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I have an interface device which communicates on Modbus protocol only. If I want to communicate with simulator running on Windows PC and want to test this device using MB protocol what tool I need to develop on my simulator computer? Looking for advice.

Go to www.Modsoft.co.uk. They have Modbus master and slave utilities for Windows for 5 pounds each (can't seem to make the pound sterling sign).

There are free ones, but these are nicer. They have utilities for other platforms as well.

What determines modbus master or slave? How?
Please help

If your device has a table of registers, like 40001 or 30012 with a description of what is in the register, then it is a Modbus slave device.

If your device has provision for assigning it a slave node ID number, then it is a Modbus slave. Modbus masters do not have ID numbers.

Most 'field' devices are Modbus slaves.

Modbus masters tend to be HMI/SCADA/software packages.

The direction of information flow is irrelevant in the master/slave concept; the designation has to do with initiation and control of the comm transactions. There is usually only a single master, while there can be many slaves (with Modbus I think single-master is mandatory). A PLC might be a master with remote I/O racks, VFD's and process instruments as slaves, for example.

The master issues a request to a particular slave, which can be either sending or requesting data. All slaves connected to the network hear the request and parse it to see if it's intended for them. If it is, and if the request is valid in terms of function request, target register address within the slave and so on, the slave will respond, either with an acknowledgement if the original request was a write, or with the data in an appropriately framed packet if it was a read.

You are correct Steve. In the strict definition of Modbus there can be only one master BUT you can extend that definition to be "one master at a time" and that opens a door to multiple Modbus Masters communicating with the same slave(s) and does not "break" the protocol. You just have to have a "traffic cop" in the middle to buffer/relay the messages when time is available.

We sell such a product.
Thanks,
Jerry Miille

if you are using modbus tcp then modbus-cli is free and may be sufficient for your needs: https://github.com/tallakt/modbus-cli

For basic interaction between your master and a slave running on a PC, you don't need to develop anything. Automated Solutions Modbus Slave ActiveX Control comes with a ready to run example application that your Modbus master can readily communicate with. Modbus RTU, ASCII, and TCP are supported.

Free, fully functioning, 30-day trial edition is available for download.

-Mark
http://automatedsolutions.com

Here you find our Modbus class

http://pvbrowser.org/pvbrowser/sf/manual/rlli/html/classrlModbus.ht ml

It is part of the open source HMI/SCADA framework

http://pvbrowser.org