Length of read in in python-gpib

[Madhusudan Singh]

python-gpib provides Gpib.py (see end of post) for Linux.

I am trying to use the method called read. I usually use it without
arguments (the default length being 512). However, I am trying to read in a
string with some 16,000 comma separated floating point numbers.

So, I have to pass a length parameter much much larger than 512. As is
stands, the default read takes in only about 35-40 numbers, so I need about
512/35*16000 ~= 230,000. Not being sure if I could make python read in
something that big, I started small - with 1024. This produces an list of
numbers that has a malformed first number (I get 88309e-9 instead of
something like 6.788309e-9.).

Could this be a bug ?

Second, does the length need to be a power of two for some reason ?

python-gpib is an excellent library, but it suffers from an utter lack of
documentation.

------------------------ Gpib.py -----------------------------
import gpib

RQS = (1<<11)
SRQ = (1<<12)
TIMO = (1<<14)
class Gpib:
def __init__(self,n ame='gpib0'):
self.id = gpib.find(name)
def write(self,str) :
gpib.write(self .id, str)

def writebin(self,s tr,len):
gpib.writebin(s elf.id,str,len)
def read(self,len=5 12):
self.res = gpib.read(self. id,len)
return self.res

def readbin(self,le n=512):
self.res = gpib.readbin(se lf.id,len)
return self.res

def clear(self):
gpib.clear(self .id)

def wait(self,mask) :
gpib.wait(self. id,mask)

def rsp(self):
self.spb = gpib.rsp(self.i d)
return self.spb

def trigger(self):
gpib.trg(self.i d)

def ren(self,val):
gpib.ren(self.i d,val)

def ibsta(self):
self.res = gpib.ibsta()
return self.res

def ibcnt(self):
self.res = gpib.ibcnt()
return self.res

def tmo(self,value) :
return gpib.tmo(self.i d,value)

[Madhusudan Singh]

Further, if I use 131072 (2^17) as the length, I get an error :

Traceback (most recent call last):
File "takedata.p y", line 74, in ?
x=sr.querytrca( srs,1,0,4000,13 1072)
File "/home/labmonkey/code/oledpmt/sr830.py", line 62, in querytrca
trca=sr.read(n)
File "/usr/lib/python2.3/site-packages/Gpib.py", line 22, in read
self.res = gpib.read(self. id,len)
gpib.error: Read Error: ibrd() failed

Following this, an attempt to run the script again chokes at the first
(harmless - just setting some instrument parameter - ibwrt() fails) gpib
command and then locks the bus, freezing the whole system unless I reboot
the system. This has happened twice.

I am using the Agilent82357a USB to GPIB convertor. Have used it for
numerous purposes with the same setup, but the library seems to be choking
on this new application. The instrument I am trying to use is an SR830
lockin amplifier.

[Dietmar Schwertberger]

In article <43************ ***********@new s.sunsite.dk>, Madhusudan Singh
<URL:mailto:sp* *************@s pam.invalid> wrote:

python-gpib provides Gpib.py (see end of post) for Linux.

I am trying to use the method called read. I usually use it without
arguments (the default length being 512). However, I am trying to read in a
string with some 16,000 comma separated floating point numbers.

I've never used python-gpib, but I'm using National Instrument GPIB
cards with a wrapper that loads the driver DLL using ctypes.
Usually for such long data you have to perform multiple read operations and
check the card/bus status whether the read operation is finished
(Code "END" - "END or EOS detected").
So with the National Instrument drivers I'm checking whether ibsta is
something like 0x2000, i.e. 1<<13.

The following code is a snippet from my sources.
Hope this helps. From your post I can see that python-gpib also provides
an ibsta method. I'd expect that END is also 1<<13.

Regards,

Dietmar

import ctypes

gpib = ctypes.windll.L oadLibrary("gpi b-32")

# read buffer
_bufsize = 1024
_buffer = ctypes.c_buffer (_bufsize)

_readstatus = 0x2000
_errorstatus = 0x8000

def get_ibcntl():
return ctypes.c_int.in _dll(gpib, "user_ibcntl"). value

def ibrd(handle):
ret = []
ibsta=0
global _buffer

while not (ibsta & _readstatus):
ibsta = gpib.ibrd(handl e, _buffer, _bufsize)
new = _buffer.raw[:get_ibcntl()]
ret.append( new )
if ibsta & _errorstatus:
_raise(GPIBIOEr ror,
"ibrd(handle=%d ): Could not read data."%handle, ibsta )
ret = "".join(ret )
if '\012' in ret:
ret = ret[0:string.find(r et, '\012')]
if '\015' in ret:
ret = ret[0:string.find(r et, '\015')]
return ret