Instrument Control (GPIB, Serial, VISA, IVI)

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Assigning parameters for write_ascii_values in pyvisa for serial port

Hi all,

 

I want to control my Keithley 6485 Picoammeter externally by connecting it via RS232toUSB to my Linux PC (CentOS 6.9) and writing a code in python (version 2.7.13) with pyvisa:

 

import sys
import visa
from visa import constants

rm = visa.ResourceManager('/usr/local/vxipnp/linux/lib64/libvisa.so')
#open serial connection and set baud to 9600, 8 data bits, CR termination, one stop bit, none parity, no flow control
amm = rm.open_resource('ASRL2::INSTR', baud_rate = 9600, data_bits = 8, write_termination= '\r', read_termination = '\r')
constants.VI_ASRL_STOP_ONE     
constants.VI_ASRL_PAR_NONE
constants.VI_ASRL_FLOW_NONE                  
amm.write("*RST")                       # Return 6485 to RST default
amm.write("SYS:ERR:ALL?")               # Return error message 
amm.write("TRIG:DEL 0")                 # Set trigger delay to zero seconds
amm.write("TRIG:COUNT 2500")            # Set trigger count to 2500
amm.write("SENS:CURR:RANG:AUTO OFF")    # Turn auto range off
amm.write("SENS:CURR:NPLC .01")         # Set integration rate to NPLC 0.01
amm.write("SENS:CURR:RANG 2e-7")        # Use 200 nA range 
amm.write("SYST:ZCH OFF")               # Turn zero check off
amm.write("SYST:AZER:STAT OFF")         # Turn auto zero off
amm.write("DISP:ENAB OFF")              # Turn Display off
amm.write("*CLS")                       # Clear status model
amm.write("TRAC:POIN 2500")             # Set buffer size to 2500
amm.write("TRAC:CLE")                   # Clear buffer
amm.write("TRAC:FEED:CONT NEXT")        # Set storage control to start on next reading
amm.write("STAT:MEAS:ENAB 512")         # Enable buffer full measurement event
amm.write("*SRE 1")                     # Enable SRQ on buffer full measurement event
amm.write("*OPC?")                      # operation complete query (synchronize completion of commands)    
amm.write("INIT")                       # start taking and storing readings wait for GPIB SRQ line to go true
amm.write("DISP:ENAB ON")               # Turn display on
print(amm.query_ascii_values("TRAC:DATA?")) # Request data from buffer

The problem when I run this script I just get "1" as the print output, although it should be returned in ASCII like this: Reading, Timestamp, Status and the error message after amm.write("*RST"): -113 undefined header. So I think the messages and not transferred correctly.

I know over the RS-232 interface, only the ASCII format is allowed. But when I follow the example in the pyvisa instruction with write_ascii_values(text, values) and assigning it a list, I only get an error message from the device -100 Command error.

Can somebody please tell me how to set the variables in write_ascii_values correctly or what I am doing wrong? Are my settings for the serial device wrong? Sometimes when I execute 2 times I get the error "VI_ERROR_ASRL_FRAMING (-1073807253): A framing error occurred during transfer ." too. I just do not know what to do.

Thank you!

Regards, Roland

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Message 1 of 7
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Try adding a "\n" to every command and query. 

amm.write("*RST\n")

Also, how are you handling the queries?  All I see are amm.write commands.  Where are the amm.read commands following a write command with a "?" within the quotes?  Once you send a command containing a "?" in the message, the instrument will then want to respond to that request.  The "1" that you receive in your print function is probably from the earlier command

amm.write("SYST:ERR:ALL?)

I do know that when you send the

 

amm.write("*OPC?")

command that it will respond with a "1" to signify that the instrument has completed processing all earlier commands.

 

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Message 2 of 7
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Hi,

 

thanks for your answer. The printed "1" comes indeed from the command

amm.write("*OPC?").

When I erase OPC? and SYS:ERR:ALL? from the code I get the error:

File "./AMDAQ.py", line 31, in <module> print(amm.query("TRAC:DATA?"))

pyvisa.errors.VisaIOError: VI_ERROR_TMO (-1073807339): Timeout expired before operation completed.

 

The adding of "\n" did not help (still get the -113 error). I assign the read and write termination as "\r" here:

amm = rm.open_resource('ASRL2::INSTR', baud_rate = 9600, data_bits = 8, write_termination= '\r', read_termination = '\r'

 Adding \n would lead to \n\r, so a LFCR termination for the device. However I tried every termination possibility with the device: CR, LF, CRLF, LFCR, but it does not change anything.

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Message 3 of 7
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I am not that familiar with this instrument, but typically, the first thing to do after a reset is to configure the instrument for the type of measuring.

CONF:CURR:DC

Also, did you try using "READ?" for your print function

 

print(amm.query("READ?"))
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Message 4 of 7
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The manual is confusing as it sometimes refers to the same command with or without the prefix of ":"

:SYST:CLE
or
SYST:CLE

Not sure if it matters?

 

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Message 5 of 7
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BTW: Keep the *OPC? statement where it is, just change it to a query as it allows the system time to configure itself properly before responding.

amm.query("*OPC?")
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Message 6 of 7
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Thanks again for your help. I amended my script according to your change advises. It does not change anything.

 

I also change from RS232 to GPIB, but I get the same VI_ERROR_TMO (-1073807339) error. Also when I change the language from SCPI to 488.1. So there is definitely something wrong with the translation when it wants to read the data out of it. But I am out of ideas...

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