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Invalid slave id
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As I recall Modbus only has an 8-bit address, so one master can only talk to 255 slaves via 1 port, so valid addresses are in the range 0..247 where 0 is used as a broadcast address.
However you can use another port, though.
If have any doubts about it I recommend you google for modbus documentation.
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Actually, max slave id is 240.
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Thanks for fmunhoz's recommendation.
In MODBUS Messaging on TCP/IP Implementation Guide, it says that
On TCP/IP, the MODBUS server is addressed using its IP address; therefore, the MODBUS Unit Identifier is useless. The value 0xFF has to be used.
When addressing a MODBUS server connected directly to a TCP/IP network, it’s recommended not using a significant MODBUS slave address in the “Unit Identifier” field. 0xFF is recommended for the “Unit Identifier" as non-significant value.
But 0xFF is greater than 247 (decimal), how can I assign 0xFF to the slave device address?
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@mlohbihler said:
Actually, max slave id is 240.
really?
Is it not assigned from 1 to 247 (decimal) ?
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@HenryDUT said:
Thanks for fmunhoz's recommendation.
In MODBUS Messaging on TCP/IP Implementation Guide, it says that
On TCP/IP, the MODBUS server is addressed using its IP address; therefore, the MODBUS Unit Identifier is useless. The value 0xFF has to be used.
When addressing a MODBUS server connected directly to a TCP/IP network, it’s recommended not using a significant MODBUS slave address in the “Unit Identifier” field. 0xFF is recommended for the “Unit Identifier" as non-significant value.
But 0xFF is greater than 247 (decimal), how can I assign 0xFF to the slave device address?
Who can help me?
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Is the slave id actually causing problems for you?
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Amazing. Here i have PLC General Electric F650, with slave Modbus TCP and the slave id is 254 (HEX FE). Great job Modbus4J assuming a slave id between 0 to 247. JAR to TRASH
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I will refer you to the protocol specification.
http://modbus.org/docs/PI_MBUS_300.pdfPage 19 (#10) is probably the page you'd be interested in, first paragraph. Some people suggest there are extra functions mapped onto units addressed 248-255, but I can't find a spec for that.
http://www.modbus.org/docs/Modbus_over_serial_line_V1.pdf
page 8http://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf
page 23 -
villadaniel_, since it is a PLC why don't you change its slave id to something in the range that will work?
