I want to read all my 5-inch floppies, using Sprinter.
These were created on TRS-80 Model 4, and have 256-byte
sectors.

What is the I/O port address for 1793, and what is I/O port
address for "drive-select" for A and B drives?
(I wish to use a 5-inch diskette for drive B).
How is interrupt provided to the CPU from WD1793
(e.g. NMI or other INT)?

0Fh Command/Status register
3Fh Track register
5Fh Sector register
FFh Drive Control register

In Spectrum configuration the port 0Fh also mapped to 1Fh for TRDOS
compatibility.
But I would recommend to you don't use direct port access for it.
A Sprinter have BIOS functions for reading/writing any disk sectors.

for example:

At first, make reset disk and to set required parameters.

INIT_DISK

	LD	D,n	; drive number 0-FDD A, 1-FDD B, 80h-HDD C...

	LD	C,#51	; reset disk

	RST	#08	; BIOS call

;

	LD	D,n	; drive number

	LD	E,n	; sectors per track.

	PUSH	DE

	LD	A,D

	LD	C,#58	; get disk parameters

	RST	#08	; BIOS call

	LD	A,H

	POP	HL

	PUSH	HL

	LD	H,A

	POP	AF

	LD	IX,256	; sector size

	LD	C,#59	; set disk parameters

	RST	#08	; BIOS call

	RET

 

DRV_READ:               ; reading from disk

        LD A,drv_type   ; bit 0..3 - drive number

                        ; bit 4..7 - drive type

                        ;     0 - FDD

                        ;     6 - ram-disk (reserved)

                        ;     8 - HDD

                        ;     C - CD-ROM (reserved)

        LD HL,sec_h     ; sector number (high)

        LD IX,sec_l     ; sector number (low)

			; (HL:IX - 32bit sector number, like LBA)

        LD B,n_sec      ; sectors count

        LD DE,bufer_adr ; memory address for reading

        LD C,#55        ;

        RST #08		; CF=0 - read OK

		; CF=1 - read error