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MODBUS master problem connecting remote slave(s)


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I searched the forum, but didn't found something to help me.

I have a unistream, connected with +-12 modbus gateway's (via TCP/IP).

when I look after I downloaded my program to the PLC, then I see that some modbus slaves are not connected...


Is there a limit on how many remote slaves IP's ?




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It has something to do with slave nr. 255....

What, I don't know, -> when I remove slave 255 then I have a good working modbus "bus"....

When I add remote slave 255, then the problems begin -> no updating of some slaves

a bug ? or maybe is adress 255 not a good idea in modbus ???

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  • MVP 2023

I'm no expert on MODBUS, but I've found that the Slave ID usually only has significance if there is a conversion to serial somewhere in the pathway. If everything stays purely in the Ethernet (TCP, as opposed to RTU) domain then it can be ignored. However, there are a myriad of MODBUS slave implementations by various manufacturers and obviously their recommendations should be followed. I would say that in no case should multiple MODBUS TCP slaves have the same IP address. It looks to me, in your implementation, that you think that you can distinguish between MODBUS TCP slaves merely by Slave ID. You cannot do that.

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let's describe the system I want to communicate with :

In my electrical cabinet, I have several Schneider Compact NSX circuit brakers (industrial fuse 😁)

these are equipped with a micrologic that can be read by  a device, where you can set the adress of. (IFM modbus interface module)

all these IFM modules are coupled with an IFE ( modbus TCP interface for compact NSX).

this IFE has an IP adress, where you can communicate with modbus TCP/IP

to collect the data I have to adress this IFE with the slave nr. of the IFM module....

the problem is that you can connect 1 NSX directly to the IFE module, standard with adress 255 (can not be changed). Altough maybe I should find some documentation about this device, it's very hard to find...

I reprogrammed the unilogic software, without slaves on adress 255 -> worked all night without problem !

now I have 14 slaves, without problems....



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