83Plus:BCALLs:50E9

Synopsis

Unofficial Name: KeyboardGetKey

BCALL Address: 50E9

Minimum OS Version: 1.15

Attempt to receive a keypress from the TI-Keyboard.

A contains status code
The actual data retrieved from the keyboard appears to be trashed in the process of popping the error handler ?!

This B_CALL was introduced in OS 1.15. Programs should check the OS version before attempting to use this routine.

Keep in mind that the keyboard communicates using a truly horrible perversion of the DBUS protocol, which works as follows:

The keyboard sends the byte E0.
It pulls both link lines low simultaneously for a short time (i.e., flags a DBUS error.)
It sends the byte 01h.
Finally, it sends the scancode or modifier mask.

As a result, there are a variety of possible error conditions.

Possible status codes returned are:

00 = No link activity detected.
01 = Success.
02 = Link activity detected, but the first byte was not E0h, or E0h was not followed by a DBUS error.
0F9h = Link assist error, and no buffered data or link activity.
0FAh = Link assist error and buffered data detected, but the buffered byte was not E0h.
0FBh = Link assist error and buffered byte E0h detected, and two more bytes might or might not have been read.
0FEh = Second byte was not 01h (possibly unsupported future commands)
0FFh = Protocol error other than the one expected.

A full disassembly of this entry point on OS 2.41 is here.