================================================================================================
===================================Link Tutorial by Merlijn=====================================
===========================================for ti83=============================================

._______________________________________________________________________________________________.
|												|	
|	I'm programming for the ti83 for a year now and I always wanted to make a link		|
|	game. But there was no link tutorial availeble. I asked around and everybody said	|
|	that the Ztetris source should help me. Sadly enough doesn't zeteris' link work at	|
|	all. Therefor did I decide to make a link tutorial for all you out there.		|
|	Aim of this tutorial send and receive bytes for multiplayer games.			|
|	E-mail me for questions, bugs, modifications and improvements. Mezzy30@hotmail.com	|
|	Special thanx to Tijl Coosemans for his routines.					|
|_______________________________________________________________________________________________|

	Disclaimer:
	I'm not responsible for damage to your calc, health ect. No one may publish this tutorial
	whithout my permission. If you use this routine give me credits.

	First you should know that the Ti-83 and other calcs use three cables to link. A white,
	a Red and a Yellow. The Yellow one the zero, so you can't set power on that one.
	The Red and the White can be changed into high or low. Usually the lines are both set
	low. This also accures after every romcall. The linkport is port number '0'.
	
	
	Now to set the lines use the following code:

	ld a,0d1h	;load value of lines into a, set red high
	out (0),a	;syntax for linkport, linkport has been set

	for other set values look in the table below.

	0d0h	both lines low
	0d1h	red high, white low
	0d2h	white high, red low
	0d3h	both lines high


	If you want to read the port use this:

	in a,(0)	; check linkport
    	and 3		; upper bits are not important
    	cp 2		; 2 means white is active

	for other read values look in the table below.

	3	both lines low
	2	red high, white low
	1	white high, red low
	0	both lines high

	
	It is important to know that if one of the calcs sets the red wire high and the other sets
	the white wire high it will result that when you read the linkport both lines are high (0).

	0d0h	3	both lines low
	0d1h	2	red high, white low
	0d2h	1	white high, red low
	0d3h	0	both lines high

	0d1h+0d2h=>0	(every calc have to set one of the wires)

	I've written a program, 'READSET' to set and to read the port. 'READSET' can be found at ticalc.org


	Now we want to send bytes and receive bytes. I used the send/receive code from venus and added a
	synchronising routine made myselve to it. Take a look at the source it will explain itselve. 
	

	-----header-----
	#define     player      saferam1


	start: 
		in a,(0)		;check linkport
		and 3			;upper bits are not important
		cp 2			;1 means red is high
    		jp z,youplayertwo	;if true other player wants to receive
    		ld   a,0D1h		;ld set red wire high
    		out  (0),a		;required syntax

	youplayerone:  
    		ld a,1			;set you player one
    		ld (player),a		;save it
		in a,(0)		;check linkport
    		and 3			;upper bits are not important
    		cp 0			;0 means both high, wait until player two has entered
    		jp z,intro		;if true other player wants to rec

		ld a,0ffh		;resets the keypad.
		out (1),a
		ld a,KeyRow_Pad		;enables the arrow keys
		out (1),a
		ld a,KeyRow_Top		;enables the top keys
		out (1),a  

    		in a,(1)		;check keys
    		cp KMode		;quit if mode pressed
    		ret z
    		jp  youplayerone

	youplayertwo:
		ld a,2			;set you player two
		ld (player),a		;save it
		ld   a,0D2h		;set white wire high, so playerone continues
		out  (0),a		;required syntax

	delay:
		in a,(0)		;check linkport
		and 3			;upper bits are not important
		cp 1			;1 means white high, this happens if playerone set both low
		jp nz,delay		;check again

	intro:
		bcall(_cleargbuf)	;this will set both lines low and clear the graphbuffer

	mainloop:

	----your program----

		call sendreceive	;call sendreceive routine
		jp mainloop		;loop again

	sendreceive:
		ld a,(player)
		cp 1
		jp z,send_than_receive	;to have a fast link calcs have to do the opposite
		cp 2
		jp z,receive_than_send	;to have a fast link calcs have to do the opposite

	send_than_receive:
		ld a,(byte_to_send)	;the byte you want to send
		ld c,a			;we actually send c
		call sendbyte		;call send routine
		call receivebyte	;call receive routine
		ld a,e			;we actually receive e
		ld (received_byte)	;save received byte
		ret			;return to mainloop

	receive_than_send:
		call receivebyte	;call receive routine
		ld a,e			;we actually receive e
		ld (received_byte)	;save received byte
		ld a,(byte_to_send)	;the byte you want to send
		ld c,a			;we actually send c
		call sendbyte		;call send routine
		ret			;return to mainloop

;====================send routines from venus and synchronising by Merlijn=======================

	sendbyte:			;input: white high, output: continue if red and white high
		IN   A,(0)		;check linkport
    		AND  3                  ;upper bits are not important
    		CP   1			;1 means white is high
		ret z			;if true other player wants to send
		ld a,0D2h		;ld set white wire high
        	out (0),a		;required syntax to send a to the linkport
	sendbytewait:
 		IN   A,(0)		;check linkport
    		AND  3                  ;upper bits are not important
    		CP   0			;0 means red and white are high
		jp   nz,sendbytewait	;not true check again

		ld   a,0		;set timer to zero
	wait2:				;create small delay so other can check port for double activety
		inc a			;increase timer 
		cp 20			;compare to 20
		jp nz,wait2		;not true increase more
        				;-----------end of synchronising-------------
 		di
		ld	b,8
	vnSendByteLoop:
		rr	c		;c is the byte to send
	vnSendBit0:
		ld	a,0D1h
		jr	nc,vnSendBit
	vnSendBit1:
		inc	a
	vnSendBit:
		out	(0),a
		ld	de,0
	vnSendBitLoop1:
		dec	de
		ld	a,d
		or	e
		ret	z
		in	a,(0)
		and	0Ch
		jr	nz,vnSendBitLoop1
		ld	a,0D0h
		out	(0),a
		ld	de,0
	vnSendBitLoop2:
		dec	de
		ld	a,d
		or	e
		ret	z
		in	a,(0)
		and	0Ch
		cp	0Ch
		jr	nz,vnSendBitLoop2
		djnz	vnSendByteLoop
		or	a	

		ld a,0D0h		;ld set no wire high for next transmission
       		out (0),a		;required syntax
	
		ret

;=========================End of send routine====================================================
;=========================receive routine from venus and synchronising by Merlijn================

	receivebyte:			;input: red high, output: continue if red and white high
		IN   A,(0)		;check linkport
    		AND  3                  ;upper bits are not important
    		CP   2			;2 means red is high
		ret z			;if true other player wants to receive
		ld   a,0D1h		;ld set red wire high
        	out  (0),a		;required syntax
	receivebyte2:
 		IN   A,(0)		;check linkport
    		AND  3                  ;upper bits are not important
    		CP   0			;0 means red and white are high
		jp   nz,receivebyte2	;not true check again

		ld   a,0		;set timer to zero
		
	wait:				;create small delay so other can check port for double activety
		inc a			;increase timer
		cp 40			;compare to 40 
		jp nz,wait		;not true increase again
        				;-----------end of synchronising-------------
 		di
		ld	b,8
	vnReceiveByteLoop:
		in	a,(0)
		and	0Ch
		ld	d,8
		cp	d
	vnReceiveBit1:
		ld	a,0D1h
		jr	nz,vnReceiveBit
	vnReceiveBit0:
		inc	a
		ld	d,4
	vnReceiveBit:
		rr	e		;e is the received byte 
		out	(0),a
		ld	hl,0
	vnReceiveBitLoop:
		dec	hl
		ld	a,h
		or	l
		ret	z
		in	a,(0)
		and	d
		jr	z,vnReceiveBitLoop
		ld	a,0D0h
		out	(0),a
		ld	d,16
	vnBitReceivedLoop:
		dec	d
		jr	nz,vnBitReceivedLoop
		djnz	vnReceiveByteLoop
		or	a

		ld a,0D0h		;ld set no wire high for next transmission
      		out (0),a		;required syntax

		ret

;=======================End of receive routine===================================================

	Test it and make great link games.