not simply interconnected with a passive length of cable; generally their connection needs at least to simulate the normal phone network. SR modems are on the other hand designed to be simply interconnected.]
Local – existing LAN infrastructure
Serial tunnelling over cabled Ethernet
The Ethernet-based local-area network is now commonplace in businesses and organisations and also in many private PC installations. A network of LAN cabling may therefore already exist in many parts of a building. While this is Ethernet and not directly RS232 serial compatible, there is a well-established technique for creating a serial ‘tunnel’ across the Ethernet and thereby being able to transport serial data transparently. With this approach, existing cabling can be used to pass the weather data between any two points on the LAN with no need of additional cable installation. This technique requires purchase of a single serial-to-Ethernet interface device costing around £150+VAT but once connected, a PC on the LAN can, by running a special small Windows driver, have access to the connected serial device just as if it were linked via a direct serial connection. We use this approach routinely at our base installation and can confirm that it operates well.
Serial Tunnelling over WiFi LAN
A logical extension of serial tunnelling over cabled Ethernet is to apply the same technique over the increasingly popular wireless Ethernet links often referred to as WiFi links. In principle of course any technique for extending Ethernet range could be used to carry a serial tunnel, but WiFi links have undergone intensive development for PC-related use and have some especially suitable characteristics. WiFi devices are inexpensive, widely available and have been allowed to use a relatively high maximum transmitter power (100mW) for a wireless use not requiring a special licence. This high power translates into the potential for longer range outdoors operation. WiFi links up to 20 miles or more have been successfully tested; this has required the use of special very high gain aerials (that may not be legal for use in Europe) but using only standard WiFi network devices. While such a long range may not be relevant to many weather data installations, it does show promise for links eg up to one mile or so (with clear line of sight between both ends of the link). Although we’ve discussed this technique in the context of a LAN configuration, it could equally be used in a direct point-to-point link, ie where the data logger is connected to the serial-to-Ethernet interface thence to a WiFi transmitter linking to a similar WiFi receiver at the far end of the wireless link connected direct to a single PC. In other words, there is no need of any additional LAN infrastructure. As with the dedicated cabled links discussed above, this would also be an always-on link in the sense that once the link was installed there would be zero subsequent running costs. However, this type of WiFi link has two disadvantages. First the cost of the link is starting to grow noticeably and might reach £300 or thereabouts. Second, WiFi transmitters require significant amounts of electrical power and in practice need to be mains-powered; they are not really suitable for isolated field installation where battery or solar panel power might be all that’s available.
Local – wireless
Another well-established technique involves the use of what are known as wireless modems (and which are distinct again from PC and SR modems). Wireless modems are similar in operation to the WiFi units discussed above in that one each of a pair is used at each end of a wireless link. However these modems are specially designed to handle serial data traffic, which brings a number of advantages. All that is necessary for a wireless modem link is a pair of modems; one will accept an input direct from the data logger and the other will connect direct to a PC. Range using a simple dipole outdoors aerial is estimated at up to 1-2km line-of-sight and range will be slightly better by using a lower serial baud rate (eg 9600 rather than the default VP logger 19200) which in practice is likely to be of little consequence. Longer range with the same modems is possible with use of directional aerials, but care is needed not to exceed the maximum permitted wireless output power. The advantages of this approach include: Relatively straightforward installation; An always-on connection with no direct running costs post-installation; Lower power consumption means battery or solar panel operation is feasible; Short cable length recommended between modem and aerial due to 2.4GHz operation; Please note that the Radio Communication options referred to in the Davis catalogue are often relevant to UK use in principle only. Specific wireless models may not be suitable for UK resale; alternative types are usually available but to EU rather than US pricing.
Modern digital technology provides many methods for passing data from point to point over long distance. Any conceivable method that can handle packetised data could probably be pressed into use to pass weather data from logger to PC, perhaps with the assistance of the serial tunnelling techniques described above and with a little further development. For example, any methodology used to bridge could probably be used to pass logger data. And VPN-type links across the Internet again could likely be used. The one caveat is that long distance links with their intrinsic time delays would probably be better suited to the periodic download of batches of archive data from the logger rather than viewing a constantly-updating stream of current data across a long-distance link. But in any event these grander types of link will probably be set up and administered by corporate IT departments with appropriate expertise and so, beyond acknowledging their existence, we’ll not discuss these methods further here. For individual users, of much more relevance are likely to be methods using one particular familiar and readily available form of long-distance data link – the telephone system, which can be accessed in two ways: landline and wireless
Public telephone landline
Well, after a discussion of some relatively exotic technologies, we’ve arrived at a very straightforward and inexpensive option – the standard PC modem connected to a standard telephone landline. Of course for modem operation we’re talking about two modems, one at each end of the link. At the logger end we simply need one particular type of PC modem – a standard external serial modem. It needs to be external because it has to be a self-contained unit and it should preferably be serial rather than eg USB because the logger has a serial connection and it’s always best to use the simplest solution rather than be forced into using adapters etc. But beyond these simple minimum requirements, nothing more needs to be specified. At the PC end, any modem, internal or external, that’s compatible with the PC should be suitable.
In this sort of configuration, the link is not dedicated, ie is not always on. It’s important to note that the connection is always initiated from the PC end. The modem attached to the logger only ever answers the incoming call from the PC and never itself calls out. The logger modem must therefore be attached to a phone line with its own phone number, or at least be set as the first device to answer any incoming call on a shared line. What happens in practice is that the phone number for the logger line is entered into the Weatherlink software, which – either manually on request or automatically to a schedule – dials out to the logger’s number, which could obviously be a long-distance number. The logger modem should answer and a link between the two modems should become established. The PC can then request data, which the logger supplies. This type of link can operate successfully over very long distances.
Setting up such a link is straightforward, though does require some careful configuration, which we will be happy to advise on. For this type of phone-modem link. a timed payment is typically required every time the connection is made, unless operated under an inclusive calls scheme. Once again therefore a modem connection is primarily suited for downloading archive data and not for continuous viewing of current conditions.
A GSM modem refers to a modem type designed to connect to a mobile phone network via a wireless link. This operates in a similar way to the landline modem described above, but with the final link to the logger modem made by a cell-phone type connection. This is obviously more flexible in not requiring a landline installed at the logger location, though cell-phone reception must clearly be acceptable.
Although there a variety of ways in which a cell-phone handset could be used to participate in this type of link, in general a special self-contained GSM modem, similar to a standard external modem but obviously with cell-phone capability, is preferable. The GSM modem link tends to be favoured for installation at remote sites, where a phone landline is not available and other means of connection are not feasible, but do be aware that these modems are relatively power hungry and are best powered from a mains adapter. It is in principle possible for power to be taken from some alternative low voltage source or from a solar panel power, but such systems need to be comfortably specified so as to be able to provide an ample reserve of power under inclement weather conditions.