From Newsgroup: comp.lang.tcl


Hello,
I am getting back into Tcl after many years and am struggling how to make up a command. The script is used to communicate with a voltmeter using a visa library.
This code works:

set rm [visa::open-default-rm]
set inst [visa::open $rm "GPIB0::22::INSTR"]

The "22" in the inst variable is the meter's GPIB address. With the rest of the code,
I can capture readings, save them to a file etc.

What I am trying to figure out is how to get this line formatted to be able to use an entry box feeding a variable named gpib. I can get the variable populated
via the entry box, but getting the variable contents to work in the set inst command
is giving me trouble.

This does -not- work:

set gpib 22 (via the entry box)
set rm [visa::open-default-rm]
set inst [visa::open $rm "GPIB0::$gpib::INSTR"]

I'm thinking maybe using the format command is the key, but can't figure it out.
any thoughts?

Thanks!
Bob

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From Newsgroup: comp.lang.tcl

try it as "GPIB0::${gpib}::INSTR"

otherwise tcl thinks you are substituting a variable $gpib::INSTR, the braces protect the variable name from what surrounds it.


On 7/20/2026 5:45 PM, Bob Liesenfeld wrote:

Hello,
I am getting back into Tcl after many years and am struggling how to make up a
command. The script is used to communicate with a voltmeter using a visa library.
This code works:

set rm [visa::open-default-rm]
set inst [visa::open $rm "GPIB0::22::INSTR"]

The "22" in the inst variable is the meter's GPIB address. With the rest of the code,
I can capture readings, save them to a file etc.

What I am trying to figure out is how to get this line formatted to be able to
use an entry box feeding a variable named gpib. I can get the variable populated
via the entry box, but getting the variable contents to work in the set inst command
is giving me trouble.

This does -not- work:

set gpib 22 (via the entry box)
set rm [visa::open-default-rm]
set inst [visa::open $rm "GPIB0::$gpib::INSTR"]

I'm thinking maybe using the format command is the key, but can't figure it out.
any thoughts?

Thanks!
Bob

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From Newsgroup: comp.lang.tcl

Bob Liesenfeld <user21406@newsgrouper.org.invalid> wrote:

set gpib 22 (via the entry box)
set rm [visa::open-default-rm]
set inst [visa::open $rm "GPIB0::$gpib::INSTR"]

I'm thinking maybe using the format command is the key, but can't figure it out.
any thoughts?

et99 gave you the solution to your code above.

If you prefer the format command instead it would be:

set inst [visa::open $rm [format "GPIB0::%d::INSTR" $gpib]]

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From Newsgroup: comp.lang.tcl

Am 21.07.26 um 05:58 schrieb Rich:

Bob Liesenfeld <user21406@newsgrouper.org.invalid> wrote:

set gpib 22 (via the entry box)
set rm [visa::open-default-rm]
set inst [visa::open $rm "GPIB0::$gpib::INSTR"]

I'm thinking maybe using the format command is the key, but can't figure it out.
any thoughts?

et99 gave you the solution to your code above.

If you prefer the format command instead it would be:

set inst [visa::open $rm [format "GPIB0::%d::INSTR" $gpib]]

This is better, because it normalizes the number. E.g. if you put spaces
into the entry like " 22 ", it would not work with ${gpib}

Christian


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From Newsgroup: comp.lang.tcl

* Bob Liesenfeld <user21406@newsgrouper.org.invalid>
| This does -not- work:

| set gpib 22 (via the entry box)
| set rm [visa::open-default-rm]
| set inst [visa::open $rm "GPIB0::$gpib::INSTR"]

In addition to what others already contributed, it is always a good idea
to include any error message or diagnostic in the question - even a
"nothing happens at all" (an error dialog popping up or a message on the console is NOT nothing) helps others to rule out possible causes.

R'
http://www.catb.org/~esr/faqs/smart-questions.html
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From Newsgroup: comp.lang.tcl


Ralf Fassel <ralfixx@gmx.de> posted:

* Bob Liesenfeld <user21406@newsgrouper.org.invalid>
| This does -not- work:

| set gpib 22 (via the entry box)
| set rm [visa::open-default-rm]
| set inst [visa::open $rm "GPIB0::$gpib::INSTR"]

In addition to what others already contributed, it is always a good idea
to include any error message or diagnostic in the question - even a
"nothing happens at all" (an error dialog popping up or a message on the console is NOT nothing) helps others to rule out possible causes.

R'
http://www.catb.org/~esr/faqs/smart-questions.html

Thanks to all for the help! Script working fine now. Also, Ralf, >great< point
about the result displayed when the script does not work. Troubleshooting code is an art, and I'm a bit rusty. :-)
Bob
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From Newsgroup: comp.lang.tcl

On 7/20/2026 8:45 PM, Bob Liesenfeld wrote:

Hello,
I am getting back into Tcl after many years and am struggling how to make up a
command. The script is used to communicate with a voltmeter using a visa library.
This code works:

set rm [visa::open-default-rm]
set inst [visa::open $rm "GPIB0::22::INSTR"]

The "22" in the inst variable is the meter's GPIB address. With the rest of the code,
I can capture readings, save them to a file etc.

...

set gpib 22 (via the entry box)
set rm [visa::open-default-rm]
set inst [visa::open $rm "GPIB0::$gpib::INSTR"]

I'm thinking maybe using the format command is the key, but can't figure it out.
any thoughts?

For the sake of full error checking, if the GPIB is alphanumeric,
formatting it as %d may not work.

Here is a solution that avoids many pitfalls:

set inst [visa::open $rm "GPIB0::[string trim $gpib]::INSTR"]

You can also add a check for when/if gpib is empty and print a message.


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