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<== Date ==> <== Thread ==>
Subject: StreamDevice and negative hex values
From: Mark Davis via Tech-talk <tech-talk at aps.anl.gov>
To: "tech-talk at aps.anl.gov" <tech-talk at aps.anl.gov>
Date: Thu, 16 Apr 2020 14:26:47 -0400
I have a question about conversion of hex input strings in StreamDevice.

If I have in statements in StreamDevice protocols that, when reading the input strings "FFFF" and "8000" using the string "%4X" conversion, results in the value 65535 and 32768 (at least when using a longin record).  Given that the end result IS a long integer, this makes some sense.

However, from the point of view of the device that produces these strings, they are 2-byte signed values, so they represent the values -1 and -32678.

Is there some way to get StreamDevice to interpret these as negative values?

I see that later versions of the StreamDevice doco includes this statement regarding the Standard LONG Converters:

    "With the - negative octal and hexadecimal values are accepted."

But what does that mean?  I tried changing the protocol to -4X rather than 4X but it made no difference.

How would the '-' option be used, and under what circumstances would it result in a negative long integer value that (without the "-" prefix) would have been a positive value?

NOTE:  For now I am just using a calc record to handle the 2-byte 2's complement format used by the device.

--
Mark Davis
NSCL/FRIB Control Systems Software Engineer
davism50 at msu.edu
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