Printers often employ their own character set, usually independently of a parent computer. The printer driver software provided with a printer should perform any necessary code conversion and should also send special commands to the printer in the form of escape sequences.
8-bit coding can be non-standard, although the codes from 0 to 127 are set aside for ASCII characters. The remainder are used to suit a particular device, although often borrowed from standard IBM character sets. In some instances, higher codes are used for an alternative character set.
The character sets described below are used in older printers manufactured by Epson. In most instances code 255 has the same effect as 127, a null character. Codes 35 to 36, 38, 64, 79, 91 to 96, 105 and 123 to 126, as well as matching values in the alternate set, can also be used for other characters as defined in the appropriate Epson National Character Set for your geographical area.
Normally, codes in the range of 0 to 31 and 128 to 159 operate as standard control codes. However, they can also be used to generate extra characters, as shown in blue in the following tables. To do this, your computer must initially send the printer an escape sequence in the form of
ESC I SOH.
This provides ASCII characters and an alternate set in italic form, as shown below:-
ESC t NUL ESC 7.
This set is also known as the Epson International Set. It provides standard ASCII characters and an alternate set in italic form, as shown below:-
Unlike the Epson Normal Character Set, the codes from 128 to 144 are always used for the characters shown in this diagram.
ESC t NUL ESC 6.
This set, derived from the IBM All Character 437 Set, is shown below:-
ESC t SOH ESC 7.
This set, also derived from the IBM All Character 437 Set, is shown below:-
ESC t SOH ESC 6.
As with Epson printers, other vintage dot-matrix printers often use the higher codes for a second set of characters with a different style, usually italic. Each character in this alternate character set uses the standard ASCII number plus 128. An example of such coding is shown in the table below:-
This type of character set is often used by older devices that can only accept 7-bit data. In this type of equipment the standard character set (codes 0 to 127) is normally available. However, the device can be easily switched into the alternate set (codes 128 to 255) by simply sending it a
SI (Shift In) control code. It can then be switched back to the standard set by sending a
SO (Shift Out) code.
Apple’s ImageWriter II dot-matrix printer also uses a non-standard set, as shown below. It includes codes for inverse characters (popular in older computer systems) and special graphics.
©Ray White 2004.