Installation guide

10 PV Clinics Installation Manual 21/07/11 01.00
1.4 Modbus
1.4.1 Overview
Modbus is a simple and robust, polled industrial communications protocol that has become the de-facto
standard for industrial applications and is now among the most commonly used systems for transferring data
between devices. PV Softclinic uses Modbus communications which allows it to monitor maximum possible
variety of measurements around the plant, from string currents and voltages to Inverter values to meteoro-
logical measurements all in one system. The connection is either serial via RS485 or via Ethernet.
For a complete description of Modbus Protocols, please refer to the
www.modbus.org website.
1.4.2 Modbus Maps
Modbus allows reading and writing of ‘I/O registers’ and ‘addresses’ within any device that supports the pro-
tocol. The content of these addresses is defined by the device manufacturer and a complete map of the con-
tent is described by the devices ‘Modbus Map’ which gives tables of addresses and their meanings. An ex-
ample for a simple Meteorological station might be:
Holding Register
Address
No. of
registers
Description Data Multiplier Units
8 1 Wind Speed 1 m/s
9 1 10Min Average Wind Speed 1 m/s
10 1 Wind Direction 1 degrees
19 2 Outside Humidity 1 %
24 1 Rain Rate 0.01 mm/Hr
25 1 UV Index 1
26 1 Solar Radiation 1 W/m2
27 1 Storm Rain .01 mm
28 1 Current rate Of Storm Rain .01 mm/Hr
29 1 Day Rain .01 mm
30 1 Month Rain .01 mm
31 2 Year Rain .01 mm
42 1 Temperature 1 °C
49 1 Console Battery Voltage 1 Volts
52 1 Time of Sunrise 1 HHMM
53 1 Time of Sunset 1 HHMM
Addresses and Offsets
In this example, the register addresses are given as offsets. Modbus uses two address spaces:
• Holding registers, which are 16-bit words from the start address of 30000
• Input Registers, which are 16-bit words from the start address of 40000
So you would enter address of the temperature as 30042.
Data size
The data size is written as ‘No. of registers’, since input and holding registers are 16-bit words the device
uses mostly 16-bit values and some 32-bit values (for ‘year rain’ and ‘outside humidity’).