Adaptec AHA1542 SCSI controller

neese at adaptex.UUCP neese at adaptex.UUCP
Thu Dec 21 09:04:00 AEST 1989


>A few months back I saw some patches to change the DMA rate (bus speed) on
>the AHA 154x cards for 386/ix.  I didn't save it, unfortunately.

I don't have the patches for the ISC SCSI driver.  Only for SCO.  As soon
as I get my other system in here, I plan to get ISC up on it and see if I
can find them.

>Now, with the new version of the SCSICNTL.EXE program, I've managed to
>enable read-ahead cache.  It's obvious from the lights that the drive is
>faster than the bus is running at the moment (the green drive LED goes off
>while the bus activity light is still on at times :-).  

Bus activity light?  SCSI bus activity light?

>I have checked out the controller/system combination up to 8Mhz.  It works
>great.  386/ix, however, appears to >ignore< the jumpers on the controller
>(grrrr...)... thus the need for the patch.

Actually, ISC may be reprogramming the DMA speed.  This overrides the jumper
settings.  Do you have the J1 pin 1 jumper installed?  That is the
synchronous enable jumper (actually a misnomer, it just dictates who will
negotiate for synchronous protocol).

>If someone has it, I would appreciate the particulars.  

You could look through the kernel init routine for the driver and look for
a 'push 0x9'.  This is the DMA program command.  Something shortly after
that will be the speed byte.  If it is hardcoded, then it will be a 'push
0x0 and then an out instruction.  This is the byte that needs to be changed.

>(Ps: Is it unusual for systems to work at 8Mhz DMA rates?  Our 20Mhz systems
>     do just fine even at full speed..... although the 25 cache boards we 
>     have tried ALL fail with the cache enabled!)

If you are refering to the host adapter DMA rate.  It is unusual for any
system to work at 8Mbytes/sec.  The only one I knew that would do that was
the Tandy 4000LX.  You really don't want to run the adapter at that rate.
The best rate is 6.7MBytes/sec.  This is due to the speed that data will
move across the SCSI bus.  At a max rate of 4MBytes/sec, with the CDC, you
will be starving the data buffer much faster and causing more disconnects/
reconnects to occur and thus a slow down.  The 6.7MByte rate is a good rate
to keep the SCSI bus in balance with the AT bus.  It is fast enough to keep
the data moving across the SCSI bus at max speed, while not emptying the
host adapter buffer so fast that an unneccessary disconnect will occur.

Karl, send me some mail, and let's see about a straight connection.

			Roy Neese
			Adaptec Central Field Applications Engineer
			UUCP @ {texbell,attctc}!cpe!adaptex!neese
				merch!adaptex!neese
				uunet!swbatl!texbell!merch!adaptex!neese



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