adccommunitymod (AutomationDirect) asked a question.
Modbus maximum block size
Created Date: March 20,2003
Created By: Michel
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I'm transfering data from a DL205 (250CPU) to my PC using Modbus and I am limited to a block size of 127 elements (127 * 2 bytes = 254 bytes) per request. Is this a limitation of the Modbus protocol or the Koyo PLC? Is there a way to increase the block size? Thanks Michel.
Created Date: June 04,2003
Created by: Mike McClanahan
I believe that I read somewhere that the 254 byte limit was part of the original modbus protocol. You would have to ask one of the techs at ADC if their PLC would allow you to go past that but I don't believe it does from what I read in the specs.
Mike
Created Date: June 04,2003
Created by: jackson
This would be a limitation of the MODBUS protocol because the "Byte Count " field is only one byte, therefore max. number is 0xFF (255 decimal).
I 've always thought this was a little strange, considering that the preceding fields are called "Number of Registers " and there are 2 bytes for this....
Created Date: March 20,2003
Created by: Michel
I'm transfering data from a DL205 (250CPU) to my PC using Modbus and I am limited to a block size of 127 elements (127 * 2 bytes = 254 bytes) per request.
Is this a limitation of the Modbus protocol or the Koyo PLC?
Is there a way to increase the block size?
Thanks
Michel.