Modbus Accessories

Introduction

   

Most automatic weather stations are not designed with a Modbus interface as a standard configuration option. However, there are third-party products that allow sensors and data streams from, for example, the Davis weather station range to be interfaced into a Modbus-speaking SCADA or BMS system and we currently offer two such Modbus interface cards.

Please note that both of these items are DIN-rail mounting, bare interface cards, which will typically need mounting in an existing equipment cabinet and supplied with a suitable 12v DC supply. The cards will communicate with the SCADA/BMS equipment via an RS232 or 2-wire RS485 communications link.

KTA/B-250 Wind speed and direction monitoring

Wind Mobus Interface CardThis card is solely dedicated to monitoring wind speed and direction and covers no other weather parameters. However, within the context of wind data, it offers considerable power and flexibility making the card suitable for many applications.

There are three output modes available, only one of which is Modbus:

  • Modbus mode: See below for details;
  • Analogue values: Both wind speed and wind direction are available translated into analogue values, on either 0-20mA/4-20mA or on 0-5V/1-5V scales;
  • Relay circuit switching; The board contains two relays which can be pre-programmed to switch on or off when certain threshold values of wind speed and direction are passed to allow the controlling of external circuits. Obviously this mode allows the sounding of external audible or other alarms when specific wind conditions are exceeded;

Available Modbus interface items can be summarised as follows:

  • Wind speed - Holding Register #1;
  • Wind direction - HR #2;
  • Maximum wind speed seen in polling interval - HR #3;
  • Minimum wind speed seen in polling interval - HR #4;
  • Numerous further configuration and status settings are accessible in other holding registers and coils;

This HTA250 card is compatible with Davis anemometers 7911 and 6410 (These two are electrically identical, but 6410 has a different arm shape allowing slightly better exposure of the cups and vane and is slightly higher priced.) In either case, attaching the anemometer to the Modbus requires baring the conductors at the end of the cable from the anemometer and connecting these correctly into the screw terminal block on the Modbus card.

GWY-141 Vantage Pro Modbus Interface Card

This GWY141 card fulfills a completely different role from the preceding anemometer-only interface card in that it  provides a Modbus interface to the full logger output of a Davis Vantage Pro weather station. In other words, rather than interfacing to the raw data output of a single sensor, the GWY141 provides access to essentially the complete set of processed data fields from a VP2 logger and makes them available on an RS232 or RS485 Modbus interface.

The GWY141 card polls the Vantage Pro logger every 10 seconds and retrieves a complete set of current data values, which effectively corresponds to the 'Loop' record that the VP loggers can generate on request from an external device (see the VP Technical Reference PDF for further details, but note that this is a largish and detailed document only suitable for technical readers).

Vantage Pro Modbus Interface CardWe won't go into complete detail here but, in brief, the data retrieved comprises four categories of information:

  • All of the primary current weather readings measured as standard by a VP station, ie wind speed/direction, temperature, humidity, pressure and rainfall;
  • Readings from the optional solar and UV sensors, including associated values such as evapotranspiration (ET);
  • Readings from any optional Supplementary Stations that may be installed, such as additional temperature, humidity, soil moisture and leaf wetness sensors;
  • Various other summary, calendar-dependent (eg sunrise/sunset times), alarm and forecast parameters, plus some system status values;

All of these various readings are made available on the Modbus interface in a total set of 60 Holding Registers (which need to be read as two blocks of 30 registers). Please note that the one limitation in the overall data set retrieved at each poll is that the current wind speed and direction values returned are the latest values measured in the polling interval. In other words, it is only sampling about one in four of all available wind gust values. However, there is also a 10-minute mean wind speed value available, which will be calculated from the complete  set of individual wind speed gusts measured by the weather station in the past 10 minutes.)

The GWY141 card is supplied as a DIN rail mounting part and needs to be connected to 9-15v DC supply.

Further details of this fascinating product are available on request.

What is Modbus?

If you're not already familiar with Modbus terminology then this page is possibly not for you. But for the curious, Modbus is a serial communications protocol which has become a de facto standard for interconnecting electronic devices in industry.

Its relevance here lies in the interfacing of weather sensors to electrical control systems as might be used, for instance, in SCADA (Supervisory Control And Data Acquisition - basically any system where sensor inputs are used to control a process) applications and BMS (Buildings Management Systems) systems in industrial/commercial/communal premises.

Modbus essentially defines a simple and highly standardised  interface specification and language by which control devices can interrogate sensors such as weather sensors. So the key attraction of Modbus is the fact that it represents an interface standard or lingua franca for device communication. It may not always be the most efficient interface type but any electronics engineer familiar with Modbus will typically be able easily to interface external sensors to some new or existing piece of control equipment by using a Modbus interface as an intermediary.

Modbus interface cards are generally designed for adding to existing SCADA/BMS control cabinets and so are typically bare cards requiring DC voltage connections rather than being separate, free-standing and self-powered units.

Modbus Card Prices

KTA-250 Anemometer Interface Card
(DIN mounting)

Card alone = £139 + VAT

Card + 7911 anemometer = £269 + VAT

GWY-141 Vantage Pro Interface Card
(DIN mounting)

Modbus Interface Card = £139 + VAT