2's complement convertion

GhostWolf

Hello,

I have build a programm that gets some data from a modbus device.
The modbus device sends its data with the 2's complement way.

How can i convert the incoming value to a normal integer?

This is what is given with the device:

Extended addressing
The panels use extended (32-bit) addressing format.
http://www.simplymodbus.ca/faq.htm#Ext
Value = (65536 * MSW) + LSW
How to interpret the values
The two's complement of the value is used.
http://en.wikipedia.org/wiki/Two%27s_complement
32 bit example: 1 is represented as 0b00000000_00000000_00000000_00000001
32 bit example: -1 is represented as 0b11111111_11111111_11111111_11111111

This is the code i have now:

auto replyIR = qobject_cast<QModbusReply *>(sender());
const QModbusDataUnit unit = replyIR->result();
uint32_t test2 = (unit.value(1)<<16) | unit.value(0);
ui->r1->setText(QString::number(test2);

(this works the same as (65536 * unit.value(1)) + unit.value(0); )

This works fine for positive numbers. The problem is the negative numbers.
How can i read all the numbers?

JKSH

@GhostWolf said in 2's complement convertion:

uint32_t test2 = (unit.value(1)<<16) | unit.value(0);

Just store the result as a signed integer (int32_t) instead of unsigned (uint32_t).

You don't need to change the bit pattern (0b11111111_11111111_11111111_11111111).

Interpreting this pattern as Unsigned gives you ‭4294967295
Interpreting this pattern as Signed gives you -1.