Geometrics Inc. G-882 Cesium Marine Magnetometer Page
Commands that are echoed through the counter chain are received and sent as
unmodified ASCII strings. Thus all commands will appear in the binary data set
after the next '*' data terminating character and will be terminated itself by a
carriage return line feed sequence. Binary transmission then resumes with the
next sample.
Excess 3 format:
Excess three format is very similar to packed BCD. In fact the only difference is
that each byte has 33 hex added to it after converting to Packed BCD. The
reason for adding 33 hex to each packed BCD number is to avoid some difficult
pitfalls with Packed BCD:
Packed BCD is a very common format but has many potential problems that can
arise. ASCII digits are combined to form bytes with hexadecimal values in the
control character range (less than 20 hex) which must be handled very carefully
by the logging program. Examples of these characters include the Ctrl-S and
Ctrl-Q software handshake controls (11 hex and 13 hex), the bell character (Ctrl-
G, 07 hex), and the ASCII null (Ctrl-shift-@, 00H). Most terminal emulation
programs can be configured to handle these characters as data instead of
commands, but this is not the way the typical default configuration is set up.
Packed BCD eliminates this by shifting all numbers up by 33 hex. This moves all
possible output values out of the control character range. It also makes them
printable to a screen or printer without bells, beeps, screen clears, form feeds,
etc. However they will still look like gibberish without translation.
Sandia/G822A format:
This is a printable ASCII format that mimics the output from a one or two channel
G822A magnetometer. Its output is limited to one counter module, with the Mag
and signal level values as the only data being sent out. The Mag reading is
preceded with an 'A' followed by 10 characters of ASCII Mag data. The G822A
format sometimes has a second Mag reading following the first which is preceded
with an ASCII 'B'. If selected the CM-221 counter places the signal level in the
first 4 significant characters of the second Mag data slot. The sample is
terminated by a carriage return line feed sequence.
The purpose of this format is to allow customer with existing G822A Sandia
logging software to be able to use the CM-221 without upgrading to new logging
software.
The single channel Sandia format is selected with the command string "OS" or
"OS0". The dual channel output is selected by the command string "OS1".
Example outputs:
This is the ASCII output example from earlier, but with 3 A/D channels:
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