83:Ports:00

Synopsis

Port Number: 00h

Function: Link port and Rom page swapping

This port has two functions. It controls the calculator's serial link port and together with 83:Ports:02, the ROM page that is swapped into 4000h-7FFFh.

Read Values

• Bit 0 and 1: The inverse of the values output to the link port.
• Bit 2 and 3: These two 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 4: Bit 3 of the current ROM page. See also 83:Ports:02.
• Bit 5-7: Zero

Write Values

• 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 4: Bit 3 of the current ROM page. See also 83:Ports:02.
• Bit 6 and 7: These bits should be 1 for unknown reasons.

Comments

Ti-OS interference

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.

Data transfer

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:

• TI Link Protocol & File Format Guide
• Ti-83 Link Port Tutorial

Example

Sending/Setting

ld a,$D0     ; Set both lines high
out (0),a

ld a,$D2     ; Set tip high, ring low
out (0),a

ld a,$D1     ; Set tip low, ring high
out (0),a

ld a,$D3     ; Set both low
out (0),a

Receiving/Reading

in a,(0)        ; Get the link port value

bit 2,a         ; Check tip
jr z,tip_low
jr nz,tip_high

bit 3,a         ; Check ring
jr z,ring_low
jr nz,ring_high