Phil_666

MODBUS Newbie!

8 posts in this topic

Hi Guys just a quick one before I begin thinking about a new project using MODBUS RTU

i want to have a MODBUS network over RS485 consisting of a HMI (Master) FX3G (Slave1) and a remote IO unit (Slave2) 

 

obviously  i I know the master can request information and send commands to slaves, but can Slave1 send outputs/commands to Slave2? Or can only the master communicates to each slave? 

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20 minutes ago, Phil_666 said:

Hi Guys just a quick one before I begin thinking about a new project using MODBUS RTU

i want to have a MODBUS network over RS485 consisting of a HMI (Master) FX3G (Slave1) and a remote IO unit (Slave2) 

 

obviously  i I know the master can request information and send commands to slaves, but can Slave1 send outputs/commands to Slave2? Or can only the master communicates to each slave? 

It cannot because data frame that send from slave include only that slave number (e.g. 1) to response value to master not send others slave number (e.g. 2, 3, 4, ...)

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1 hour ago, Phil_666 said:

Or can only the master communicates to each slave? 

Correct. The master controls all the data flow on the loop. If a slave could initiate communications on the loop collisions would occur. MODBUS is a very simple master/slave protocol.

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Perfect, thanks guys just wanted to confirm.

 

i will make the PLC (Master) and the HMI (Slave1) and just use the PLC to write information to the HMI

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7 minutes ago, Phil_666 said:

Perfect, thanks guys just wanted to confirm.

 

i will make the PLC (Master) and the HMI (Slave1) and just use the PLC to write information to the HMI

I think you should use HMI to be Master because it can communicate to other slave devices (Temperature Controller, VFD, etc) that you can use HMI to read/write PLC that mean you can send data from PLC to HMI too.

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Wasan thanks for the input,

i wanted to use the PLC to control Outputs on the remote IO unit, if I can’t do that with the PLC being the slave my choice is limited no?

 

with the PLC as master, I can have inputs on the remote IO causing actions on the master PLC then the PLC can send Outputs to the remote IO and update registers for the HMI to read with the HMI as master I have nothing to send programmed commands to the remote IO

 

the System is basic, it should never need to include more than a HMI, Remote IO and the PLC 

 

 

Edited by Phil_666

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3 minutes ago, Phil_666 said:

Wasan thanks for the input,

i wanted to use the PLC to control Outputs on the remote IO unit, if I can’t do that with the PLC being the slave my choice is limited no?

 

with the PLC as master, I can have inputs on the remote IO causing actions on the master PLC then the PLC can send Outputs to the remote IO and update registers for the HMI to read with the HMI as master I have nothing to send programmed commands to the remote IO

 

the System is basic, it should never need to include more than a HMI, Remote IO and the PLC 

 

 

I see it but if your PLC have two or more communication ports I recommend you to use this diagram

HMI (Master) <-> PLC (Port 1/Slave)

PLC (Port 2/Master) <-> Remote I/O (Slave)

When you need to add SCADA it is easy to use HMI Ethernet port collect data from machine.

But if it is simple your method are good enough.

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Thanks Wasan

i will only have one port on this FX3G but worth bearing in mind for another project

 

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