kraykov

FX5U Modbus communication

9 posts in this topic

Hello,

It is necessary to manage 34 inverters via serial communication RS485. If a Modbus protocol is used according to the documentation, only one RS485 channel can be master. Then the maximum number of inverters will be 32. Can anyone share their experience on this case?

Thanks in advance to all those who participated!

FX5U_Modbus Communication.jpg

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What inverters do have? Asking because if its mitsu - you can manage them via mitsu invertor communication and divide them between 2 channels (or use cc-link ie basis if inverters have ethernet).

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Hello @SolidRio

Frequency regulators are FR-D740, but for 34, two channels are few. 2x16 = 32. Not possible to use CC-link  since the inverters are available and have a serial port.

Thank you.

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You can use 1 channel as a modbus master with 32 slaves and other as a Mitsu Inverter Communication with 16 slaves. I didnt test that, but you can.

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A couple comments about Modbus in general.

1.  Consider how often the master can communicate with each slave.  The query/reply time period for multiple slaves adds up.  

If one slave is “down” then the master waits for that slave’s reply until the master “times out”.  Consider a relatively workable means of de-selecting slave(s) in the master’s polling list should a slave be out-of-service.

2.  Pushing the maximum number of nodes on a single network/bus frequently leads to communications issues where a simple work-around is to use an RS-485 isolator repeater half way down the bus so the 485 drivers do not have to drive all the loads, only half the total loads.

 

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Hello @DanW

 

I share your opinion on Modbus communication. My client insisted on finding a solution for controlling these 34 frequency inverters on a serial port.

 

The only possible solution, in my opinion, is to allocate these inverters to the four CH1 - CH4 serial ports and to use the Mitsubishi protocol for communication with inverters.

 

Still, it is a machine, not an experimental set-up.

 

Thanks for your advice.

 

The best regards,

 

K. Raykov

 

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There's somthing i can't find solid answear for...

FX5 serial communicatian manual says that max number of units controlled by RS-485 inverter communication is 16.

It's not clear if this is for each channel or for the system at all?

 

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Historically and generally, a spec for the maximum number of units on an RS-485 network is based on the RS-485 chip in the device, and the frequently followed by the disclaimer (as in embedded graphic in the first post above) like "the number of slaves varies depending on communication equipment type".

There have been advances in the 3+ decades that RS=485 has been around, and there are 485 chips that can support 64 devices, but the entire network has to be those chips.

The Modbus RTU protocol limit for addressable slaves is 247 slaves. 

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