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Q10002. How to access the modem from Sprinter computer?

Summary

This article describe ISA-8 interaction.

More information

If you want to interaction with ISA devices, you have to make following steps:
1) send 10h value to port 1FFDh(system port);
2) send control byte to port 0E2h(third memory window port);
control byte:
D7...should be 1
D6...should be 1
D5...should be 0
D4...should be 1
D3...should be 0
D2...specify number of ISA slot
D1...specify access mode (0 - ISA memory, 1 - ISA ports)
D0...should be 0
The read/write signals are forming from read/write signals memory range 0C000h-0FFFFh.
And the address lines A13...A0 has taken from processor data-BUS. The other ISA-signals such as RESET, AEN, A19...A14 can be set in port 9FBDh. And default value is 00h.
port 9FBDh:
D7...RESET
D6...AEN
D5...A19
D4...A18
D3...A17
D2...A16
D1...A15
D0...A14

ISA_DIR      EQU 9FBDh
SC_PORT      EQU 1FFDh
PAGE3        EQU 0E2h

SAVE_PAGE    DB 0           ;variable for previous status of third memory port

RESET_ISA:                  ; reset ISA device
        LD C, ISA_DIR
        LD A,0C0h
        OUT (C),A
	CALL Pause_10ms
        LD A,0
        OUT (C),A
        RET

Pause_10ms:
        LD HL,10000
Loop:
        DEC HL
        LD A,H
        XOR L
        JR NZ,Loop
        RET

OPEN_ISA_PORTS:             ; open access to ISA
        LD BC,SC_PORT
        LD A,10h
        OUT (C),A
        IN A,(PAGE3)        ; read value from memory port
        LD (SAVE_PAGE),A    ; save previous status of memory port
        LD A,0D2h           ; control byte for (first (upper) ISA slot)
        OUT (PAGE3), A      ; opening ISA port address space
        LD C,ISA_DIR
        LD A,0              ; high ISA addresses RESET and AEN signals.
        OUT (C),A
        RET

CLOSE_ISA_PORTS:            ; close access to ISA
        LD A,(SAVE_PAGE)
        OUT (PAGE3),A       ; restore value of memory port
        LD BC,SC_PORT
        LD A,0
        OUT (C),A           ;
        RET

WRITE_ISA_PORT:
        LD HL,ADRESS_PORT+0C000h     ; ISA port address + 0C000h offset 3
memory page
        LD A,DATA_OUT                ; data for writing
        LD (HL),A                    ; write data to port which pointered HL
        RET

READ_ISA_PORT:
        LD HL, ADRESS_PORT+0C000h    ; ISA port address + 0C000h offset 3
memory page
        LD A,(HL)                    ; read data from port which pointered HL
        RET

When you work with ISA, the access to third memory page are disabled. If you want to address ISA-memory more than 3FFFh, you should specify high bits in port 9FBDh.

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