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4.3- COMMUNICATIONS

ReadWatt has two communications ports, RS-232 and RS-485. Both ports can be used simul-

taneously.

The transmission speed, Baudrate, is the same for both ports and can be congured.

4�3�1�- MODBUS PROTOCOL

The MODBUS protocol is a communication standard in the industry that enables the network

connection of multiple units, where there is a master and multiple slaves. Within the MODBUS

protocol ReadWatt uses the RTU (Remote Terminal Unit) mode.

In RTU mode the start and end of the message is detected with silences with a minimum of 3.5

characters and the 16-bit CRC error detection method is used.

The MODBUS functions implemented in the unit are:

Functions 03 and 04. Reading of logs.

Function 10. Writing of multiple logs.

4�3�2�- READ COMMANDS

ReadWatt supports the integer type read functions: 0x03 and 0x04.

The unit's MODBUS variables are specied in Table 4.

Example: Reading of the unit's serial number with peripheral number 01.

We will send the following MODBUS frame:

Address

Func-

tion

Initial log Log no� CRC

01 04 0060 0002 CRC

The unit will respond to us with the next frame:

Address

Func-

tion

No� of

bytes

Serial no� CRC

01 04 04 XXXX XXXX CRC

Note: The values are shown in hexadecimal.

The number of requested logs must be the same as the size of the variable requested.

It is possible to read several consecutive addresses, if the request meets the correct format.

ReadWatt

Instruction Manual

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