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smarmengol / Modbus-Master-Slave-for-Arduino

Modbus Master-Slave library for Arduino
GNU Lesser General Public License v2.1
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MAX_BUFFER question #5

Closed 0jinjing closed 4 years ago

0jinjing commented 8 years ago

Hi, I have a question with MAX_BUFFER. Default MAX_BUFFER is 64 bytes, but I'm add this value to 512. It's goes well, and give right response. However, MAX_BUFFER more than 512. My modbus Master get nothing. Is there some limit? I'm tried to change T35's value but it's not work.

Could you illustrate it ? Thanks.

smarmengol commented 8 years ago

Hi, I haven't tried it. Anyway the MODBUS standard doesn't allow more than 253 bytes:

http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf

According to this web

http://www.hobbytronics.co.uk/arduino-serial-buffer-size

you could. Maybe the problem is inside my code.

Regards,

0jinjing commented 8 years ago

Hi, I'm trying to got au16data that au16data's size is 1024bytes and modbus master can polling 10 times to get all data, eg. first time is get 0x0~0x78, second time is 0x79~0xF0.....

"http://www.hobbytronics.co.uk/arduino-serial-buffer-size"=> Increasing the buffer size in HardwareSerial.cpp is not help save this problem.

I think it's because RAM is not enough to save "au16data" that size more than 256. I check RAM with this and print au16data's data: int free_ram() { extern int heap_start, *brkval; int v; return (int) &v - (brkval == 0 ? (int) &__heap_start : (int) brkval); } It shows au16data[2xx] is not be "0", maybe it means memory conflict.

shivalingaraju commented 7 years ago

hi Ojinjing, I am trying to read 200 bytes of data from the RS485 is it possible. if yes how can I achieve it?