Oki B6250n Reference Manual page 160

In general:
If the barcode numeric only data, the end of barcode data is
indicated by space/CR/LF/FF/Escape code.
If the barcode is alphanumeric, end of data is indicated by
CR/LF/FF/Escape code.
Some bar code formats support use of the full 128-character
ASCII character set. (128A, 128auto, EAN/UCC128). If you
want to print special characters (ASCII code < 32) with those
bar codes, a Transparent Print Data PCL sequence (Esc&p#X,
where # is replaced with the number of following data bytes)
must follow immediately the font selection sequence. This is
the only way to know how many characters have to be
printed as bar codes. Please analyze the sample Code/EAN/
UCC 128 test files for more information.
3. Once the data is gathered, it is analyzed to verify it fits in the
bar code specs:
Correct data size EAN 8/13 and UPC A/E have fixed lengths
etc...).
Valid data: some systems, like UPC-E, accept only specially
structured data.
4. Then the barcode data is compressed, if necessary. Some
code systems, like UPC-E, print compressed data, while ours
allows the barcode data to be precompressed (6-characters
input) or uncompressed (10 characters input). If
uncompressed data is provided, but the barcode requires the
data to be compressed, then compression is done
automatically.
5. If necessary, check digits are generated. Most barcode types
require check digits in the generated barcode. The user has
the option to either provide the checksum with the barcode
data characters, or have them generated automatically. In
the case of UPC/EAN barcodes, if the user provides checksum
characters, they are discarded, and one will be generated.
6. This feature then renders the barcode itself at the current
cursor position, in the current text direction (only 90-degree
rotations are supported. The parameters specified by the
data stream determine the final properties of the rendered
barcode.
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