IBM 3270 Differences and Limitations

When using VPLUS to design or modify applications for IBM 3270 users, it is important to understand the limitations that restrict forms design for IBM 3270 display stations. The information in the following sections explain how an IBM 3270 differs from an HP 3000 terminal and the impact of these differences when running VPLUS applications using SNA DHCF.

Keyboard Differences

Table K-1 compares HP 3000 terminal keys with equivalent IBM 3270 keys.

Table K-1 Comparison of Major Keys

HP 3000 Key	Equivalent IBM 3270 Key and Explanation
AIDS	No equivalent key exists.
BREAK	PA1 - Equivalent to a system break. A VPLUS application can disable this key. If disabled, SNA DHCF ignores the key when pressed and re-enables the keyboard.
CLEAR DISPLAY	CLEAR - Functions as a refresh key by clearing the entire screen, including data and the form itself. Consequently, this key is not a local function, like the equivalent HP key.
CLEAR LINE	ERASE EOF - Clears data in an unprotected field from the cursor to the end of the field, regardless of the number of lines in the field. This differs slightly from CLEAR LINE, which performs the same function only if the field does not exceed one line.
CTRL	No equivalent key exists.
CTRLY	PA2 - Equivalent to a subsystem break. A VPLUS application can disable this key. If disabled, SNA DHCF ignores the key when pressed and re-enables the keyboard.
DEL LINE	No equivalent key exists.
ENTER	ENTER - Locks the keyboard and transmits all modified data. The cursor remains in the same position.
ESC	No equivalent key exists.
INS LINE	No equivalent key exists.
MODES	No equivalent key exists.
NEXT PAGE	No equivalent key exists.
PREV PAGE	No equivalent key exists.
RESET	RESET - Unlocks a locked keyboard.
ROLL DOWN	No equivalent key exists.
ROLL UP	No equivalent key exists.
USER KEYS	No equivalent key exists.

In addition to the keys listed above, IBM 3270 function keys differ from HP 3000 function keys. The IBM keyboard has 12 or 24 function keys; the first 8 function keys map to HP keyboards. Table K-2 shows the mapping of HP function keys to IBM 3270 function keys.

Table K-2 Mapping of Keys

HP Function Key	IBM 3270 Function Key
ENTER (0)	ENTER
f1 (1)	PF1, PF13
f2 (2)	PF2, PF14
f3 (3)	PF3, PF15
f4 (4)	PF4, PF16
f5 (5)	PF5, PF17
f6 (6)	PF6, PF18
f7 (7)	PF7, PF19
f8 (8)	PF8, PF20
ENTER (0)	PF9, PF21
ENTER (0)	PF10, PF22
ENTER (0)	PF11, PF23
ENTER (0)	PF12, PF24
BREAK	PA1
CTRLy	PA2
REFRESH	CLEAR

Screen Differences

SNA DCHF affects the screen design of VPLUS applications for IBM 3270 display stations in the following areas:

Screen attributes
Screen enhancements
Function key labels
Fields
Character translation

Screen Attributes

IBM 3270 display stations have the following screen attributes and limitations:

Display Area

Depending on the IBM 3270 display station model, the display area is either 24, 32, or 43 lines long. The maximum length for a VPLUS form is 24 lines. Consequently, if you display a VPLUS form on an IBM 3270 that has 32 or 43 lines, the lines following line 24 will be blank.

SNA DHCF reserves the last two lines of the display area for function key labels. Therefore, a form can consist of either 21 lines including a window line, or 22 lines without the window line.

Status Line

A status line appears below the display area. SNA DHCF applications, including VPLUS, cannot access the status line. Only the display controller can update this line.

Window Line

A window line defined in a forms file consumes a line of the available display. If you define a window line in the forms file, you must place the window line on line 1 or line 22, because IBM 3270 limitations prohibit inserting a line in the middle of a form. Selecting a window line reduces the form length to 21 lines.

You must reserve a fixed line if you define a window line in a forms file, because window lines cannot be inserted.

SNA DHCF limits the text in a window line to 79 bytes instead of 80, because space must be reserved for the attribute byte. Text exceeding 79 bytes becomes truncated. (Refer to the section entitled "Fields" for information about the attribute byte.)

Modified Data Tag

An IBM 3270 functions as a modified data tag (MDT) device. It transmits to the HP 3000 only the data that the end user has modified.

Limitations

The following IBM 3270 limitations are important to note when designing or modifying VPLUS applications. The IBM 3270 does not have:

Forms caching capability
Terminal memory other than display memory, which consists of a 24-line length and 80-column width
Scrolling capability

In addition, the IBM 3270 does not support:

The line-drawing character set provided on some HP terminals
Local field edits that occur in the terminal firmware, such as edits for numeric fields

Screen Enhancements

Several HP terminal display enhancements are unavailable on the IBM 3270. You do not have to alter any enhancements specified in an existing forms file for IBM 3270 usage, however.

Table K-3 summarizes the mapping of HP display enhancements to IBM 3270 display stations. The codes provided for some of the HP display enhancements correspond to the value specified in FORMSPEC for the display enhancements of fields, errors, and the window line. Note that the half-bright enhancement (code=H) overrides all other enhancements except for security.

Table K-3 Display Enhancements

HP Display Enhancement	IBM 3270 Display Enhancement
Security (code=S)	Non-display
Half-bright (code=H)	Normal display
No enhancement (code=NONE)	Intensified display
Blinking (code=B)	Intensified display
Underlining (code=U)	Intensified display
Inverse video (code=I)	Intensified display
Color pair 1 (red/black)	Intensified display
Color pair 2 (green/black)	Intensified display
Color pair 3 (yellow/black)	Intensified display
Color pair 4 (blue/black)	Intensified display
Color pair 5 (magenta/black)	Intensified display
Color pair 6 (cyan/black)	Intensified display
Color pair 7 (black/yellow)	Intensified display
Color pair 8 (white/black)	Intensified display

Function Key Labels

VPLUS simulates function key labels by "reserving" the last two lines of the screen. This reduces the available screen area to 22 lines.

Function key labels for IBM 3270 display stations appear similar to those for HP terminals. The label text appears on an IBM 3270 display station as intensified display on a blank background. The default labels on HP terminals and IBM 3270 display stations are identical, and cannot be configured by users.

Fields

You should not design a form that has contiguous fields, because the IBM 3270 display station requires an attribute byte preceding each field or text area. The attribute byte uses a physical screen location and appears as a blank space on the screen. The attribute byte indicates whether the field can be or has been changed.

You can create a space for both leading and trailing attribute bytes by inserting visible brackets before and after all fields. SNA DHCF can convert these visible brackets to invisible attribute bytes on IBM 3270s. For instance, if you defined the following three fields:

[08][05][52]

they would appear as shown below when the VPLUS form is displayed on an IBM 3270 screen.

*08**05**52*

The * represents an attribute byte. On the screen, the attribute byte appears as a blank.

If you use invisible brackets to delimit fields, you must provide a leading space before each field or text area. When the field or text begins in the first column of a line, insert the space at the end of the previous line. The "Home" position is an exception to this guideline; the attribute byte should be inserted in the bottom right position instead (reverse "Home").

Although the attribute byte is the most important concept regarding fields for IBM 3270 display stations, you should also be aware of the following information about fields.

Character Translation

As mentioned earlier, HP computer systems use ASCII character sets, whereas IBM computer systems use EBCDIC character sets. Consequently, SNA DHCF must use system default translation tables from the Native Language Subsystem (NLS) to translate characters from one system to the other.

If Native-3000 is the native language used, SNA DHCF translates all ASCII characters without alterations, except for the characters shown in table K-4. Note that the HP terminal bracket characters equate to text, not actual FORMSPEC brackets.

Table K-4 Differences Between HP and IBM Character Displays

HP Terminal Character	ASCII Hex Value	IBM 3270 Character	EBCDIC Hex Value
[	5B	¢	4A
]	5D	!	5A
!	21		4F
^	5E		5F

To display one of these characters on one system, you must specify the corresponding character on the other system. For instance, if you want to send an exclamation point (!) to an IBM 3270 user, your VPLUS application must specify this character as a right bracket (). If an IBM user wants to input a left bracket (), he or she must specify ¢.