Some MODBUS related queries

I am trying to develop a MODBUS compliant device that will be a slave unit in a MODBUS network.

I have these basic queries...

1. Say I connect my slave unit to a MODBUS network, how will the MODBUS master (in my case a PLC) know the address of the slave unit. Is the address given to the slave dynamically by the master or is it set by the user during installation ??

2. When I take my slave unit for installation, does it need any GSD files (like in PROFIBUS) or any other configuration files ?? Are these files generated by the PLC ??

3. The slave unit is a automation controller, commands from the MODBUS master are sent to the slave unit that does the necessary motion control as dictated by the command.

So the command list for the slave unit will have to given to the user during installation of the system ?? How should this list be given as a specification file or as configuration file to be entered in a PLC ??

Sorry if these queries sound basic, but I am new to the area of PLC and I am curious to know how systems are setup in the industry.

thanks
a

These are very good questions. Modbus is very simple and relies on a lot more manual settings than some more complex protocols.

The authority on Modbus serial protocol is the Modbus.org website.

1. The Modbus slave address is set by the user during installation. Sometimes DIP switches or rotary switches are used, but usually it's a parameter setting. There is no Modbus equivalent of BOOTP/DHCP or any auto or default slave address setting mechanism.

2. Modbus doesn't use GSD or EDS or other configuration files. Modbus addresses are generally part of the user manual.

3. It is crucial that the slave device vendor provide clear and complete descriptions of the Modbus function codes and offsets that correspond with the functions they want the PLC device to be able to command.

I prefer devices that present exactly which Modbus function codes they support, as well as which element offset matches which function of the slave. Modbus function codes and offsets are more clear and precise than Modicon PLC addresses like "40001".

Re:

thanks for the reply.

I will be providing DIP switches for slave address configuration. I will also provide DIP switches for baud rate selection.

Is the baud rate fixed for MODBUS ? or does this depend on the user ?

thanks
a

These sound like fundamental questions that you should understand if developing a modbus compatible slave.

Basic knowldege of modbus implementation would answer most of your questions.

I downloaded the 'MODBUS over serial Line specification and implementation' guide, I read the guide and could not understand the 'Implementation Classes' section on page 34.

According to this section "two implementation classes are proposed, basic and regular..' a table of implementations Regular, Basic is given with the default values, my question is should my slave device implement both these classes or only the basic class implementation ??

Is the baud rate fixed for MODBUS ?

the answer to this is exacltly the same that Kroach gave you for the id question...

The Modbus *BAUDRATE* is set by the user during installation. Sometimes DIP switches or rotary switches are used, but usually it's a parameter setting.

i didnt understand your last post: can't comment anything about that.

arbj said:

I downloaded the 'MODBUS over serial Line specification and implementation' guide, I read the guide and could not understand the 'Implementation Classes' section on page 34.

According to this section "two implementation classes are proposed, basic and regular..' a table of implementations Regular, Basic is given with the default values, my question is should my slave device implement both these classes or only the basic class implementation ??

Click to expand...

This question I do believe that only you can reply to.

It depends on what kind of master you will use and how it can be configured.

Will your slave be on a RS485 with other slaves? Then you probably need the Regular.