Data acquisition

[spintronic]

Dear friends,

I have a trouble with understanding the following. I have a very short
script (shown below) which works fine if I "run" step by step (or line
by line) in Python shell (type the first line/command -> press Enter,
etc.). I can get all numbers (actually, there are no numbers but a
long string, but this is not a problem) I need from a device:

'0.3345098119,0.01069121274,0.02111624694,0.03833379529,0.02462816409,0.0774275008,0.06554297421,0.07366750919,0.08122602002,0.004018369318,0.03508462415,0.04829900696,0.06383554085, ...'

However, when I start very the same list of commands as a script, it
gives me the following, which is certainly wrong:

[0.0, 0.0, 0.0, 0.0, 0.0,...]

Any ideas? Why there is a difference when I run the script or do it
command by command?

===========================
from visa import *

mw = instrument("GPIB0::20::INSTR", timeout = None)

mw.write("*RST")
mw.write("CALC1ATA? FDATA")

a=mw.read()

print a
===========================
(That is really all!)


PS In this case I use Python Enthought for Windows, but I am not an
expert in Windows (I work usually in Linux but now I need to run this
data acquisition under Windows).

 

[Jean-Michel Pichavant]

Dear friends,

I have a trouble with understanding the following. I have a very short
script (shown below) which works fine if I "run" step by step (or line
by line) in Python shell (type the first line/command -> press Enter,
etc.). I can get all numbers (actually, there are no numbers but a
long string, but this is not a problem) I need from a device:

'0.3345098119,0.01069121274,0.02111624694,0.03833379529,0.02462816409,0.0774275008,0.06554297421,0.07366750919,0.08122602002,0.004018369318,0.03508462415,0.04829900696,0.06383554085, ...'

However, when I start very the same list of commands as a script, it
gives me the following, which is certainly wrong:

[0.0, 0.0, 0.0, 0.0, 0.0,...]

Any ideas? Why there is a difference when I run the script or do it
command by command?

===========================
from visa import *

mw = instrument("GPIB0::20::INSTR", timeout = None)

mw.write("*RST")
mw.write("CALC1ATA? FDATA")

a=mw.read()

print a
===========================
(That is really all!)


PS In this case I use Python Enthought for Windows, but I am not an
expert in Windows (I work usually in Linux but now I need to run this
data acquisition under Windows).

Just in case you have a local installation of visa and it silently fails
on some import,

try to add at the begining of your script:
import sys
sys.path.append('')

When using the python shell cmd line, '' is added to sys.path by the
shell, that is one difference that can make relative imports fail in
your script.

If it's still not working, well, it means the problem is somewhere else.

JM

 

[Nick Dokos]

Dear friends,

I have a trouble with understanding the following. I have a very short
script (shown below) which works fine if I "run" step by step (or line
by line) in Python shell (type the first line/command -> press Enter,
etc.). I can get all numbers (actually, there are no numbers but a
long string, but this is not a problem) I need from a device:

'0.3345098119,0.01069121274,0.02111624694,0.03833379529,0.02462816409,0.0774275008,0.06554297421,0.07366750919,0.08122602002,0.004018369318,0.03508462415,0.04829900696,0.06383554085, ...'

However, when I start very the same list of commands as a script, it
gives me the following, which is certainly wrong:

[0.0, 0.0, 0.0, 0.0, 0.0,...]

Any ideas? Why there is a difference when I run the script or do it
command by command?

===========================
from visa import *

mw = instrument("GPIB0::20::INSTR", timeout = None)

mw.write("*RST")
mw.write("CALC1ATA? FDATA")

a=mw.read()

print a
===========================
(That is really all!)


PS In this case I use Python Enthought for Windows, but I am not an
expert in Windows (I work usually in Linux but now I need to run this
data acquisition under Windows).

Shot in the dark: could it be that you have to add delays to give the
instrument time to adjust? When you do it from the python shell, line by
line, there is a long delay between one line and the next.

Nick

 

[John Gordon]

Any ideas? Why there is a difference when I run the script or do it
command by command?

Are you running the same python program in both cases?

Are you in the same directory in both cases?

Does PYTHONPATH and/or sys.path have the same value in both cases?

Show us an exact transscript of both executions.

 

[spintronic]

Shot in the dark: could it be that you have to add delays to give the
instrument time to adjust? When you do it from the python shell, line by
line, there is a long delay between one line and the next.

Nick

Hi, Nick!

Thanks! You are right but it was the first thing I thought about. So I
have tried to delay using sleep(t) from the time module (I also sent
"*OPC?" or "*WAI" commands to a device for synchronization). However,
it does not help ...

Best,
AS

 

[spintronic]

Thanks, John!

Are you running the same python program in both cases?

Yes, the same.

Are you in the same directory in both cases?
Does PYTHONPATH and/or sys.path have the same value in both cases?

It looks that yes but how can it matter? All I need it is to import
the visa module and it works well.

Show us an exact transscript of both executions.

There is nothing but "numbers". Or do you mean something else? I do
not receive any errors, only different results ...

Best,
AS

 

[spintronic]

Dear friends,

I have a trouble with understanding the following. I have a very short
script (shown below) which works fine if I "run" step by step (or line
by line) in Python shell (type the first line/command -> press Enter,
etc.). I can get all numbers (actually, there are no numbers but a
long string, but this is not a problem) I need from a device:

'0.3345098119,0.01069121274,0.02111624694,0.03833379529,0.02462816409,0..0774275008,0.06554297421,0.07366750919,0.08122602002,0.004018369318,0.03508462415,0.04829900696,0.06383554085, ...'

However, when I start very the same list of commands as a script, it
gives me the following, which is certainly wrong:

[0.0, 0.0, 0.0, 0.0, 0.0,...]

Any ideas? Why there is a difference when I run the script or do it
command by command?

===========================
from visa import *

mw = instrument("GPIB0::20::INSTR", timeout = None)

mw.write("*RST")
mw.write("CALC1ATA? FDATA")

print a
===========================
(That is really all!)

PS In this case I use Python Enthought for Windows, but I am not an
expert in Windows (I work usually in Linux but now I need to run this
data acquisition under Windows).

Just in case you have a local installation of visa and it silently fails
on some import,

try to add at the begining of your script:
import sys
sys.path.append('')

When using the python shell cmd line, '' is added to sys.path by the
shell, that is one difference that can make relative imports fail in
your script.

If it's still not working, well, it means the problem is somewhere else.

JM

Hi!

Thanks! I have just tried. Unfortunately, it does not work ...

Best,
AS

 

[John Gordon]

It looks that yes but how can it matter? All I need it is to import
the visa module and it works well.

If you run the two cases from different directories, and the current
directory is in PYTHONPATH or sys.path, and one of the directories
contains a python file named "visa.py" and the other doesn't, that
culd account for the difference in output.

Do you have access to the visa.py source code? Can you add a simple
print statement near the top of the module so that we know the same
visa.py module is being imported in both cases?

There is nothing but "numbers". Or do you mean something else? I do
not receive any errors, only different results ...

I was more interested in the exact commands you used to run both cases,
rather than the output.

 

[Dietmar Schwertberger]

Am 25.10.2011 19:22, schrieb spintronic:

Thanks! You are right but it was the first thing I thought about. So I
have tried to delay using sleep(t) from the time module (I also sent
"*OPC?" or "*WAI" commands to a device for synchronization). However,
it does not help ...

RST is resetting all data and CALC is somehow calculating and returning
data. Without a trigger between RST and CALC, I would not expect any
data...

Maybe the equipment is triggering continuously e.g. every second.
When you were using the shell, you had a good chance to see a trigger
between RST and CALC. With a script, it's not so likely.
OPC won't help, as it would wait for completion of a measurement, but if
you don't trigger, it won't wait.

What kind of instrument are you using? Check for the trigger command.
It may be something like INIT:IMM

Regards,

Dietmar

 

[Paul Simon]

Dear friends,

I have a trouble with understanding the following. I have a very short
script (shown below) which works fine if I "run" step by step (or line
by line) in Python shell (type the first line/command -> press Enter,
etc.). I can get all numbers (actually, there are no numbers but a
long string, but this is not a problem) I need from a device:

'0.3345098119,0.01069121274,0.02111624694,0.03833379529,0.02462816409,0.0774275008,0.06554297421,0.07366750919,0.08122602002,0.004018369318,0.03508462415,0.04829900696,0.06383554085,
...'

However, when I start very the same list of commands as a script, it
gives me the following, which is certainly wrong:

[0.0, 0.0, 0.0, 0.0, 0.0,...]

Any ideas? Why there is a difference when I run the script or do it
command by command?

===========================
from visa import *

mw = instrument("GPIB0::20::INSTR", timeout = None)

mw.write("*RST")
mw.write("CALC1ATA? FDATA")

a=mw.read()

print a
===========================
(That is really all!)


PS In this case I use Python Enthought for Windows, but I am not an
expert in Windows (I work usually in Linux but now I need to run this
data acquisition under Windows).

I'm almost certain that there is a turnaround timing issue that is causing
the problem. These are common problems in data aquisition systems. The
simplest solution is to loop and wait for end of line from the sending end
and if necessary put in a time delay. After receiving the data, check the
received data for correct format, correct first and last characters, and if
possible, check sum. I've worked through this problem with rs-485 data
collection systems where there is no hand shaking and would not be surprised
to expect the same even with rs-232.

Paul Simon

 

[Dennis Lee Bieber]

script (shown below) which works fine if I "run" step by step (or line
by line) in Python shell (type the first line/command -> press Enter,
etc.). I can get all numbers (actually, there are no numbers but a
long string, but this is not a problem) I need from a device:

However, when I start very the same list of commands as a script, it
gives me the following, which is certainly wrong:

[0.0, 0.0, 0.0, 0.0, 0.0,...]

mw = instrument("GPIB0::20::INSTR", timeout = None)

Does "timeout=None" mean "wait forever", or "return with whatever is
available"?

If the latter, it would mean the script is running too fast for the
device to respond, and you should add some sleeps.

From the PyVisa documentation:

doc> Here, my device may be a device, an interface or whatever, and its
timeout is set to 25 seconds. Floating-point values
doc> are allowed. If you set it to zero, all operations must succeed
instantaneously. You must not set it to None. Instead, if
doc> you want to remove the timeout, just say
doc> del my_device.timeout
doc> Now every operation of the resource takes as long as it takes, even
indefinitely if necessary.

Note the warning: "YOU MUST NOT SET IT TO NONE"

mw.write("*RST")
mw.write("CALC1ATA? FDATA")

Does the write() buffer? Do you need to flush the output (something
that may be occurring in the background with the interactive session).