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 or 24v DC supply. The cards will communicate with the SCADA/BMS equipment via TCP/IP (KTA-282 only) or via an RS232 or 2-wire RS485 communications link.

KTA-282 Vantage Pro Modbus TCP Interface Card

KTA-282The KTA-282 Modbus card is the updated successor to the previous GWY-141 card (see below).This KTA-282 card provides a Modbus interface to the full logger output of a Davis Vantage Pro 2 or Vantage Vue weather station. In other words, rather than interfacing to the raw data output of a single sensor, the KTA-282 provides access to essentially the complete set of all sensor data fields from a VP2/Vue Weatherlink logger and makes them available via a Modbus TCP/IP over Ethernet or Modbus RTU over RS485 interface.

The KTA-282 card polls the Vantage Pro 2 logger every 2.5 seconds and retrieves a complete set of real-time data values, which effectively corresponds to the ‘Loop’ and ‘Loop2’ records that the Weatherlink loggers can stream to an external device. Details of the Modbus register-set and other interface details will be found in the short KTA-282 PDF manual and we suggest that prospective users browse through this manual to appreciate the comprehensive capabilities offered by the KTA-282 card.

(Full details of the primary data interface for the Davis loggers can be found in the VP Technical Reference PDF, but note that this is a largish and detailed document only suitable for technical readers).

 We won’t go into complete detail here but, in brief, the data available from a KTA-282 card comprises four categories of information:

  • All of the primary current weather readings measured as standard by a VP2/Vue 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 125 Holding Registers.

The main enhancements offered by the KTA-282 over its predecessor GWY-141 card include:

  • Support for Modbus TCP/IP over Ethernet;
  • Polling data every 2.5 secs (GWY-141 was limited to every 10 secs);
  • Support for the Weatherlink ‘Loop2’ record, providing access to extra data values;
  • Allows interrogation over HTTP to return any Modbus register values in XML format;
  • Web-page control interface to allow configuration across a network;
  • Can directly write the internal memory of the weather station;
  • The KTA-282 card is supplied as a DIN rail mounting part and needs to be connected to 9-24v DC supply. This card requires that the weather station be fitted with a 6510SER Weatherlink data logger.

Further details of this fascinating product are available on request.

GWY-141 Vantage Pro Modbus Interface Card

GWY-141The original GWY141 card remains available as a lower priced part, but please remember that GWY-141 has significant limitations relative to the newer KTA-282 part:

  • Modbus RTU over RS485 interface only (No Ethernet support);
  • Polls the Weatherlink logger only every 10 seconds, which limits the accuracy of wind speed data;
  • No Loop2 support, with restricts the number of available weather parameters;
  • More restrictive command and control features;

The PDF manual specifically for the GWY-141 card can be found here.

KTA-250 Wind speed and direction monitoring

KTA-250_250This 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 KTA250 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.

NB We currently offer the KTA250 card from stock, which is the 24v version of this card with DIN-rail mount. This version will run off lower voltages unless the relay functions are to be used, in which case the supply voltage is recommended to be 20-24v. Other versions of the card (eg the nominal 12v version) are available to order.