HP and IBM Terminal Characteristics

HP terminals and IBM 3278 display stations differ in some of their characteristics and functionality. This section describes the differences between HP and IBM terminals.

Non-Display Input Fields

On an IBM 3278 display station, you can enter data into a non-display unprotected field, and the data does not appear on the screen. Although HP terminals do not support non-display fields, SNA IMF has several ways of implementing the non-display feature.

SNA IMF supports security video on the following HP terminals:
2624A 2624B 2620A 2626A 2392A 2392X 2394A 2397A 150A 700/92 700/94
Security video emulates the non-display feature of 3278 terminals. Data that you type into a non-display field is retained in terminal memory, but it is not displayed on the screen.
On terminals that do not support security video, SNA IMF/V displays meaningless symbols from alternate character set C in non-display fields. If your HP terminal does not have a character set C, but it has an A or B character set, no symbols are displayed and the terminal functions like a 3278 display station. If your HP terminal does not have an alternate character set, but has only the standard character set, the emulated IBM 3278 non-display feature is not available with SNA IMF/V; anything you type in a non-display field is printed on the screen.
If you are using non-display input fields with a 264x terminal, slot C must be empty.
Press the terminal [TEST] key to determine whether your terminal has an alternate character set. The resultant test pattern displays any alternate character sets that are installed in your terminal.
On HP graphics terminals, data that you type into non-display fields is printed black-on-black.
If the IBM host sends data to a non-display unprotected field, SNA IMF does not send the data to an HP terminal. Anything you type into a non-display field on an HP terminal replaces the original data from the IBM host. An IBM 3278 changes only that part of the field in which data has been entered. This can be a problem for some applications.

Cursor Addressing

When an IBM 3278 display station user presses the transmit key, the current cursor address is sent to the IBM host. If your HP terminal is connected to the IBM host over a Datacommunications and Terminal Controller (DTC), an Asynchronous Data Communications Controller (ADCC), or an Asynchronous Terminal Processor (ATP), the current cursor address is sent to the host when you press the transmit key.

For SNA IMF/V, HP terminals connected to the HP 3000 over a multipoint line always send the cursor address as though the cursor were positioned in the upper left corner of the screen; that is, buffer position (0,0). This pseudo cursor address causes problems if the IBM host application is expecting to receive the exact cursor address. For example, Pass Thru cannot support the split screen mode of the IBM Structured Programming Facility (SPF) on multipoint terminals.

Cursor Positioning

On an IBM 3278 display station, the IBM host can position the cursor within a protected field after you enter the first character of input. Pass Thru cannot place the cursor within a protected field. If the IBM host attempts to position the cursor within a protected field, Pass Thru positions the cursor at the start of the next unprotected field before enabling input. This produces the same results as pressing the [TAB] key as the first character on an IBM 3278 display station.

If you type a character into the last position on the screen (the lower right-hand corner), your HP terminal temporarily rolls the screen up one line. If you type another character, the screen rolls back down to its correct position. The cursor homes, and your character is placed correctly in the first character position on the screen (the upper left-hand corner). However, if you cause the screen to roll up, you must home the cursor before you send the screen's data to the host. If you attempt to send data while the screen is rolled up, you will receive an error message, and no data will be sent to the host.

Unprotected Fields

Typing a character into an unprotected field of an IBM 3278 display station sets the Modified Data Tag (MDT) for that field. Later, when you press the transmit key, only the fields with the MDT set are sent to the host.

Hewlett-Packard makes both MDT and non-MDT terminals. MDT terminals have a Modified Data Tag associated with each field, just like IBM 3278 display stations. Non-MDT terminals do not support Modified Data Tags.

MDT and non-MDT terminals differ in the way they handle null and blank characters. The HP 700/94 terminal now has a ROM module (part #5062-1306) that allows it to support null characters, but all of the older HP terminals represent null characters as blanks. For more information on null character translation on MDT and non-MDT terminals, see "Leading Blanks, Trailing Blanks, and Trailing Nulls," in Chapter 2 “Configuring Pass Thru”

Unprotected Fields on HP MDT Terminals

When you type a character into an unprotected field on an HP MDT terminal, the terminal sets the Modified Data Tag for that field. Only the fields with the MDT set are transmitted to the HP 3000 when you press a transmit key. If you type identical data over the existing data in the field, it is transmitted to the HP 3000.

Unprotected Fields on HP Non-MDT Terminals

Striking a transmit key on an HP non-MDT terminal that is under Pass Thru control causes the HP 3000 to read all unprotected fields in the screen. Pass Thru then sets the Modified Data Tags for the unprotected fields. Each unprotected field received from the terminal is compared with the unprotected field last written to the terminal by Pass Thru. If the data in a field has changed, the MDT is set for that field, and the field is then sent to the IBM host. If you type identical data over the existing data in a field, the HP 3000 compares it with the original data, finds no difference, and does not transmit it to the IBM host.

Displayed Symbols

All characters written to an HP Pass Thru terminal by an IBM program must correspond to ASCII characters (40 through 377 octal, or 20 through FF hexadecimal). If they do not, the HP terminal cannot display them. Certain characters displayed on an HP terminal will differ from those displayed on an IBM display station. Table 5-5 “IBM to HP Character Display Differences” summarizes these differences.

Table 5-5 IBM to HP Character Display Differences

IBM Display Station Character	EBCDIC Values		HP Terminal Character	ASCII Values
	Octal	Hex		Octal	Hex
¢ (cent sign)	112	4A	[ (left bracket)	113	5B
! (exclamation)	132	5A	] (right bracket)	135	5D
\| (logical OR)	117	4F	! (exclamation)	041	21
(not sign)	137	5F	^ (caret)	136	5E




	NOTE: IBM host programs should not send characters that equate either to HP terminal escape sequences or to control characters that perform device-dependent functions on HP terminals.

Transmitted Printer Commands

The IBM host can send printer commands within a data stream to an IBM 3278 display station. New Line (NL), End of Message (EM), Carriage Return (CR), and Form Feed (FF) are examples of printer commands. When the IBM host transmits printer commands to an IBM display station, the display station displays either a blank or a character (depending on the display station model and the command sent). If the display station sends the printer command back to the host within a modified field, the host receives the correct code for the printer command.

If a printer command is sent to an HP terminal during a Pass Thru session, SNA IMF saves the printer command in the internal screen image and displays a character on the HP terminal screen. The character displayed on the HP terminal may not be the same as the character displayed on an IBM display station.

If you modify a field that contains a printer command, when SNA IMF transmits that field to the host, it will transmit the character representing the printer command, not the printer command itself.

You cannot enter a FIELD MARK (FM) in Pass Thru. If the host sends an FM, Pass Thru displays it as a semicolon ( ; ). An IBM 3278 display station also displays a semicolon for FM. However, if you send a semicolon within a modified field back to the host, the host receives a semicolon instead of the FM code.

Intensified Fields

Unlike IBM display stations, HP terminals cannot turn off inverse video for part of a field. The white stripe of inverse video extends the full length of the field, even if the data does not fill the field.

Blinking Cursor

The cursor of an HP terminal always appears as a blinking underline. The user of an IBM 3278 display station may choose to have the cursor appear as a solid underline, a box, a blinking underline, or a blinking box.

Function Indicators

An IBM 3278 display station has a row of indicator symbols, including (1) SYSTEM AVAILABLE, (2) INSERT MODE, and (3) INPUT INHIBITED, at the bottom of the display station screen. This row of symbols is called the operator information area. The HP indicator light associated with the [INSERT CHAR] key is the only equivalent to the IBM operator information area.

Numeric Fields

Unlike IBM terminals, HP terminals do not differentiate between numeric fields and alphanumeric fields. Pass Thru treats numeric and alphanumeric fields the same. Pass Thru does not provide a numeric-only check in fields defined as numeric, so you are responsible for making sure the data you enter in numeric fields is numeric data.

Field Manipulations

On an HP terminal, you should not do any explicit field manipulations, such as FORMAT MODE ON, START FIELD, or STOP FIELD. Pass Thru does not support such field manipulations.

Automatic Skip

An HP terminal's automatic skip feature does not function the same as the IBM 3278 display station's automatic skip feature.

On an IBM display station, the automatic skip feature positions the cursor at the next field. On an HP terminal, the automatic skip feature positions the cursor at the next unprotected field.

Card Reader, Light Pen, and Cursor Select

An HP terminal does not have a magnetic card reader, auxiliary card reader, light pen, or cursor select function.

Screen Sizes

The IBM host specifies the screen size of an HP terminal as 480, 1920, or 3440 characters. No other screen sizes can be emulated. IBM application programs can use the ERASE/WRITE ALTERNATE command of the IBM 3278 display station to change screen sizes from 480, 1920, or 3440 characters (the defaults) to 960, 2560, or 3564 characters. This feature does not work on HP Pass Thru terminals. ERASE/WRITE ALTERNATE commands received from the IBM host by SNA IMF are not supported; however, they are not rejected. ERASE/WRITE ALTERNATE commands are mapped to ERASE WRITE commands.