I'm french student and I have been playing with SimpleModbus for 1 week, and it worked fine... Until I tried to implement a slave with a large amount of registers (1000 needed). Registers beyond 255 using slave 1 cannot be accessed. Whatever I do I get a timeout error as soon as the registers reading go beyond 255 :
control.modbus.CommunicationException: While reading holding registers 255 on slave 1
The documentation says nothing about such a limitation. Reading SimpleModbusSlave.cpp didn't help ... maybe I need to change the starting address in the "modbus update" function but I am too beginner to understand...
unsigned int modbus_update() {
if (*(ModbusPort).available())
{
unsigned char buffer = 0;
unsigned char overflow = 0;
while ((*ModbusPort).available())
{
// The maximum number of bytes is limited to the serial buffer size of 128 bytes
// If more bytes is received than the BUFFER_SIZE the overflow flag will be set and the
// serial buffer will be red untill all the data is cleared from the receive buffer.
if (overflow)
(*ModbusPort).read();
else
{
if (buffer == BUFFER_SIZE)
overflow = 1;
frame[buffer] = (*ModbusPort).read();
buffer++;
}
delayMicroseconds(T1_5); // inter character time out
}
// If an overflow occurred increment the errorCount
// variable and return to the main sketch without
// responding to the request i.e. force a timeout
if (overflow)
return errorCount++;
// The minimum request packet is 8 bytes for function 3 & 16
if (buffer > 7)
{
unsigned char id = frame[0];
broadcastFlag = 0;
if (id == 0)
broadcastFlag = 1;
if (id == slaveID || broadcastFlag) // if the recieved ID matches the slaveID or broadcasting id (0), continue
{
unsigned int crc = ((frame[buffer - 2] << 8) | frame[buffer - 1]); // combine the crc Low & High bytes
if (calculateCRC(buffer - 2) == crc) // if the calculated crc matches the recieved crc continue
{
function = frame[1];
unsigned int startingAddress = ((frame[2] << 8) | frame[3]); // combine the starting address bytes
unsigned int no_of_registers = ((frame[4] << 8) | frame[5]); // combine the number of register bytes
unsigned int maxData = startingAddress + no_of_registers;
unsigned char index;
unsigned char address;
unsigned int crc16;
// broadcasting is not supported for function 3
if (!broadcastFlag && (function == 3))
{
if (startingAddress < holdingRegsSize) // check exception 2 ILLEGAL DATA ADDRESS
{
if (maxData <= holdingRegsSize) // check exception 3 ILLEGAL DATA VALUE
{
unsigned char noOfBytes = no_of_registers * 2;
// ID, function, noOfBytes, (dataLo + dataHi)*number of registers,
// crcLo, crcHi
unsigned char responseFrameSize = 5 + noOfBytes;
frame[0] = slaveID;
frame[1] = function;
frame[2] = noOfBytes;
address = 3; // PDU starts at the 4th byte
unsigned int temp;
for (index = startingAddress; index < maxData; index++)
{
temp = regs[index];
frame[address] = temp >> 8; // split the register into 2 bytes
address++;
frame[address] = temp & 0xFF;
address++;
}
crc16 = calculateCRC(responseFrameSize - 2);
frame[responseFrameSize - 2] = crc16 >> 8; // split crc into 2 bytes
frame[responseFrameSize - 1] = crc16 & 0xFF;
sendPacket(responseFrameSize);
}
else
exceptionResponse(3); // exception 3 ILLEGAL DATA VALUE
}
else
exceptionResponse(2); // exception 2 ILLEGAL DATA ADDRESS
}
else if (function == 16)
{
// Check if the recieved number of bytes matches the calculated bytes
// minus the request bytes.
// id + function + (2 * address bytes) + (2 * no of register bytes) +
// byte count + (2 * CRC bytes) = 9 bytes
if (frame[6] == (buffer - 9))
{
if (startingAddress < holdingRegsSize) // check exception 2 ILLEGAL DATA ADDRESS
{
if (maxData <= holdingRegsSize) // check exception 3 ILLEGAL DATA VALUE
{
address = 7; // start at the 8th byte in the frame
for (index = startingAddress; index < maxData; index++)
{
regs[index] = ((frame[address] << 8) | frame[address + 1]);
address += 2;
}
// only the first 6 bytes are used for CRC calculation
crc16 = calculateCRC(6);
frame[6] = crc16 >> 8; // split crc into 2 bytes
frame[7] = crc16 & 0xFF;
// a function 16 response is an echo of the first 6 bytes from
// the request + 2 crc bytes
if (!broadcastFlag) // don't respond if it's a broadcast message
sendPacket(8);
}
else
exceptionResponse(3); // exception 3 ILLEGAL DATA VALUE
}
else
exceptionResponse(2); // exception 2 ILLEGAL DATA ADDRESS
}
else
errorCount++; // corrupted packet
}
else
exceptionResponse(1); // exception 1 ILLEGAL FUNCTION
}
else // checksum failed
errorCount++;
} // incorrect id
}
else if (buffer > 0 && buffer < 8)
errorCount++; // corrupted packet
}
return errorCount;
}
Slave: SimpleModbusSlave Any thoughts ?
Edit : I think someone had the same problem but I really did not understand what he changed :( here
Thanks in advance to those who can enlighten me!
