Advise needed for SASI controller board replacement
Date: 2006-10-19 09:40:03

Hallo allemaal,

I am busy with disassembling the controller part of the CBM 90x0. There is a piece of software, actually one line, of which I0m not sure if I understand it completely. But first some information:

The 90x0 contains two boards and a MFM hard disk. The controller board drives the hard disk. The main board contains two 6502's, one for the IEEE communication and others things (as we are used to with the other IEEE drives) and the other 6502 for controlling on its turn the controller board. This is done using the SASI bus, the predesessor of SCSI.
The 6522 involved is used controling the SASI bus. 

.eq PortB	= $80			; some control lines
.eq PortA	= $81			; data
.eq DDRB	= $82
.eq DDRA	= $83
.eq PCR	= $8C			; CA1 = REQ  (request, in)
					; CA2 = ACK  (acknowledge, out)
All used signal are active (H).

For transferring data from or to the controller board, the next four lines are the start of the used routines:
		lda	PCR
		ora	#$0A		; <====================
		and	#$FB
		sta	PCR		; reset ACK

If I understand correctly, "ora #$0a" puts the 6522 in pulse mode. CA2 is (H) allt he time. But the moment Port A is written to or read from, a (L) pulse is sent towards the controller board. Is this correct?

If this is the case indeed, I cannot use my 'off-the-shelf' solution, my self-built IEEE interface, as it lacks the means to detect that pulse. But replacing this board + HD with a PC wasn't meant as a permanent solution anyway, just as a starter to figure out how things exactly work. 

I looked at Andre's site and found out that the only difference between SASI and SCSI is that SCSI has an extra ATN line. So my first question for the SCSI experts amongst you: could it be so simple by replacing the board + HD by a SCSI hard disk ??? But be aware, the condition is that there is no hardware change needed on the 6502 board. Later I want the drive to be able to handle bigger hard disks but I want to realize that by altering the ROMs and only as less as possible (I don't mind to be stuck to 16 MB).

Another idea: replacing things with a micro controller + IDE-HD (or Flash card). Problems: what uC to use, how to program it? I myself am thinking of using an old 1541-II board as base. The advantage: I know 6502 ML and the onboard 6522 can handle the pulse problem for sure. 

Any input is welcome!

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