Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!usc!samsung!munnari.oz.au!bruce!herbie
From: herbie@bruce.cs.monash.OZ.AU (Andrew Herbert)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: ISA SCSI adapters - summary of replies
Keywords: ISA, SCSI, adapter
Message-ID: <2757@bruce.cs.monash.OZ.AU>
Date: 29 Jul 90 03:56:59 GMT
References: <2683@bruce.cs.monash.OZ.AU> <8527@inco.UUCP>
Distribution: comp
Organization: Monash Uni. Computer Science, Australia
Lines: 46

In article <2683@bruce.cs.monash.OZ.AU> I wrote:
>...
>Adaptec AHA1540B/1542B
>----------------------
>* Maximum SCSI->AT transfer rate: 10M bytes/s (motherboard-dependent).
>	(Bus master DMA, => card drives bus directly, not via the PC's DMA
>	controller.)
>...

In <8527@inco.UUCP> pauls@inco.UUCP (Paul Stygar) writes:
>	[ ref. to above information ]
>	Where did this info come from ????
>	I didn't see any info in the User Manual for the 1542B 
>	to directly confirm this ...

>	The board can be jumpered to set the DMA Transfer Speed
>	to 5.0, 5.7, 6.7,  or 8.0 MB/s (via PINS 12,13 of the J5 Jumper Block)
>	(p. 2-9 of AHA-1540B/1542B User's Manual, Rev 1.0, April 1990).

The 1540A/1542A manual states:

	2.1.1 Bus Master DMA 

	The AHA-1540A/1542A controls the host AT bus as a master and
	transfers data directly to and from main system memory. This
	implementation is known as Bus Master DMA. Bus Master DMA
				   ^^^^^^^^^^^^^^
	greatly reduces the host software overhead because the host CPU
	is no longer required to maintain the DMA channel's address
	pointers and word counts. ...  The Adaptec AIC-560L is the DMA
	controller.  Adaptec's implementation of Bus Master DMA can
	achieve a 10 MB/second burst data rate. ...
		  ^^^^^^^^^^^^

I must admit I don't understand how this is achieved in view of the maximum
jumper-selectable DMA speed of 8 Mbytes/s (perhaps the software-selected
speed can go to 10Mbytes/s?), but there it is.

>	Not to mention,  I don't understand this info,  since the 1542B
>	seems to run OK on an ancient (circa 1988) MICRONICS 20 Mhz
>	motherboard which only supports 4 Mhz DMA transfer rate.

This rate probably refers to the motherboard's DMA controller, not the AT
bus limit.

Andrew