Port Number: 00h
This port controls the calculator's serial link port (the standard link port present on the 83+, 83+ SE, 84+ and 84+ SE - do not confuse this with the 84+/84+SE's USB link port).
- Bit 0 and 1: The low 2 bits indicate the state of the link port's two lines. A 1 bit indicates a high line, and a 0 bit indicates a low line. When idle (no transfer in progress, no cable plugged in, etc), both lines are usually high (1). When a cable is connected on both ends, a line reads high if and only if both ends have set the line high. The line will read low if either calculator sets it low.
- Bit 5 and 6: Bits 5 and 6 indicate which lines are pulled low by the calculator (unlike bits 0 and 1 they are not influenced by the other calculator). A 1 bit indicates your calculator is holding the line low. A 0 bit indicates your calculator is not holding the line low. (When both calculators have a 0 bit here, the corresponding line will read 1.) In other words, these bits reflect bits 0 and 1 from the most recent write to this port.
- Bit 2: I have absolutely no idea what this means or does...
- Bit 0 and 1: The low 2 bits indicate what state the lines should be put into. A 1 bit will pull the line low. A 0 bit will stop holding the line low (allowing it to go high if the other calculator is not holding it low).
- Bit 2: I have absolutely no idea what this means or does, except that any change of this bit causes both lines to be released, and writing with this bit set causes bit 2 to read 1, and the next write causes it to read 0 and both lines are released.
As mentioned bit 2's effect is unknown. An example of it's behavior follows:
Start, read 03. Write 02, read 01. Write 07, read 07. Write 07, read 03. Write 07, read 07. Write 00, read 03. Write 09, read 07. Write 01, read 03.
One thing to keep in mind when writing link port related software is that the Ti-OS checks for silent transfers in the background. When two calculators are connected and one pulls a line low, the other calculator will respond by pulling the other line low to acknowledge that it has received a bit. This phenomenon is known to cause severe headaches for programmers who attempt to write synchronization routines :). It's unclear if this is only when the other calculator is in the homescreen, or if it is part of the default interrupt routine at $0038.
Transferring an entire byte requires you to implement some form of protocol. Examples include TI's official linking protocol, and Michael Vincent's TachyonLink protocol.
Other useful information on linking in general:
ld a,0 ; Set both lines high out (0),a ld a,2 ; Set tip high, ring low out (0),a ld a,1 ; Set tip low, ring high out (0),a ld a,3 ; Set both low out (0),a
in a,(0) ; Get the link port value bit 0,a ; Check tip jr z,tip_low jr nz,tip_high bit 1,a ; Check ring jr z,ring_low jr nz,ring_high