And testing reveals that the answer is that it IS interpreted as a 3270 
datastream, partially. The following Rexx, running under ISPF

/* Rexx */                                            
say 'some stuff ************************************' 
say 'f5c311404040'x||'Hellow clear world!'            
say 'more stuff'                                      
say 'and still more'                                                  

produces the following at the top of the screen.

Hellow clear world! ***************************
5C                                             
more stuff                                     
and still more                                 
***                                              

My interpretation is

- the F5C3 is ineffective. The command has to be the first thing in the 3270 DS 
and Rexx and/or TSO already provides that.
- However, the SBA (0,0) has the desired effect. Notice that my first SAY has 
been (partially) overwritten
- I can't explain why the F5C3 comes out *after* "Hellow clear world!" Can 
anyone else?

I am not sure if I have not made a mess of TSO's unformatted screen management. 
I wonder if TSO's "time to clear the screen" line count may be thrown off.

On Thu, 18 Jun 2026 15:46:18 +0200, salva <[email protected]> wrote:

>  It is easier to test it—it's just half a line—than to drop the dogma.
>
>El jue, 18 jun 2026 a las 10:34, Seymour J Metz (<[email protected]>) escribió:
>
>> It's irrelevant; it won't be interpreted as a 3270 display stream.
>>
>> --
>> Shmuel (Seymour J.) Metz
>> http://mason.gmu.edu/~smetz3
>> עַם יִשְׂרָאֵל חַי
>> נֵ֣צַח יִשְׂרָאֵ֔ל לֹ֥א יְשַׁקֵּ֖ר
>>
>>
>>
>>
>> ________________________________________
>> From: IBM Mainframe Discussion List <[email protected]> on behalf
>> of Martin Packer <[email protected]>
>> Sent: Thursday, June 18, 2026 4:24 AM
>> To: [email protected] <[email protected]>
>> Subject: Re: Two TSO terminal I/O from Rexx questions
>>
>>
>> External Message: Use Caution
>>
>>
>> For those of us unfamiliar with 3270 data streams can somebody decode the
>> hex before the readable text. Thanks!
>>
>> From: IBM Mainframe Discussion List <[email protected]> on behalf
>> of salva <[email protected]>
>> Date: Thursday, 18 June 2026 at 07:43
>> To: [email protected] <[email protected]>
>> Subject: [EXTERNAL] Re: Two TSO terminal I/O from Rexx questions
>>
>> 1. You can play with:
>> say 'f5c311404040'x||'Hellow clear world!'
>> (your result may change based on your Logmode, I can't do an  empty say
>> 'f5c311404040'x.

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