How to read strings and negative floats?

[Jaco Fourie]

Hi all.

 

I am able to read float values from my VSD. How do I read strings and negative floats using this lib with serial RTU.

I can read the values using Python with the minimal modbus lib. But I am developing and application with a free pascal binding.

 

This is the Python code using Minimalmodbus

 

amps = vsd.read_register(registeraddress=0x0000,number_of_decimals=2, functioncode=3 ,signed=True)

serial_number = vsd.read_string(170, number_of_registers=10, functioncode=3)

 

 

This is the free pascal code using the libmodbus binding

 

ret := modbus_read_registers(ctx, 0, Length(regbuf), regbuf);

amps :=  regbuf[0] / 100;

 

amps is fine as long as it stays positive. As soon as I have a negative value it will give me the wrong value.

 

I have no idea how to get strings with this lib as I do not see a function for it.

 

Thanks.

[Jaco Fourie]

I had a look at the way it is done in Python and this is the comment

Some manufacturers allow negative values for some registers. Instead of
an allowed integer range 0 to 65535, a range -32768 to 32767 is allowed.
This is implemented as any received value in the upper range (32768 to
65535) is interpreted as negative value (in the range -32768 to -1).

Use the parameter ``signed=True`` if reading from a register that can hold
negative values. Then upper range data will be automatically converted into
negative return values (two's complement).

============== ================== ================ ===============
``signed`` Data type in slave Alternative name Range
============== ================== ================ ===============
:const:`False` Unsigned INT16 Unsigned short 0 to 65535
:const:`True` INT16 Short -32768 to 32767 ============== ================== ================ ===============

As the buffer type is unsigned 16-bit integer how will I be able to get a - value?

[Jaco Fourie]

OK I have got it working