LabVIEW drivers - Carlo Gavazzi WM14 power analyzer and Modbus

Carlo Gavazzi - WM14 power analyzer

The information provided by Carlo Gavazzi is a good starting point for creating a vi to control the WM14 power analyzer. The following text describes the requirements for controlling several power analyzers.

Making the connection

The WM14 can be connected to a PC trough a RS485 connection. Your PC should be equiped with a special add-on card or you should apply a RS232-to-RS485 converter. You can select either a 2 wire or a 4 wire connection. A proven connection is 2 wire with a 2 channel Advantech RS422/485 card.

Establishing the right protocol

The protocol the WM14 uses, is a subset of the Modbus protocol. The settings for the serial connection are as follows:

• baudrate 9600 (1200-38400),
• 8 databits,
• 1 stopbit and
• no pariy.

Commands and responses are allways followed by an error check.

A command consists of:

• the address,
• the command number (usually 3 or 4, which both request information from the WM14 memory registers),
• the memory location to read from,
• the number of bytes to read (no more than 12) and
• an error check.

After sending the command the power analyzer will give the following responses:

• the address,
• the command number and
• the answer string containing the original command (echo).

The right address

Several power analyzers or modules can be connected to a single RS485 port when each module has been given an unique address. The modules can be given an address from 0 upto 255; the default address is 0.
The address can be changed with the front panel menu.

Downloads

LabVIEW library
wm14.llb
Library file containing:

Note: This library has been made with LabVIEW 6.1 Full Development System. It allows reading both 2 and 4 byte data.

See also

Power management application for methanol factory.

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