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2's complement convertion
GhostWolf last edited by
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?
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.