Modbus slave

[MTZOVARAS]

Hello,

master is a device that wants
to write to my software that acts as slave

what should i do to my software (slave) to take the values from master

i have sent you my source code

[Roman Novgorodov]

Hello

Modbus protocol does not consider such usage. If you use real hardware devices slave can not ask master to read or write values.

If you do not have real hardware modbus devices and you develop app for both sides of TCP connection, you can implement Master1 <-> Slave2 on 502 port and Slave1 <-> Master2 on other TCP port.

But, in that case, better to use ordinary socket communication without modbus at all.

I do no understand what do you do and why.

Roman Novgorodov

[MTZOVARAS]

Hello,

sorry for the misunderstanding !

i develop a software as Modbus slave
there is a device (wcclite) that acts as a Modbus master that collects values from "holding registers" and "coils" from my application
after some calculations the device wants to return an "unsigned16" value, and an "signed32" value to my application
should i define "input registers" and wait the device to write to them ?

Or how this can be done ?

[Roman Novgorodov]

Yes, it is ordinary behaviour. Master device polls slave app for coils and regs and, when needed, changes that regs and coils.

Roman Novgorodov

[MTZOVARAS]

Hello,

thank you for your help !

can you please send me some lines of code or some tips for my slave application to take values from Modbus Master ?

Do i need "holding registers" or "input registers"

Until now i have "holding registers" where i give initial values but i do not see Modbus Master to give them the new values !

[Roman Novgorodov]

Hello

Slave app code can read or change current values for registers or coils as follows

Code Select Expand

nrModbus1.SlaveRegisters.Register[1234] :=  456; // access and change register 1234;
 nrModbus1.SlaveCoils.Coil[88] :=  true;          // access and change coil 88;


If you need to handle moment writing value you need to handle following event:

Code Select Expand

procedure TForm1.nrModbus1ModbusRequest(sender: TObject; Reply: TnrModbusPacket;
  Registers: TnrModbusRegisterList; Coils: TnrModbusCoilList; isWrite: Boolean;
  var error_code: TnrModbusError);
var
    sCoil : string;
    i:integer;
begin
  sCoil := '';
  for I := 0 to Coils.Count - 1 do
  begin
   sCoil := sCoil + 'Device: ' + IntToStr(Reply.Address) + ' Coil:' + IntToStr(Coils.GetCoilNumber(i)) + ' = ' + IntToHex(Coils.GetCoilValue(i).ToInteger(), 4) + #13#10;
  // listbox1.items.add('in modbus request : '+sreg);
  end;
  mmoErrorLog.Lines.Add(sCoil);
end;

Roman Novgorodov

[MTZOVARAS]

Hello,

master send reg 200 value 1
                201 value 2

slave receives reg 100 value 0
               reg 101 value 0

[Roman Novgorodov]

Hello

Yes it is correct.

Th 200 and 201 regs are not 100 and 101

Roman Novgorodov

[MTZOVARAS]

Hello,

is it a bug ?

should i wait for a correction ?

[Roman Novgorodov]

Hello

If MASTER writes to 200 register, the SLAVE gets that value in its 200 register. It works correct.
Why do you want to see value in register 100 in slave, if MASTER writes to 200?
I do not understand.

Roman Novgorodov