83Plus:Ports:00

2006-01-16T17:38:39Z

24.119.3.181:

[[Category:83Plus:Ports:By_Address|00 - Link]] [[Category:83Plus:Ports:By_Name|Link]]
== Synopsis ==
'''Port Number:''' 00h

'''Function:''' Link

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).

=== Read Values ===
* Bits 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.
*'''83+ only:''' Bit 2: Set means link receive assist is active.
*'''83+ only:''' Bit 3: Set when link assist has received a complete byte. The only way to reset this bit is to read [[83Plus:Ports:05|port 5]].
* Bit 4 and 5: Bits 4 and 5 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.

=== Write Values ===
* Bits 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).
*'''83+ only:''' Bit 2: Set this bit to enable the link receive assist. After setting this bit, poll port 0 until bit 3 is high, at which point read from [[83Plus:Ports:05|port 5]] to get the byte.
<br>

== 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 [http://www.ticalc.org/archives/files/fileinfo/277/27718.html Michael Vincent's TachyonLink protocol].

Other useful information on linking in general:
* [http://www.ticalc.org/archives/files/fileinfo/247/24750.html TI Link Protocol & File Format Guide]<br>
* [http://www.ticalc.org/archives/files/fileinfo/294/29418.html Ti-83 Link Port Tutorial]
* [http://www.ticalc.org/archives/files/fileinfo/242/24244.html All about the Ti-86 link port]

== Example ==

=== Sending/Setting ===
<nowiki> 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</nowiki>

=== Receiving/Reading ===
<nowiki> 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</nowiki>

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83Plus:Ports:00