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Is 0x9B (155decimal) a special control character? Why is it missing from ascii tables?

Tags:

c

embedded

control-characters

communication

serial-port

I'm working on an embedded system, and i'm having dramas getting it to send a certain chunk of data across the serial port. I narrowed it down and found that if a 0x9B is present in the message, it corrupts the message.

So i then look up 0x9b (155) on http://www.asciitable.com/, and it's missing! Isn't that a bizarre coincidence!

Any ideas, is this a special character or something?

-edit- Okay sorry guys, it wasn't the 0x9b causing this, it was an 0x11 character. Which...drumroll... is an XON/XOFF character. I mistakenly had flow control as xon/xoff on the computer, and no flow control on the device! Thanks for the help anyway.

like image 826
Chris Avatar asked Apr 15 '10 04:04

Chris

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What are the ASCII special characters?

Special Characters(32–47 / 58–64 / 91–96 / 123–126): Special characters include all printable characters that are neither letters nor numbers. These include punctuation or technical, mathematical characters.

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Non-ASCII characters are those that are not encoded in ASCII, such as Unicode, EBCDIC, etc. ASCII is limited to 128 characters and was initially developed for the English language.

2 Answers

In ANSI escape sequences, 0x9B is the one-character Control Sequence Introducer (the multi-character version which is more familiar is ESC-[.

like image 149
caf Avatar answered Oct 07 '22 00:10

caf

0x9B is the CSI or "Control Sequence Introducer" its part of the C1 set of control codes, see here: http://www.search.com/reference/C0_and_C1_control_codes

Assuming the data is passing through a layer that processes C1 control codes it not surprising that there are a few bytes missing after this character as it is used to indicate the beginning of an ansi escape sequence. The bytes are disappearing because some layer is pealing them off as part of an instruction. More info on that here: http://en.wikipedia.org/wiki/Control_Sequence_Introducer

Obviously can't guarantee this is your problem but its where I would start digging in the api documentation based on the symptoms you described.

like image 22
Mark Avatar answered Oct 07 '22 01:10

Mark
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