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I want control rtu in modbus protocol via gprs internet

The relationship between master and slave is permanently established in modbus. my connection is through Gprs. I want only when changes in master or slave was made to use Gprs. How is the low-speed Internet could be used for modbus?

You will either require a very intelligent GPRS device, or that your RTU acts as the MASTER, not the slave.

It is of course impossible for a central 'master' to know when a remote RTU/slave has a meaningful change. You need your RTU to detect that and act in a 'mobile-originated' manner to write data into the hots/SCADA.

So you need to do 1 of 2 things:

1) have the RTU monitor data, and use Modbus writes to 'call out' when there is significant change. The central device it calls to can be a simple PLC acting as shared Modbus memory, with the main SCADA system polling the central PLC - many OPC vendors also make Modbus 'slave' drivers.

2) have a smart/programmable GPRS device poll the RTU locally and act as a Modbus master to write relevant changes.

Hello,

For GPRS communication with Modbus you can use Embedded computer like Moxa. Custom software enables event-driven communications.

Regards,
Andrzej
www.modbus.pl

You can use our GT-541 to create a virtual com port connection to your Modbus RTU slaves through GPRS.

GT-541:
http://www.icpdas-usa.com/products.php?PID=3934