Error Situations

The following gives a description of some more important error situations which a user may encounter when operating an ASCII terminal.

3.2.1 Errors on Communication Lines

Errors can occur in data transmitted between the ASCII terminal and the IBM 7171. When the connection is made using normal phone lines, the error can be caused by background noise produced by the telephone equipment. For locally connected terminals such errors are less common, but may still be caused by elec-trical interference.

When a transmission error is detected on input data (bad parity, framing error, or overrun) the terminal is marked in an input error state. Every subsequent character received is ignored and a BEL character is echoed back to the terminal causing an audible alarm ("beep") to inform the user of the error state. The terminal user must explicitly acknowledge the error by entering the Character Error Reset func-tion (see 3.1.7, "Local Reset and Control Facilities" on page 3-10). After receiving the key sequence for this function, the IBM 7171 clears the error indi-cation for the terminal and normal input can continue.

The BREAK key on the keyboard transmits a sequence which the IBM 7171 will regard as a character transmission error. Therefore, if a terminal user accidentally hits the BREAK key, he must correct the error by typing in the key sequence of the Character Error Reset function. Should connection with the terminal be lost due to modem or phone line errors, the line will be disabled and reenabled to permit the user to dial back in and logon.

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3.2.2 Trying to Change Protected Data

There are certain parts of the screen into which the user cannot type data. For example, in a full-screen editor there are usually sections in the top line of the screen which contain status information which cannot be modified. There are also field separators called "attribute bytes" that separate the various fields of each screen line, and the user cannot modify or delete these field separator locations.

Finally, when the terminal is in insert mode, and the cursor is positioned in a field which is full (the last character in the field is neither 'null' nor 'blank'), then no additional characters can be added to the field.

In all three cases, the IBM 7171 responds to an illegal attempt to change protected data, or to add to a full field, with an audible alarm ("beep") and by leaving the screen unchanged.

Once the problem is corrected by moving the cursor out of the protected field, the IBM 7171 will continue normal operation. It is not necessary to explicitly press a

"reset" key in any of these circumstances as would be required on a "real" 3270.

Note, however, that the IBM 3101 Model2x terminals will generate their own Lock/Reset condition when one of the block-mode-only keys is pressed.

3.2.3 Pressing the Wrong Key

If any control request is entered out of context, the request will be ignored and the ASCII terminal will respond with an audible alarm ("beep"). For example, a Char-acter Error Reset function issued when there is no error situation present, or a Keyboard Unlock function issued when the keyboard is already unlocked will gen-erate a "beep." Similarly, any attempt to use an undefined CTRL key (ALT key on the IBM 3101), ESC or PFK sequence will also generate a "beep."

Once the problem is corrected by pressing the correct key, the IBM 7171 will con-tinue normal operation. It is not necessary to explicitly press a "reset" key as would be true on a "real" 3270. Note, however, that the IBM 3101 Model2x ter-minals will generate their own Lock/Reset condition when one of the block-mode-only keys is pressed.

3.2.4 Errors on Output Data Stream

Errors may also be introduced into the output data stream transmitted from the host to the ASCII terminal. These errors appear as "garbage" characters some-where on the screen. When the user suspects that the contents of the screen is in error, he can request that the screen image be retransmitted. The "Redisplay"

control function (see "Control and Cursor Movement Keys" for the specific ter-minal) is used to request such retransmission. Note, however, that this function will not take effect until the keyboard is unlocked.

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3.2.5 Full Type-Ahead Buffer

Within the IBM 7171 each ASCII terminal has a 340 character buffer for

typeahead. When this buffer is filled and additional characters are typed, then the IBM 7171 has no place to store them and responds with an audible alarm

("beep"). Since some data has been lost there is no automatic way to continue normal processing. The user could simply wait for the entire contents of the typeahead buffer to be processed. When the cursor moves out of the lower right corner, then the buffer is empty and the user can see which characters were received and which were lost. Then the Character Error Reset function (see 3.1.7, "Local Reset and Control Facilities" on page 3-10) will reset the error state and the user can enter additional data.

3.2.6 Manually Unlocking the Keyboard

3.2.7 Master Reset

There are times when the keyboard is logically locked, but the terminal user wants to manually unlock it. This is the case, for example, when he wants to interrupt an executing program with a program attention. On a 3270 this function is performed by the RESET key. For an ASCII terminal on the IBM 7171, there exists a Key-board Unlock function, which can be found under "Local Reset and Control Func-tions" in this chapter. If there are any characters in the type-ahead buffer when the Keyboard Unlock function is entered, they will immediately be processed and may appear on the screen in the current cursor position. Should an attention gener-ating key (ENTER or a PFK) be present in the type-ahead buffer it will cause the keyboard to lock again. For example, if while the keyboard is locked the user has typed

abc<ENTER>def

and now a Keyboard Unlock function is typed, the letters "abc" will appear on the screen and an ENTER notification will be sent to the host. The keyboard is log-ically locked by this ENTER and the "def" will remain in the type-ahead buffer. A second Keyboard Unlock function can be entered to unlock the keyboard again.

For simplicity reasons, a Master Reset function has been defined, which combines all the possible reset services in a single function. When issued, it purges the type-ahead buffer, clears error indicators, ends insert mode and rewrites the screen. The Master Reset function is much easier to remember than the group of other reset functions which it can replace. When a Master Reset is done, any problem in the terminal communications should have been cleared up. If subsequent data con-tinues to generate an audible alarm ("beep"), the cursor is probably in a protected field (refer to 3.2.2, "Trying to Change Protected Data" on page 3-12). See

"Local Reset and Control Functions" for the specific terminal in

Appendix B, "IBM 7171 Supplied Terminal Definition Tables" for the key sequence to be issued to generate a Master Reset.

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3.2.8 Other Errors

For more information describing IBM 7171 error conditions refer to Chapter 5, "IBM 7171 I/O Interface to Terminals" for terminal errors,

Chapter 6, "IBM 7171 I/O Interface to the Host System" for channel errors and Chapter 8, "Problem Determination" for general system errors.