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Bizzare serial port output

Posted by neilvp (neilvp), 23 March 2007
Can anyone shed any light on what is going on here?

I am writing a script to control a piece of test equipment via
the serial port (COM1) on a Windoze machine under XP, running

One of the commands didn't work - this command included the value 0x8B
and in trying to figure out why I wrote the following test
code that generates a count of 0-255 and outputs each byte to
the serial port.

The serial port is being monitored by a utility within the PC the
captured output of which is shown below.

If I use the supplied demo (compiled C) code to control the
instrument then the offending command works and is correctly
shown in the serial port utility. (Therefore the problem is not a quirk of the monitoring utility)

If one takes a look at the values between 0x82 and 0x9F the
majority of them are 0x3F !
The same happens under wish or tclsh.

The code...

set serial_port [open com1: RDWR]
fconfigure $serial_port -mode 9600,n,8,1 -handshake none

# Generate a 0-255 count, convert to its single byte hex value,
# place into the TxQueue on Com1 and then flush the lot

   set loop 0
   while {$loop < 256} \
       puts -nonewline $serial_port [binary format c $loop]
       incr loop
   flush $serial_port

close $serial_port

The results...

00 01 02 03 04 05 06 07 08 09 0D 0A 0B 0C 0D 0E
0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E
1F 20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E
2F 30 31 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E
3F 40 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E
4F 50 51 52 53 54 55 56 57 58 59 5A 5B 5C 5D 5E
5F 60 61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E
6F 70 71 72 73 74 75 76 77 78 79 7A 7B 7C 7D 7E
7F 3F 81 3F 3F 3F 3F 3F 3F 3F 3F 3F 3F 3F 8D 3F
8F 90 3F 3F 3F 3F 3F 3F 3F 3F 3F 3F 3F 3F 9D 3F
3F A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 AA AB AC AD AE
AF B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 BA BB BC BD BE
BF C0 C1 C2 C3 C4 C5 C6 C7 C8 C9 CA CB CC CD CE
CF D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 DA DB DC DD DE
DF E0 E1 E2 E3 E4 E5 E6 E7 E8 E9 EA EB EC ED EE
EF F0 F1 F2 F3 F4 F5 F6 F7 F8 F9 FA FB FC FD FE

Any suggestions as to what is going on and more importantly
how to fix it would be much appreciated.



Posted by neilvp (neilvp), 23 March 2007

That will teach me not to cut and paste without thinking about it.  

fconfigure -translation binary

is the answer.

Puzzled as to why it only shows around 0x8? & 0x9? though.

Posted by admin (Graham Ellis), 24 March 2007
Perhaps because they're the control codes but with the extra bits set?  The codes that really shouldn't occur anywhere except in a binary string?

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