Epson 4100 manual

Operator's Guide D-3

Note: Notice that the right and left halves of the ASCII character set are identical. That is because the ASCII set was created for a 7-bit data byte (0000000 to 1111111 in the binary numbering system, or 0 to127 in decimal, or 00 to 7F in hexadecimal) back in the days of the teletypewriter. Because computers and printers now commonly use an 8-bit data byte (00000000-11111111 binary, 0-255 decimal, or 00-FF hexadecimal), the ASCII character and control codes are repeated on the upper half of the character set to cover the possibilities of the 8th bit being either a binary 0 or a binary 1.

The printer contains literally hundreds of "standard" character and control code map combinations. You can also modify the standard character set using one or more overlays. An overlay can be used to map over some or all of the character and control possibilities with other characters from ROM or RAM character libraries.

This process of creating and modifying characters sets is shown graphically on Page D-8.