Keyboard and character set of my 3178 terminal

KEYBOARD

Data Entry Keypunch keyboard from the IBM 3178 terminal

CHARACTER SET FROM 0x00 to 0xBF

The reason that the main screen characters are restricted to the range of 0x00 to 0xBF is that the way the controller and terminal distinguishes an attribute byte from a data byte is through the high bit being 1. A screen on a 3270 is typically broken into fields, each beginning with an attribute byte. This byte specifies how the subsequent screen positions behave, up until the next attribute byte. It may protect a field from update by the operator, make the characters intensified, or restrict the typed information to be numeric. 

Screen character codes on the 3178

The bottom line of a display is a special area, called the Operator Information Area. It is always protected so that the operator cannot type in it and the arrow/tab keys won't move the cursor there. Because it is not programmable, it does not need nor use attribute bytes. Thus, for this special last line, the codes from 0xC0 to 0xFF can represent displayable characters that are only used in this area. 

Special characters for OIA line, courtesy Andrew Kay