Input from a 3270 terminal
Keyboard input reaches the host through the buffer. There are many different keyboard arrangements available for 3270 terminals, but in any arrangement, a key falls into one of three categories, data key, keyboard control key, or attention key.
Data keys
The data keys include all the familiar letters, numbers, punctuation marks, and special characters. Pressing a data key changes the content of the buffer (and therefore the screen) at the point indicated by the cursor. The cursor is a visible pointer to the position on the screen (that is, in the buffer) where the next data keystroke is to be stored. As the operator keys data, the cursor advances to the next position on the screen, skipping over fields defined with the autoskip attribute on the screens that have been formatted.
Keyboard control keys
Keyboard control keys move the cursor to a new position, erase fields or individual buffer positions, cause characters to be inserted, or otherwise change where or how the keyboard modifies the buffer.
Attention keys
The keys in the previous groups, Data, and Keyboard control keys, cause no interaction with the host; they are handled entirely by the device and its control unit. An attention key, signals that the buffer is ready for transmission to the host. If the host has issued a read to the terminal, the typical situation in CICS® , transmission occurs at this time.
- ENTER
- F (function) key
- CLEAR
- PA (program attention) key
- CNCL (cancel key, present only on some keyboard models)
- Using an identification card reader
- Using a magnetic slot reader or hand scanner
- Selecting an attention field with a light pen or the cursor select key
- Moving the cursor out of a trigger field
Trigger field capability is provided with extended attributes on some terminal models, but all the other actions listed previously require special hardware, and in most cases the screen (buffer) must be set up appropriately beforehand. We talk about these features in Support for special hardware . For this section, we concentrate on standard features.
The AID
The 3270 identifies the key that causes transmission by an encoded value in the first byte of the inbound data stream. This value is called the attention identifier or AID.
Ordinarily, the key that the terminal operator chooses to transmit data is dictated by the application designer. The designer assigns specific meanings to the various attention keys, and the user must know these meanings in order to use the application. (Often, there are only a few such keys in use: ENTER for normal inputs, one function key to exit from control of the application, another to cancel a partially completed transaction sequence, for example. Where there are a number of choices, you can list the key definitions on the screen, so that the user does not have to memorize them.)
There is an important distinction between two groups of attention keys, which the application designer must keep in mind. The ENTER and function keys transmit data from the buffer when the host issues a “read modified” command, the command normally used by CICS. CLEAR, CNCL and the PA keys do not, although you do get the AID (that is, the identity of the key that was used). These are called the short read keys. They are useful for conveying simple requests, such as “cancel”, but not for those that require accompanying data. In practice, many designers use function keys even for the nondata requests, and discard any accompanying data.