Let's communicate with MSX on Windows "Joy Joy File System"

Have crammed together the 'RS232 to TTL' cable just for testing, and using a USB to RS232 converter.
it works like a charm.
Very fun project...

This code is actually better... runs at 57600bps. Credits: https://github.com/rolandvans/msx_softserial
Ported by me to SDCC.

Also was thinking about flow control.
any pin 2,3,4 can be used as to receive RTS and Pin 7 can be used to send CTS.
Was thinking of using the VDP interrupt to check if RTS is active, then signal CTS, and receive data.

void recv(char* data, int bytes)
{
	__asm
	
	    LD IY,#2
		ADD IY,SP
		LD L,0(IY)
		LD H,1(IY)
		LD C,2(IY)
		LD B,3(IY)
	
		;TRANSFER 'BC' BYTES FROM JOY2, 'UP', PIN1 TO [HL]
		;MSX, Z80 3.58MHz 57600bps
		;SETS CARRY BIT ON ERROR. A HOLDS ERROR CODE:
		;A=1 RS232 LINE NOT HIGH,A=2 STARTBIT TIMEOUT
		RECBYTES:	
			DI			;NO INTERRUPTS, TIME CRITICAL ROUTINE
			LD A,#0x0F	;PSG REGISTER 15, SELECT JOYSTICK PORT 2
			OUT (0xA0),A
			IN A,(0xA2)
			SET 6,A		;SELECT JOY2
			OUT (0xA1),A

			LD A,C	; FAST LOOP WITH BC (GRAUW, FAST LOOPS)
			DEC BC	; COMPENSATION FOR THE CASE C=0
			INC B
			LD C,B	; SWAP B AND C FOR LOOP STRUCTURE
			LD B,A

			LD A,#0x0E	;SET PSG #14
			OUT (0xA0),A
			LD E,#0x01	;FOR FASTER 'AND' OPERATION 'AND r'(5) VS. 'AND n'(8)
			IN A,(0xA2)
			AND E
			JR NZ,.FIRSTSTARTBIT	;RS232 LINE SHOULD BE HIGH, OTHERWISE STOP
			LD A,#0x01	;ERROR, RS232 LINE NOT READY
			SCF
			EI
			RET
		;THE NEXT PART IS TIME CRITICAL. EVERY CYCLE COUNTS
		.FIRSTSTARTBIT:		;WAIT FOR FIRST STARTBIT
			IN A,(0xA2)
			AND E
			JP NZ,.FIRSTSTARTBIT
			JP .READFIRSTBIT	;START READING BIT0, COMPENSATED FOR 12 CYCLES, 1 JP
		.RECVSTARTBIT:	
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITS
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITS
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITS
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITS
			LD A,#0x02	;ERROR, STARTBIT TIMEOUT
			SCF
			EI
			RET
		.READBITSNEXTBLOCK:
			INC HL
			DEC C	
			JR NZ,.INITBITLOOP	;NEXT BLOCK UNLESS WE ARE DONE (C=0)
			JR .RECVEXIT
		.READBITSNEXTBYTE:
			INC HL 
			NOP	;DUMMY
			JR .INITBITLOOP
		.READBITS:
			IN A,(0xA2)	;DUMMY
		.READFIRSTBIT:	;ALT TIMING TO COMPENSATE FOR 1 JP
			ADD A,#0x00	;DUMMY
			NOP	;DUMMY
		.INITBITLOOP:
			LD D,B
			LD B,#0x08
		.RECVBITLOOP:	
			INC HL	;DUMMY
			DEC HL	;DUMMY
			IN A,(0xA2)
			RRCA		;SHIFT DATA BIT (0) -> CARRY
			RR (HL)		;SHIFT CARRY -> [HL]
			DJNZ .RECVBITLOOP
			LD B,D
			DJNZ .RECVSTARTBITNEXTBYTE	;NEXT BYTE, SKIP STOPBIT AND WAIT FOR STARTBIT
			NOP	;DUMMY
		.RECVSTARTBITNEXTBLOCK:
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITSNEXTBLOCK
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITSNEXTBLOCK
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITSNEXTBLOCK
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITSNEXTBLOCK
			DEC C	;POSTPONED CHECK
			JR Z,.RECVEXIT	;IF C=0, WE ARE DONE SO EXIT OTHERWISE ERROR
			LD A,#0x02;	ERROR STARTBIT TIMEOUT
			SCF
			EI
			RET
		.RECVSTARTBITNEXTBYTE:	
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITSNEXTBYTE
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITSNEXTBYTE
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITSNEXTBYTE
			IN A,(0xA2)	;WAIT FOR THE HIGH->LOW TRANSITION (STARTBIT)
			AND E
			JP Z,.READBITSNEXTBYTE
			LD A,#0x02
			SCF
			EI
			RET
		.RECVEXIT:
			OR A	;RESET CARRY
			EI
			RET
	__endasm;
}

void send(char* data, int bytes)
{
	__asm

	    LD IY,#2
		ADD IY,SP
		LD L,0(IY)
		LD H,1(IY)
		LD C,2(IY)
		LD B,3(IY)
		
		;SEND 'BC' BYTES FROM [HL] TO PIN6, JOY2
		;MSX, Z80 3.58MHz 57600bps
		SENDBYTES:	
			DI	;NO INTERRUPTS
			LD A,#0x0F	;SELECT PSG REG #15
			OUT (0xA0),A
			IN A,(0xA2)	;SAVE VALUE
			PUSH AF
			SET 6,A		;JOY2
			RES 2,A		;TRIG1 LOW
			LD E,A		;0V VALUE (0) IN E
			SET 2,A		;TRIG1 HIGH
			LD D,A		;5V VALUE (1) IN D
		.BYTELOOP:	
			PUSH BC
			LD A,E
		.SENDSTARTBIT:	
			OUT (0xA1),A
			LD C,(HL)
			LD B,#0x08
		.SENDBITLOOP:	
			RRC C
		;ASSUME BIT=1		
			LD A,D
			JR C,.SETBIT
		;NO, BIT=0		
			LD A,E
		.SETBIT:	
			ADD A,#0x00	;DUMMY
			OUT (0xA1),A
			DJNZ .SENDBITLOOP
			LD A,E
			POP BC
			DEC BC
			NOP	;DUMMY
			ADD A,#0x00	;DUMMY
		.STOPBIT:	
			LD A,D
			OUT (0xA1),A
			INC HL
			LD A,B
			OR C
			NOP	;DUMMY
			JP NZ,.BYTELOOP
		.SENDEXIT:
			NOP	;DUMMY
			POP AF
			OUT (0xA1),A		
			EI
			RET
	__endasm;
}

Very interesting project... Could we imagine a Nextor driver using it ? MSX would send sector number to read / write to PC, and PC would reply / apply the change ? A common hard-disk image could be stored on PC.

https://youtu.be/pLSxEonacbc
Do you want LEDs?