Signed Byte

[feri79]

Hi,

I'm using Visa to read the packet from my hardware (sensor).

And as you know in Read Serial returns a string. I can convert the string to unsigned byte using

( Sring to unsigned byte array operator).

But I need to read them as a signed byte.

Any idea ?

Thank you.

[Norm Kirchner]

What data are you trying to retrive from the string (Integer, float, 8bit, 32bit, complex,...).

I'm assuming that the data returned is in raw form and not an ascii string.

if you take the output which you say gives you an unsigned byte and wire it to an indicator that is a signed byte (U8 wired to a I8) you should see the converted data. To see this is very simple. Just place a numeric control and a numeric indicator on the screen. Change the representation of the control to U8 and the representation of the indicator to I8. Make the Control value 207, run the VI and you should see -49 in the indicator.

If you alread know this , I must have misunderstood the question. In that case reply with some more detail of the end result you are looking for.

If not than this should get you moving along the right path. :thumbup:

-Norman J. Kirchner Jr.

Engineering Specialists, Inc.

NormK@engspec.com

[feri79]

Hi njkirchner,

Thank you for your help.

I'm trying to comm. with an inertial sensor. in the documentation is written "Each sensor

data value is represented in the data packet as a 16bit signed short (the MSB-most

significant byte and LSB - least significant byte)".

and in the table that describes the packets shows for example

Temperature (MSB) 19

Temperature (LSB) 20

"numbers relate to the byte number in the packet"

these are raw data and if i want to convert them I've to do some operations on them.

====

In order to do that I've to marge the bytes to become 16bit signed.

====

my approach was to read the data using Read Visa. that returns a string of byte. Convert the string to unsigned byte array. and using Join Numbers (under ADVANCED-->DATA MANIPULATION --> joing number) I connected the MSB and LSB to HI and LO input of Joing number.

===

Is this a right way ?

Thank you.

[Norm Kirchner]

That sounds right. Take the two bytes merge them and you can wire them directly to a I16 indicator which should automatically coerce or manualy convert the type to I16. :thumbup:

[Rolf Kalbermatter]

      Temperature (MSB)      19

      Temperature (LSB)          20

      "numbers relate to the byte number in the packet"

these are raw data and if i want to convert them I've to do some operations on them.

Your approach will work too, but since your data is really a 16 byte short in big endian format you could take advantage of the fact that LabVIEW's flattened stream format is also normalized to be in big endian.

Just pick out the interesting bytes and wire them to the Type Cast function in Advanced->Data Manipulation. Wire a int16 constant to the middle terminal e voila you get your numeric 16 bit signed value. This is simple fast and will work on any LabVIEW platform independant of the underlying endianess of the CPU, since LabVIEW takes care of the necessary (if necessary) byte swapping for you.

If the number in the stream would be in little endian format you would just have to add a Byte Swap node from the same palette to the wire after the Type Cast. Really swaps bytes twice on little endian machines (only Intel CPUs ;-) but it's most probably still faster than doing the byte extraction and joining on your own.